Recently, the world celebrated an International Day of Climate Action, called “350″, which is based on lifting public awareness on the need for an international climate treaty to reach a 350 parts per million CO2 level as a target threshold. I didn’t really join in on the fun or follow it in any detail, but from what I understand it was a pretty big deal, and I hope that they had some success in raising awareness.

In any case, I just wanted to say a word about the communication of climate science. It is very easy to justify, scientifically, that the globe is warming and that anthropogenic (human-induced) activities are responsible for at least a large part of that warming trend. It is easy to justify that continued use of fossil fuels under “business-as-usual” scenarios will eventually lead to consequences which can be dangerous to socio-economic and ecological welfare. The first two Working Groups of the IPCC 2007 report have an extensive background into these issues, as does many other reports and papers. It is therefore not necessary to over-inflate the consequences of climate change or to assume “worst-case” scenarios in analysis, and I urge people to consider this, since the consequences of this for a public attention, policy, and scientific viewpoint can be just as bad as those who downplay or misrepresent the urge to take climate action.

This brings me to a report online entitled The Economics of 350: The Benefits and Costs of Climate Stabilization (PDF file). The report is written by a group of economists (Ackerman et al.) who focus on the subject of emission targets, which urge quick action, and while I agree with much of what they write (and their concern about climate change and the urgency to act), much of the science in the paper is flawed or incomplete to emphasize their point that “climate change is occurring faster, and its consequences could be more severe, than previously expected” (Pg. 3). Such broad statements should be much more detailed (e.g., sea ice loss might be going up faster than expected, but temperatures are not) and the Copenhagen Synthesis report is a suitable citation for post-AR4 findings on this topic. I would just like to touch upon some points:

– Ackerman et al. describe climate sensitivity values cited in the IPCC AR4 (2 to 4.5 C per doubling of CO2) and sensitivity values in Hansen et al (2008) (~ 6 C per doubling of CO2) in “Target Atmospheric CO2: Where Should Humanity Aim?” in The Open Atmospheric Science Journal 2: 217-231. Ackerman et al. believe these these values are inconsistent, and specifically “That is, [Hansen et al] argue that the global warming likely to result from any given atmospheric concentration of CO2 is approximately twice as great as AR4 projected.”

This is not correct and is an apples-to-oranges comparison. IPCC 2007 uses the so-called Charney sensitivity, which refers to an equilibrium state in which CO2 is doubled. This accounts only for so-called “fast feedbacks” which respond relatively rapidly to climate change, such as sea ice, or water vapor content. The Hansen et al. value refers to a much longer term sensitivity value which takes into account slower feedbacks such as ice sheet and long-term vegetation changes. Hansen et al does not argue for a sensitivity outside the IPCC range when you compare the two appropriately.

– The assumption of a 6 degrees per 2x CO2 sensitivity is too high and unwarranted. The authors do explore alternative scenarios, however.

–Ackerman et al. continue to assert that scientific findings are becoming too troublesome. For instance,

The “climate sensitivity,” that is the amount of warming that will result from a doubling of the atmospheric concentration of greenhouse gases, may be inherently uncertain — because in a system such as the earth’s climate with strong positive feedbacks, small errors in estimating the size of the feedbacks inevitably cause large errors in the outcome (Roe and Baker 2007).

However, Ackerman et al ignore Hannart et al (2009) which suggests the Roe and Baker analysis may be an artifact of a strange use of the word ‘uncertainty.’

– Ackerman et al state:

Low-level clouds, one of the least understood aspects of the climate system until recently, may be a source of additional positive feedback to the warming process; of the major climate models, the one that simulates clouds most accurately is also the one that predicts the most rapid warming (Clement et al. 2009).

Although it may very well be true that clouds act as a positive feedback, Clement et al. can only be taken as a very small part of the literature on this topic. In particular, they only focus on a very small region of the globe (roughly 115° to 145°W, 15° to 25°N) and focus only on lower-level clouds that are important for albedo, and not higher ones which also can have a very large impact on sensitivity due to their impact on outgoing terrestrial radiation. A more comprehensive discussion of the literature, or at least a reference to more comprehensive discussions (e.g. Bony et al 2006) should be included.

–Ackerman et al. state,

There are also some lesser known, serious threats (Lenton et al. 2008; Weitzman 2009). For example, rising temperatures could trigger abrupt, massive releases of methane from undersea geological formations (clathrates) or from permafrost; this could lead to a runaway greenhouse effect (Hall and Behl 2006).

A runaway greenhouse effect is not something that is taken seriously as a possibility right now, and it has a very precise meaning in the study of climate physics and in the evolution of planetary atmospheres. It does not simply refer to a “tipping point” or dangerous amount of warming. Certainly, I can find no support for such a proposal in the reference by Hall and Behl (I cannot even find the word ‘runaway’ in that document).

–Ackerman et al state

Hansen’s assertion that 300–500 ppm CO2 corresponds to a 25 percent chance of harm is consistent with Harvey’s results under the assumptions that non-CO2 gases either remain at their current levels or decline, and that aerosols have no effect on temperature

There’s some sketchiness about what exactly “harm” means here, but Hansen et al only quote this CO2 range once, and they simply do it in the context of referencing Harvey’s results for background on the topic. It should not be taken to be “consistent” with Harvey’s reference.  It came from Harvey’s reference!

– Ackerman et al. summarize the strategy by Hansen et al (2008) to stabilize emissions, although I personally don’t feel they do a good job of error bars (there are essentially none in their descriptions or assumptions). A lot of numbers are given in this document which should have some brackets next to them. Another example of overselling certainty is the claim that “The combined radiative forcings from aerosols is -1.18 W/m2″.  It is not until a later graph that one can inspect a see a range of about 0.3 to 2 W/m2.

These are just some of the quibbles I have with this document.  I also would recommend that documents which go into even some detail about the science (and not just the economics) have better correspondence with scientists in that field.  I do agree with their focus on how to best stabilize emissions, although I find it difficult to get quantitative information from this piece.

I haven’t been able to post much lately, so I just want to put in this post which outlines some of the basic radiative forcing and feedback physics which climatologists use to assess climate change. This is fairly standard material which should be understood by anyone with a deep interest in climate. This article is a bit lengthy so hopefully you have the patience to go through it (or put it on your favorites and come back). Also, a lot of discussion has come up recently over Richard Lindzen’s ERBE analysis in which he purports to show that global climate sensitivity is small, and that the net effect of climate feedbacks is to dampen the so-called Planck response. That basically provoked this post. I’m going to define all these terms below, so don’t worry if I’ve already lost you, and while I am going to do some math in this post, it should be accessible to most people who know a bit of algebra. Skipping over a few calculus steps won’t be detrimental and I’ve tried not to assume much climate background (although I do link to some side references for clarification on some matters). My focus is not on Lindzen’s analysis here, which I don’t feel to be robust at all, but rather building up simple mathematical models for understanding climate change. This will not be new to anyone who has followed the climate literature or discussions for some time, but hopefully it can be helpful to some, or at the very least, serve as a useful reference.

The primary focus of study within the atmospheric sciences for grasping how climate change works is in electromagnetic radiation, and how radiative fluxes interact with the surface and atmosphere interface. We can keep this simple and imagine that planets take in light only at short wavelengths of the EM spectrum from the sun (mostly in the visible region), and that planets emit light back to space only in the longer wavelengths, in the thermal infrared part of the spectrum. As seen below, we can distinguish easily between the solar radiation curves and terrestrial radiation curves, which hardly overlap at all across roughly the 4 micron threshold:

For all the terrestrial planets (Venus, Mars, Earth) we can say that the heating due to radioactive decay in the Earth’s interior is negligible, and they only gain and lose heat radiatively (since outer space is a vacuum).

The radiative balance at the top of the atmosphere acts as the fundamental boundary condition which constrains the global climate. In this model, we assume that the energy input by the sun at the top of Earth’s atmosphere ends up being balanced by the outgoing infrared radiation to space. This is true over sufficiently long timescales when the climate is not undergoing change. If it were not true, then the planet would either warm or cool depending on whether more energy was coming in than going out, or more was going out than coming in. The simplest model for radiative balance can then be written as:

The “A” terms describe the area of which the planet receives or emits radiation. For Earth, the outgoing energy leaves in all directions, so we can take the area to be however, the incoming energy only comes in like a circle (essentially the shadow that would be cast by the planet) because the sunlight comes in from just one side and is not as intense over the whole half-sphere. Thus the A₁ term is . In reality, the ratio of A₂ to A₁ is about 4.0034 (Loeb et al 2009) and not 4, since the Earth is not a perfect sphere, but the algebra is made much easier in assuming sphericity, and the radius of the Earth becomes irrelevant since those terms cancel out. S₀ here is the solar constant, which is the radiative flux (in Watts per Square meter) that would be intercepted by a flat “wall” in space which stood perpendicular to the incoming solar rays. For a planet at the mean Earth-sun distance, the solar constant is about 1370 W/m². is thus the incoming solar radiation averaged over the spherical Earth, with a factor of at the equator and declining like the cosine of the latitude angle as you move toward the poles. is the albedo of the planet, which is the fraction of incoming solar radiation that is reflected right back out to space (mostly clouds, but also by various land surfaces, brighter ones like ice or desert sand contributing strongly). The fraction of absorbed sunlight is therefore 1- . The albedo of the Earth is presently about 0.3 (i.e., 30% of the incoming solar energy is reflected back to space). is the Stefan-Boltzmann law which relates the total power output (per unit area) of a body to a given temperature, and sigma is about 5.67 * 10^-8 W/m²/K⁴. Basically it shows that hotter bodies radiate much more than cooler bodies, because of the strong fourth power dependence. Temperatures must be expressed in Kelvins. From all this, we can solve for an “effective temperature” of a hypothetical planet which essentially radiated like a perfect blackbody and had uniform temperature over the globe:

Plugging in all the relevant parameters into the equation, the effective temperature becomes about 255 K, or about 0 degrees F. The reason why the planet is not actually this cold is because the atmosphere acts to inhibit the efficiency at which the outgoing infrared radiation escapes to space. This is the greenhouse effect, and is caused by molecules (water vapor, CO2, ozone, methane, mostly) which strongly interact with infrared radiation at Earth-like conditions. I’ve already done a couple of posts (see Part 1 , Part 2 and this post) on how the greenhouse effect works, and I’m assuming most readers are either educated in that or can get a somewhat decent understanding by reading the above links (wiki actually does a good job IMO). These will also help place the preceding discussion in better context.

We’ve already established that at equilibrium, the difference between the absorbed sunlight and outgoing energy at the top of the atmosphere is zero. Now suppose we perturb the climate system and force the global temperature to change by changing the amount of sunlight we get (or the planetary reflectivity), or as is the case in modern times, changing the outgoing infrared radiation with greenhouse gases. We quantify such a perturbation in terms of radiative forcing, or loosely the difference between the incoming radiation energy and the outgoing radiation energy (there’s some caveats in here about allowing the stratosphere to adjust to equilibrium, and some people define forcing at the tropopause or Top Of Atmosphere, and other alternate definitions have come up in the primary literature, but those details are not really important right now).

We can include a number of different forcings which may have competing effects (e.g., greenhouse gas increase would warm the planet, turning down the sun would cool, increasing sulfate-based aerosols would raise the albedo and cool the planet a bit). We can also introduce an efficacy factor (described in Hansen et al 2005) which is the global temperature response per unit forcing for a given forcing agent relative to the response to a standard CO2 increase,

Where FA is the climate forcing, and the summation includes all relevant radiative forcings over a certain time period. Efficacy arises because not all forcings produce the same relative impact, and some produce stronger or weaker changes in climate than CO2 (defined so = 1) for the same forcing. The radiative forcing for CO2 (as described in Myhre et al 1998 and later papers) is

Where the constant k (derived from line-by-line radiative transfer codes) is typically taken to be 5.35 W/m², and C and C₀ are the final and initial CO2 concentrations. Using this simple relation results in a forcing of nearly 4 W/m² for each doubling of CO2. Note that the logarithmic relation suggests that the fractional change in CO2 is what is important, since every doubling produces the same effect. In that sense, adding 10 ppmv of CO2 to a background concentration of 20 ppmv would produce a much larger change than adding 10 ppm to a background concentration of 1000 ppmv. This relation holds well over relevant Earth-like conditions, however the forcing becomes stronger than logarithmic at very low or very high concentrations. Also note this is the forcing at the tropopause, not the surface. Here is a table of modern day forcings relative to 1750 values (IPCC 2007)

It makes sense to ask now what this actually means for us. In other words, how much temperature rise would you get for a given forcing? We use a matric called climate sensitivity to make sense of this. Climate sensitivity is the temperature response of the system per unit forcing. In other words, a high climate sensitivity means that it is very easy to change the global mean temperature, while a very low sensitivity would require an enormous forcing to get that same change. In the easiest case, we’ll consider what happens when you only increase some forcing (say double CO2) and allow the outgoing radiation to increase (according to the Stefan-Boltzmann law) to re-establish a new radiative equilibrium. Here, nothing else changes with the climate state (no cloud cover changes, no ice melts, etc) except for our forcing. This is the so-called Planck response. In a simple way, we can assume that the surface and emission temperature are linearly related, in which case the Planck-only feedback response can be computed as the inverse of the derivative of Stefan-Boltzmann with respect to temperature,

Which equals,

The temperature response can then be linearly related to a forcing

Where is the Planck-feedback factor described above. It is important to note now that this is an equilibrium formula, meaning that we don’t see the full temperature response to show up right away if we instantly double CO2, since it takes time for the radiative imbalance to go to zero (it’s hard to heat up the oceans quickly!). We’ll see that when we actually allow other things like clouds,water vapor, albedo, etc to vary with the climate response (as opposed to the unrealistic stefan-boltzmann only feedback), then lambda becomes a function of all those things, and describes how the total forcing is connected to the temperature response. This formula implies that for a 4 Watt per square meter forcing (remember, about a doubling of CO2 equivalent), you get roughly a 1 K temperature rise (multiply these numbers by two to get changes in Fahrenheit).

To compute a radiative forcing for an increase in solar irradiance, we do

where the 1/4 and 0.7 factor account for the geometry and albedo of the Earth, respectively. Depending on how radiative forcing is defined, this number can often be reduced further to account for ozone absorption of UV or other effects, but in general the forcing due to a realistic change in solar increase is very small. It follows that it would take about a 22 W/m² change in solar irradiance to produce a 1 K change in global temperature. This is actually a very stable climate. This also demonstrates the intellectual bankruptcy of those who claim that the solar trend over the last half century (which has pretty much been a flat-line when you remove the 11-year oscillatory signal) is responsible for most of the observed late 20th century warming, and simultaneously argue for a low sensitivity.

Paleoclimate evidence suggests much larger climate changes have occurred than is possible under realistic forcing scenarios given this sort of sensitivity. The magnitude of glacial-interglacial cycles for instance is on the order of 4-6 K in the global mean, and when you go back in time far enough, much larger climate changes are possible. Even observed trends over the 20th century do not appear to be compatible with a very small sensitivity factor. A useful summary of Earth’s equilibrium sensitivity evidence can be found in Knutti and Hegerl 2008. The best available evidence over the last few decades of research (discussed in IPCC 2007 especially) hints at an equilibrium temperature sensitivity of 2 to 4.5 K (as opposed to 1 K) per doubling of CO2. It can thus be inferred (and supported by a larger body of evidence) that other things are acting to amplify the Planck-response to create a climate which is more sensitive to changes. This will be elaborated upon briefly.

The 2 to 4.5 K value range is the so-called Charney sensitivity. Remember this is the equilibrium response, not the immediate response, and so this is probably not a realistic realization for what to expect over the course of this century. The transient response for a doubling of CO2 (this is defined by assuming that CO2 increases by 1% per year and then recording the temperature increase at the time CO2 doubles) is about 1.3 to 2.6 K in the CMIP3 archive (USCCP 2008) which is less than the longer-term response, and also features less uncertainty. There is also a very long climate sensitivity response which is relevant on timescales of many hundreds to thousands of years and included “slow feedbacks” like ice sheet changes, and is on the order of around 5 K for a doubling of CO2.

Now we consider feedbacks to understand why the actual climate response differs so much from what you’d expect with just the radiative forcing and radiative adjustment. A feedback is essentially something which acts to amplify or dampen the initial forcing. The important distinction is that the forcing “pushes” the climate into a new state, and the feedback simply responds (it doesn’t occur on its own in a stable climate), either pushing the system further in the direction of the initial forcing (positive feedback) or dampening the response which brings the system closer to the initial climate state (negative feedback). The primary radiative feedbacks are water vapor feedback, the lapse rate feedback, cloud feedbacks, and surface albedo feedbacks. Useful summaries of this science can be found in Bony et al 2006 for example, although there’s a lot of good literature here. IPCC 2007 is generally the most comprehensive. I’ll briefly summarize the individual feedbacks below. We now see that lambda is not the Planck-feedback value, but instead is a function of all the possible feedbacks which can occur, and is thus different than the no-feedback scenario (unless all the possible feedbacks happened to cancel out perfectly)

Where these things can also be separated into longwave and shortwave radiation components. Note that the Earth is very inefficient at reflecting infrared radiation at all, so this is not an important term. Reflection of visible radiation is very important however as we’ve seen in describing albedo. Gases in the atmosphere are generally not very good at absorbing incoming sunlight, but rather make the atmosphere opaque to the outgoing infrared. If we change some external variable (e.g., solar constant, more CO2), which we’ll call , and the response depends on a number of other variables, x_j (which are then related to ), then

where the summation ranges from j=1 to the number of important response variables, which could be large, with each component having different (or sometimes self-competing) effects that complicates the picture. Indeed, future projection for climate change to a given change in CO2 is much more uncertain in then it is in the forcing, since GHG forcing uncertainties are typically very small. Aerosol forcings are more uncertain (and restrict knowledge of the total 20th century forcing) but GHG changes should strongly outweigh aerosol changes over the 21st century. Accordingly, future prediction of temperature change depends very much on understanding the individual response variables to climate change and the total response. The following descriptions are far from sufficient, but just to get the feet wet

Water Vapor:

The saturation vapor pressure of water (loosely, “the amount of water the air can hold”) increases nearly exponentially with temperature. This follows from the Clausius-Clapeyron equation (Pierrehumbert et al 2007):

Where TR and are a reference temperature and saturation pressure (614 Pa at 273 K) and x corresponds to the latent heat of the phase transition divided by the gas constant, and takes a value of 5419 K for condensation into liquid and 6148 K for condensation into ice. The dominant influence of the saturation vapor pressure is temperature, increasing sharply, with a 3 K temperature resulting in roughly 20% change in saturation pressure. However, the radiative influence of water vapor depends on the fractional change of water vapor (just like CO2) and not the absolute increase, and so the absorptivity goes as the logarithm of the vapor mass change. Note that this has nothing to do with relative humidity (the fraction of vapor held in air to its saturation amount), but rather specific humidity. Why relative humidity is often discussed in water vapor feedback context is to see how the actual water vapor change scales with the upper limit provided by Clausius-Clapeyron. In the global mean, RH tends to remain roughly constant, which coupled with a change in temperature will yield a positive feedback.

The effect of water vapor is to reduce the outgoing radiation vs. temperature curve, effectively making the climate much more sensitive to forcing. It is important to note that most of the water vapor feedback occurs in the higher altitudes where it is dry and temperatures are cold, and so can be most effective at reducing the outgoing radiation to space. It is also most important in the tropics. The lower level boundary layer water vapor has relatively little to do with water vapor feedback (although it does have implications for other hydrological responses to climate change). Furthermore, water vapor also absorbs visible radiation. This component is far less important than the infrared part, although it can be important in the polar regions where you have scattering from a high surface albedo.

Observations and models are in basic agreement that the upper troposphere becomes moister in a warming climate (e.g., Soden et al 2005), global relative humidity is approximately conserved, and thus enhancing the warming. A good summary of water vapor feedback science that is up-to-date is the Dessler and Sherwood 2009 perspective piece in Science. Water Vapor feedback is the most powerful positive feedback and enhances the warming forced by CO2 by a factor of roughly two.

Lapse Rate:

Because the greenhouse effect depends on the temperature decline with height, decoupling the atmosphere from its current vertical temperature structure will also change the surface temperature. As it is, the whole troposphere is pretty much created by convection, and when it warms or cools it does so as a unit in a way that keeps it near a moist adiabatic lapse rate. The lapse rate change tends to offset some of (but not all) the water vapor feedback. Counterbalancing the water vapor feedback results in warmer temperatures at the high altitudes, more water vapor meaning more condensation, and lifting to higher altitudes so that any given layer of the atmosphere is now radiating more efficiently to space. This can be seen in a simple diagram which shows a steepening of the moist adiabat curves as the climate warms. It follows that “moist regions” will tend to be amplified at altitude relative to the surface.

See here for a larger version of this diagram. The lapse rate feedback tends to thus be negative in regions of moist convection at the tropics, but positive in the high latitudes where surface warming is expected to be amplified. The net effect globally is a negative feedback. This issue also surrounds the whole “hotspot” argument that I’ve discussed before, and whether or not the low-latitude troposphere actually has been amplified relative to the surface. Note again that the hotspot is not a manifestation of higher CO2, just higher temperatures and the fact that we see a moist adiabatic structure during El Nino, the solar cycle, etc. In reality, if such a hotspot doesn’t exist, it just means a less negative lapse rate feedback.

Surface Albedo:

Because the reflectivity of the planet is so important (see Equation. 1) since it directly relates to the absorbed solar energy, changes in the surface that accompany climate change will act as feedback. This occurs when the ratio of a high albedo surface to a low albedo surface increases or decreases in time. The best example is with sea ice, since ice extent tends to increase (decrease) in a cooling (warming) planet, and since ice is much more reflective than surrounding ocean or land, you get a positive feedback. This is one large component of “polar amplification” which describes why high latitudes are more sensitive to climate change than lower latitudes. Warm (cold) climates are therefore characterized by weak (strong) pole-to-equator temperature gradients. Less ice in a global warming situation means more solar absorption which winds up resulting in higher surface air temperatures. The same idea applies to a once forested area which becomes a desert. Sea ice is included in the Charney sensitivity, but not long-term changes in the much larger ice sheets.

It is a very robust result seen in observations and universally across models that the Arctic will warm faster than the Northern Hemisphere as a whole. Over anthropogenic timescales, the Northern amplification is also much more pronounced than at the South Pole. A somewhat more complete picture is that as the climate warms, the summer melt season lengthens which results in reduced sea ice at summer’s end. The summertime absorption of solar radiation in open areas enhances the sensible heat content of the ocean, and thus ice formation in the autumn and winter is delayed. Note that the surface is not highly amplified in the summer where excess energy goes into evaporation or melt, but enhanced upward heat fluxes in the cooler months result in increased temperatures in the lower atmosphere.

Clouds:

Clouds are the largest source of uncertainty in quantifying the extent of climate feedbacks. I’m not actually going to talk about them here (maybe soon! I’d like to explore the various hypotheses and evidence in much better detail) but suffice to say that clouds have competing effects between reflecting sunlight (low clouds mostly) and influencing the outgoing infrared radiation (high clouds mostly). It is still not clear how these two effects will balance out, and thus the magnitude and even the sign of the feedback is not well constrained. It’s probably not very big in either direction, although much of the uncertainty range in the 2 to 4.5 K values for a doubling of CO2 is because we just don’t have clouds nailed down yet in a satisfactory manner. Cloud influence also depends on latitude, optical thickness, and a host of other issues.

Feedbacks behave in a power series like fashion, with small and diminishing gains as time progresses. This looks like g + g² + g³… and so forth, so the temperature response can be related to the planck-only response and the gain factor as

Here, “the sum of g’s” must be less than unity to allow for the possibility of positive feedbacks while still not allowing a “runaway” effect that is unstable. Note also that the feedbacks operate dependently on each other, so a stronger water vapor feedback would mean warmer temperatures, still less ice, a still lower albedo, and so forth.

Below is a plot showing relative strengths of individual feedbacks for water vapor, water vapor+lapse rate, albedo, CRF (not discussed), and clouds, computed for 14 coupled ocean–atmosphere models.

_{Soden et al 2008}

References:

_{Bony, S., R. Colman, V.M. Kattsov, R.P. Allan, C.S. Bretherton, J.-L. Dufresne, A. Hall, S. Hallegatte, M.M. Holland, W. Ingram, D.A. Randall, D.J. Soden, G. Tselioudis, and M.J. Webb, 2006: How well do we understand and evaluate climate change feedback processes? J. Climate, 19, 3445-3482, doi:10.1175/JCLI3819.1}

_{CCSP, 2008: Climate Models: An Assessment of Strengths and Limitations. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research [Bader D.C., C. Covey, W.J. Gutowski Jr., I.M. Held, K.E. Kunkel, R.L. Miller, R.T. Tokmakian and M.H. Zhang (Authors)]. Department of Energy, Office of Biological and Environmental Research, Washington, D.C., USA}

_{Dessler, A.E., and Sherwood, S.C. A matter of humidity, Science, 323, 1020-1021, DOI: 10.1126/science.1171264, 2009.}

_{Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland, 2007: Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.}

_{Hansen, J., Mki. Sato, R. Ruedy, L. Nazarenko, A. Lacis, G.A. Schmidt, G. Russell, I. Aleinov, M. Bauer, S. Bauer, N. Bell, B. Cairns, V. Canuto, M. Chandler, Y. Cheng, A. Del Genio, G. Faluvegi, E. Fleming, A. Friend, T. Hall, C. Jackman, M. Kelley, N.Y. Kiang, D. Koch, J. Lean, J. Lerner, K. Lo, S. Menon, R.L. Miller, P. Minnis, T. Novakov, V. Oinas, Ja. Perlwitz, Ju. Perlwitz, D. Rind, A. Romanou, D. Shindell, P. Stone, S. Sun, N. Tausnev, D. Thresher, B. Wielicki, T. Wong, M. Yao, and S. Zhang, 2005: Efficacy of climate forcings. J. Geophys. Res., 110, D18104, doi:10.1029/2005JD005776}

_{Knutti, R. and G. C. Hegerl, 2008, The equilibrium sensitivity of the Earth’s temperature to radiation changes, Nature Geoscience, 1, 735-743, doi:10.1038/ngeo337}

_{Loeb, N.G., B.A. Wielicki, D.R. Doelling, G.L. Smith, D.F. Keyes, S. Kato, N. Manalo-Smith, and T. Wong, 2009: Toward Optimal Closure of the Earth’s Top-of-Atmosphere Radiation Budget. J. Climate, 22, 748–766}

_{Myhre, G., E. J. Highwood, K. P. Shine, and F. Stordal., 1998. New estimates of radiative forcing due to well-mixed greenhouse gases. Geophysical Research Letters, 25, 2715–2718}

_{Pierrehumbert RT, Brogniez H, and Roca R 2007: On the relative humidity of the atmosphere. in The Global Circulation of the Atmosphere, T Schneider and A Sobel, eds. Princeton University Press}

_{Soden, B. J., D. L. Jackson, V. Ramaswamy, M. D. Schwarzkopf, and X. Huang, 2005: The radiative signature of upper tropospheric moistening. Science, 310(5749), 841-844}

_{Soden, B.J., I.M. Held, R. Colman, K.M. Shell, J.T. Kiehl, and C.A. Shields, 2008: Quantifying climate feedbacks using radiative kernels. Journal of Climate, 21, 3504-3520}

Yesterday, I had the opportunity to attend a colloquium seminar where Zhengyu Liu of University of Wisconsin-Madison gave a presentation on a couple of topics. One was on his recent paper which I discussed not too long ago concerning transient simulations of the the deglacial climate evolution. He also discussed the stability issues of the Thermohaline circulation and how intermediate models of AMOC tend to exhibit hysteresis behavior while fully-coupled AOGCM’s do not. I won’t touch on that but I’ll touch briefly on a few key points concerning the time period of roughly LGM to Bolling Allerod and some background. The regional and global-scale responses of atmospheric warming and their causes are explained better in my first post which is linked above. I also discuss the time period of LGM-BA in a bit more detail in case readers are unfamiliar with these events.:

– At present, the THC is characterized by a strong North Atlantic Deep Water (NADW) and a moderate Antarctic Bottom Water. In contrast, the LGM is characterized by dominant Antarctic bottom water and shallower and weaker NADW (See Shin et al 2003, GRL).

– A number of climate models gets the thermohaline depth at the LGM wrong (without cheating a little bit of course). Some models even show enhanced LGM NADW circulation, which is inconsistent with paleoclimatic data. One model exception is CCSM which was the model used by Liu et al. and discussed in the above link.

– At the LGM, sea ice extent in the Northern Hemisphere actually changes relatively little. Sea ice thickness changes significantly, perhaps doubling, but in Antarctica the Southern Ocean is characterized by deep convective mixing, which prevents thickening of sea-ice. Instead sea ice extent changes significantly in Antarctica at the LGM. This is true even today where Antarctic sea ice extent shows much more interannual and seasonal variability than the North (and also why global sea ice extent is not really a good proxy for the cyrosphere response to global warming).

– When ice is formed from sea water, salt is rejected from the crystal structure which results in the formation of brine and adds salt to the water underneath the ice and increasing the density. A significant fraction ( > 75%) of the increase in Antarctic Bottom Water at the LGM (relative to present) is explained by increased brine injection in the Southern Ocean. This makes the deep ocean saltier relative to present day. This denser Atlantic Bottom Water penetrates into the North Atlantic along the ocean bottom and shallows the North Atlantic THC

_{Liu et al 2009}

The above also shows that the well-known “bipolar see-saw response” applies to SST’s while the subsurface ocean warms throughout the Atlantic. An increase in meltwater flux starting around 20kya induces a gradual decrease in the AMOC and freshens glacial bottom water. From 17 ka to the Bolling-Allerod, the meltwater flux decreases and thus AMOC shows a gradual recovery but resumes abruptly (see part D).

_{Liu et al 2009}

– Dr. Liu proposes that the North Atlantic THC is controlled predominantly by the climate forcing over the Southern Ocean at the long glacial-interglacial cycle timescales, but by the North Atlantic climate forcing at the short timescales. This idea is best illustrated in his 2006 paper although the idea itself is older.

– For a similar forcing (say a globally mixed tracer like carbon dioxide), the initial response is to favor a much more pronounced Northern warming due to higher heat capacity and the buffering effect of the Southern Ocean. On longer timescales, the cooling becomes more important in the South because of the stronger ice-albedo feedback, which depends on extent and not thickness. This thin and vast sea ice is more sensitive to forcing than in the North and enhanced formation leads to enhanced brine input. Just for illustrative purposes, we can look at the temperature response to 2xCO2 in a slab ocean GCM compared to a model with a full-depth ocean GCM. Without an ocean GCM, the warming is nearly symmetric (except for Antarctica). Slab ocean models just have a shallow upper ocean, so the atmosphere and ocean come into equilibrium very quickly. Models with a deep ocean take thousands of years to come into equilibrium because heat is “sequestered” into the deep ocean. Presumably the two curves would look the same if we could run the full depth ocean GCM out for thousands of years. See Bitz et al 2006 for further details.

_{Cecilia Bitz, personal correspondence.}

– CO2 is a primary forcing during the LGM and the leading causes of glacial THC anomalies. I also posted a while ago on the role of the Southern Ocean on atmospheric CO2 levels and the effects of the westerlies on upwelling, so that paper may be worth going over again.

– Under this view, the shallow/weak NADW is caused by strong Atlantic Bottom Water, caused by stronger sensitivity of sea ice in the SH to lower CO2. Faster responses (usually associated with abrupt climate changes) are dominated in the North, since it takes too much time for the Southern Ocean to change much for some perturbation. However, on much longer timescales, we have a proposed dominant Southern Ocean forcing of the North Atlantic THC.

Hopefully people interested in the blog wars have been alerted to the ongoing climate change “debate” between George Monbiot and Ian Plimer. If not, the best place to start is probably Monbiot’s blog itself (with several posts on the topic already). Greenfrye and Tamino also have some ongoing commentary, so have fun catching up on what’s going on.

Unfortunately, round 1 consisted of Plimer dodging Monbiot’s questions which ask Plimer to defend certain indefensible statements in his book “Heaven and Earth.” Maybe Plimer just “wanted to go first” so I’ll give him the benefit of the doubt, but his own set of questions intended for Monbiot are quite revealing about his intentions.

As most commenters have suspected, these questions are all ill-posed or have nothing to do with the attribution or prediction of future climate change.  In most cases, if an answer is even possible, it’s not very meaningful information.   It’s just a distraction, something Monbiot wanted to avoid in his debate conditions.  They’re an obvious way to let Plimer set the tone of the “debate” by making it sound scientific to readers who know nothing about the subject. I’m not going to “answer them” in much detail but just to address the tactics behind a handful of them I am creating a post. I will open up commenters to address some other ones.

1. From the distribution of the vines, olives, citrus and grain crops in Europe, UK and Greenland, calculate the temperature in the Roman and Medieval Warmings and the required atmospheric CO2 content at sea level to drive such warmings. What are the errors in your calculation? Reconcile your calculations with at least five atmospheric CO2 proxies. Show all calculations and justify all assumptions.

It is rather strange why Plimer requests agriculture as a proxy for paleotemperature (and what’s stranger is what this has to do with anything). The answer about the error is simply that it will be “large” and will not be indicative of global temperatures, even if temperature is the dominant climatic variable picked up by changes in vines and crops. What’s more interesting is the statements about Carbon Dioxide. Plimer plays an interesting trick here and assumes that these climate changes were in fact caused by Carbon Dioxide (which they weren’t). The question about CO2 content at sea level is a hypothetical question (how much CO2 would it take to cause the climate change?), but then he asks Monbiot to reconcile this with observations (I’m not sure why he needs five, I’d just use ice core records here as a starting choice.) But what if CO2 didn’t cause those warmings? In that case, Monbiot could answer the hypothetical first question, but there would be nothing to answer for the second part of the question since the justification itself would be expected to be void. It’s like if someone asked me how much the globe would cool if we removed all the CO2 from the atmosphere (which is an interesting question) but then asking me to back that up with proxy data, as if this actually happened before. Even to answer the first question about “CO2 content” would include some caveats since it’s the *change* in CO2 that matters for driving warming, not the absolute content, and it’s the fractional increase (not the absolute increase) that matters and so you’d need to know the baseline value of CO2. This is not difficult to get from proxies (and not much different from pre-industrial values) but the question as stated makes little sense. Furthermore, CO2 is well-mixed in the atmosphere and so there’s no reason to confine the question to sea-level, especially since CO2 in the upper atmosphere matters significantly for radiative transfer.

Tabulate the CO2 exhalation rates over the last 15,000 years from (i) terrestrial and submarine volcanism (including maars, gas vents, geysers and springs) and calc-silicate mineral formation, and (ii) CH4 oxidation to CO2 derived from CH4 exhalation by terrestrial and submarine volcanism, natural hydrocarbon leakage from sediments and sedimentary rocks, methane hydrates, soils, microbiological decay of plant material, arthropods, ruminants and terrestrial methanogenic bacteria to a depth of 4 km. From these data, what is the C12, C13 and C14 content of atmospheric CO2 each thousand years over the last 15,000 years and what are the resultant atmospheric CO2 residence times? All assumptions need to be documented and justified.

The whole point here is to sound smart. There’s nothing here remotely relevant to what Monbiot wanted to debate. One can certainly pull up tree ring or ice core data to get C13/C12 ratios as a time-series, or (probably more relevant) CO2 concentration in the atmosphere whose fluctuations will tell you something useful about net emission/uptake and whether the carbon cycle has been perturbed, as well as residency time. We don’t know emissions from every individual source and there’s also not much point in throwing the “From these data…” connection since certain proxies record CO2 content in the atmosphere to pretty high accuracy without knowing where it came from.

From first principles, calculate the effects on atmospheric temperature at sea level by changes in cloudiness of 0.5%, 1% and 2% at 0%, 20%, 40%, 60% and 80% humidity. What changes in cloudiness would have been necessary to drive the Roman Warming, Dark Ages, Medieval Warming and Little Ice Age? Show all calculations and justify all assumptions.

Changes in cloudiness at 0% humidity indeed!!

Obviously some of these situations are unphysical and so preclude any realistic calculation and cloud change is generally thought to be a feedback. It also depends not only on cloud amount but distribution of cloud type, as changing high clouds and low clouds (or for example one could decrease the area coverage but increase the cloud top altitude) would have different, even competing effects.

One crude estimate presented by Dennis Hartmann is his book “Global Physical Climatology” is that the fractional area of cloud cover is about 50% and has a net -20 W/m2 impact on the energy balance, and thus the partial derivative of the net radiative energy input at the TOA with respect to the total fractional area of clouds would imply that a 10% change in cloud fraction would either offset or double the RF for a doubling of carbon dioxide.

From ocean current velocity, palaeotemperature and atmosphere measurements of ice cores and stable and radiogenic isotopes of seawater, atmospheric CO2 and fluid inclusions in ice and using atmospheric CO2 residence times of 4, 12, 50 and 400 years, numerically demonstrate that the modern increase in atmospheric CO2 could not derive from the Medieval Warming.

Bascially in an overcomplicated way, Plimer is asking to show that the modern rise in CO2 is not a feedback from medieval warming. This is probably some offshoot of the whole “CO2 lags temperature” line. One issue here is that the perturbation lifetime of CO2 is different than the “lag time” which occurs as a response to warming. Even then no one expects an abrupt decadal scale rise of CO2 centuries after the event, which isn’t even what occurred in the glacial-interglacial cycles, which were considerably larger in magnitude and spatial extent then the MWP. The rate of change is today of CO2 is orders of magnitude larger, and the absolute concentration is also much higher than the whole ice core record show and even much longer. We also know that today’s CO2 increase is anthropogenic because CO2 is going into the ocean (ocean acidification, duh) not going out and isotopic signatures of light and heavy carbon changes.

From the annual average burning of hydrocarbons, lignite, bituminous coal and natural and coal gas, smelting, production of cement, cropping, irrigation and deforestation, use the 25µm, 7µm and 2.5µm wavelengths to calculate the effect that gaseous, liquid and solid H2O have on atmospheric temperature at sea level and at 5 km altitude at latitudes of 20º, 40º, 60º and 80ºS. How does the effect of H2O compare with the effect of CO2 derived from the same sources? All assumptions must be justified and calculations and sources of information must be shown.

I laughed very hard at this last one. It’s all a complete mess and meshes together many different problems. There’s absolutely no relation between what we burn for example and the absoprtion properties of water vapor or CO2 (and by the way, for CO2 you want to look near 15 microns and the unsaturated bands on either side). There’s also essentially no effect from water vapor from those sources since it precipitates out quickly and is thus not a climate forcing. This is of course in the gas phase, I’m not sure what information he wants about liquid or solid phases. All togther this is jumbled up nonsense and shows that Plimer is intentionally trying to mislead others.

For other of Plimer’s questions, I’ll let commenters tackle those. It’s unfortunate that skeptics wanted to “debate” for so long and now have this clown representing them, who is just throwing up sciency-sounding but intellectually vacuous smokescreens.

One of the most interesting parts of the paleoclimate record over the last 100,000 years, is the series of abrupt climate changes prior to the Holocene that have occurred on very rapid timescales, ranging from years to decades (Alley et al, 2003). These changes were large, fast, and occurred when the climate was pushed across certain thresholds.

Of particular note, is the well over 20 Dansgaard-Oeschger events since the last interglacial. Typically, a rapid warming on timescales of decades was followed by slower cooling, rapid cooling, and then a brief period of little temperature change. A value near 1500 years between these events is common, although sometimes there are skips and so the spacing could be a scalar multiple of near 1500 years. Successive D-O oscillations become progressively cooler as the cold-based ice sheet grows in Hudson Bay, and when the base of the ice thaws, you get a Heinrich event surge that dumps large number of icebergs that calved from the Laurentide ice sheet into the North Atlantic, via the Hudson Straight (or perhaps other sources such as the Icelandic and British Isles ice sheet). This succession of progressively cooler D-O events, punctuated by a Heinrich event (until the next cycle begins or the climate becomes too warm for an ice sheet to grow) is a Bond cycle. These events are common before the Holocene which led to a climate punctuated by high-frequency variations and a much more variable situation than that which humans have enjoyed over the past 10,000 years.

Heinrich event 1 (which is actually the youngest Heinrich event, not the oldest) occurred approximately 17,000 years ago was followed by an abrupt warming around 14,500 years ago, something known as the Bolling-Allerod interval (an event interrupted by the Older Dryas and the Inter-Alleroid cold periods). This warming is seen in a wide variety of proxy records including the Greenland summit. This was then followed by the cooling into the Younger Dryas, and then finally a warming into our current Holocene interglacial.

The physical mechanisms responsible for the Heinrich event 1- Bolling-Allerod transitions have been controversial. In the recent issue of Science, a study by Liu et al., 2009 uses advanced modeling to tackle this particular question in more detail. One thing is clear: These abrupt changes are related to the Atlantic Meridional Ocean Circulation (AMOC)(loosely, the conveyor-belt or thermohaline circulation).

It has been known for some time that a reduction North Atlantic sinking would warm the south while cooling the north in a bipolar see-saw (Discussed previously at RealClimate), and so understanding how the AMOC, freshwater flux into the ocean, and other atmospheric changes are related is crucial. Attributing causes to various abrupt shifts is very important for our understanding of the physical climate system and possible tipping points in the future. Models of intermediate complexity (e.g., Ganopolski and Rahmstorf 2002) model D-O like events as a threshold process involving stochastic resonance. This is one possible mechanism in which “noise” and a very weak “signal” (a weak but true 1500 year periodicity in forcing) could combine (Alley et al., 2001) although dating issues are such that placing very high confidence on a true periodicity at 1500 years is difficult. Transitions like that of the Bolling-Allerod (which is in some ways similar to a D-O event) could involve surface warming of the North Atlantic or reduced melt water influx, but Liu et al. use the first synchronously coupled atmosphere-ocean general circulation model that goes from the Last Glacial Maximum to the Bolling-Allerod, in a rather unique way to investigate that topic- a transient modeling approach that prescribes the time evolution of external boundary condition changes. They force their model changes in insolation from Milankovitch, atmospheric greenhouse gas concentrations, continental ice sheets and coastlines, and meltwater flux over the North Atlantic and Gulf of Mexico.

The authors also get a bipolar seesaw response characterized by a cooling over the Northern Hemisphere and a warming over the Southern Hemisphere into Heinrich event 1 that is caused by decreased poleward heat transport from the AMOC. They demonstrate warming that is global in spatial extent from Heinrich event 1 to the Bolling-Allerod, showing signs of polar amplification and maximum warming in the Arctic and North Atlantic. Here is a global illustration of temperature anomalies of Heirich event 1 relative to the LGM, the Bolling-Allerod relative to Heinrich 1, and then the Bolling-Allerod relative to the LGM.

 
_{Liu et al 2009}

At the Bolling-Allerod onset, Greenland warms tremendously, largely due to reduced melt water influx, and Antarctica continues to warm as a result of large increases in greenhouse gas concentration. The large increase in methane is mostly caused by an increase in wetland extent and temperature, as wetlands are the primary source of methane in the pre-industrial time period. Perhaps tropical wetlands were a major contributor since ice sheets covered the primary extratropical methane sources during this time (Chappellaz et al 1993). Changes in the position of the Intertropical Convergence Zone (ITCZ) cause rainfall suppression at Heinrich event 1 and enhancement at the Bolling-Allerod.

The unique aspect of this paper is that many studies in the past, using models of lesser complexity, show that abrupt warming from Heinrich event 1 to the Bolling-Allerod was caused by a sudden resumption of the AMOC in response to a gradual perturbation. However, Liu et al. simulates the Bolling-Allerod warming largely as a linear response to Melt water flux. When the discharge of meltwater from the retreating glacial ice sheets during Heinrich Event 1 stops suddenly, this is where there is a transition to a new state. As the Meltwater flux increases, the AMOC diminishes nearly linearly, in contrast to many intermediate climate models.

Much of the warming into the Bolling-Allerod is thus caused by the AMOC, and also by the increase of both methane and CO2 (about 40 ppmv for CO2) as well as an “overshoot” (by overshoot, they mean this is recovery beyond the glacial-state transport) of the AMOC due to convection in the Nordic sea. Whether this overshoot exists in observational records is actually unclear.

_{Graphic from the non-technical article in Science by Axel Timmermann and Laurie Menviel}

As another note, this kind of modeling needs to be performed in the future by different groups to check the robustness of Liu et al and to provide further perspective on the mechanisms and spatio-temporal extent of abrupt climate change. Unfortunately, as pointed out in the accompanying perspective piece by Timmerman and Menviel , this is a very computationally demanding task, and already involved one and a half years of model number-crunching to get initial results. They close with an insightful line,

“Ultimately, breakthroughs in our understanding of Earth’s climate evolution will come from close interactions between paleoproxy experts, paleoclimate modelers, and climate dynamicists. It is time to train a new generation of scientists familiar with all these fields.”

References:

Alley, R.B., S. Anandakrishnan, and P. Jung. 2001: Stochastic resonance in the North Atlantic. Paleoceanography, 16(2):190-198

Alley, R.B., et al, 2003: Abrupt climate change. Science, 299, 2005-2010

Chappellaz J., et al., 1993: Synchronous changes in atmospheric CH4 and Greenland climate between 40 and 8 kyr BP, Nature 366, 443-445

Ganopolski, A. and S. Rahmstorf, 2002: Abrupt glacial climate changes due to stochastic resonance, Phys. Rev. Let. 88(3), 038501

Liu Z., et al., 2009: Transient Simulation of Last Deglaciation with a New Mechanism for Bølling-Allerød Warming, Science 325: 310-314

Timmerman, A. and L. Menviel 2009: What Drives Climate Flip-Flops? Science 325: 273-274

So I was at work today, and with all the bad weather he had plenty of time to shout at the boss and another co-worker about global warming.  It was a good 2-on-1 handicap match. “Talking” debates are not really my thing since no opportunities exist to check claims, reference sources, show graphs, etc that you could do in online/text correspondence, and so basically anything goes.  Even totally wrong claims like “Volcanoes spit out more pollution than humans do.”

 The boss and the co-worker were very skeptical.  I’m not sure how scientific our exchange was– they spent most of the time trying to convince me I should be very cautious in trusting the general scientific community, and I spent most of the time telling them that they should trust physics, but no one budged.  They’re intelligent group of folk (one trained in biology) but not really familiar with the climate science literature, so I tried to avoid ideas like “radiative forcings,” “water vapor feedback,” “stratospheric cooling,” and other concepts.  So we didn’t really discuss “how CO2 influences climate” or even radiative feedbacks, and it probably was worthwhile as a philosophy of science talk if anything.

Still, I was troubled by a lot of misconceptions they had with terminology, and the basic way in which attribution of warming is (and is not) carried out.  This is something that I’ve run into with other “dinner table” quality discussions.  This post will be nothing interesting to those who have followed the issues, although many of us may lack experience talking to general people like our friends or co-workers who have not engaged in the back and forths of the “debate” or have studied documents by IPCC, National Academies, etc.  Some concepts are:

1) Cycles: This follows the traditional “we’re in a cycle” line of thought.  The justification was essentially that warmer and colder times happened before, and the co-worker reminded me of the ice core bubbles showing ups and downs in the past.  I’m pretty sure she was talking about Milankovitch variations over the last million years.  The term “cycles” in thrown around very loosely in these kind of discussions.  So a few pointers

• The term “cycle” has a precise statistical meaning.  Just because climate changed before doesn’t mean “it’s a cycle.”  The sun has a very clear cycle of roughly 11 years corresponding to changes in solar output, day-night variations are a cycle, there is a seasonal cycle, but in fact true cycles which affect the climate of the planet are not very common.  Milankovitch are probably quasi-cyclical, but also not relevant for modern global change.  Long-term changes in plate tectonics, mountain uplift, and other geologic controls are not cycles.
• Cycles are real physical phenomena, and thus if they change the Earth’s climate they did it in some real way, which needs to be defined.  Seasons for instance are caused by the tilt of the Earth and the motion around the sun, while day-night changes are caused by the Earth’s rotation.  Just saying “it’s a cycle” is not very useful: does this cycle happen to deliver the Earth more solar output, what kind of fingerprints should it leave behind, how long is the cycle, etc?
• Humans are a new part of the equation.  Life can influence the climate.  Plants and other organisms took an entire deoxygenerated atmsophere and put oxygen into it making it suitable for the life we know today (which by the way is not a cycle).  There should be nothing mystical about humans being able to change the climate, particularly as it’s easy to monitor changes in atmospheric chemistry through  emissions.
• Timescale matters: Milankovitch cycles influence climate on timescales of thousands to hundreds of thousands of years.  Day-night cycles influence people on timescales of many hours, while seasonal variations influence people on timescales of months. 
• Forcings on climate need to be added, not replaced.  Different things can change the climate.  If multiple things are changing, then you need to add them up, not pick which ones you like.  If the sun is going up and CO2 is going up, you can’t just say “it’s the sun” because you like “natural” stuff or you don’t like humans or you like big yellow balls of fire, or whatever else.  The influence of CO2 is very well defined and can be calculated with high accuracy and thus no physical justification exists for ignoring it in modern or future global change

2) Self-Correcting mechansisms: My boss was convinced that even if it got warmer, the climate would fix itself.  He was not necessarily referring to what some specialists would call “negative feedback” (some people invoke some cloud-albedo mechanism), and actually he bewildered me with some radical plate tectonics idea about warmer temperatures meaning more volcanic eruptions and a return to today’s climate (although that’s not actually what would happen).   He told me an asteroid could hit the Earth, or whatever else that was beyond prediction.  So, some points

• Asteroid impacts or more volcanic eruptions are not “self-correcting mechanisms.”  They aren’t cycles either.  They are completely random events, and various hypothetical examples could push the climate further or away from the current warming trajectory.  Relying on them for prediction is obviously kind of strange.  The Earth actually doesn’t care what the climate is, and so there is no tendency for it to be the pre-industrial climate.  The concern is us, and current ecological structures.
• Again, timescale matters: The Earth has gone from climates without ice sheets and alligators roaming around in swamps in the present day arctic circle, and there have been climates where Earth was covered (or nearly covered)  in ice.  Some might call, say, the silicate weathering thermostat hypothesis to be a “self-correcting mechanism” although this is important on hundreds of thousands and millions of year scales.  A key observation is that the climate can clearly change and stay in a new state on timescales which are far longer than we need to be worried about, and extra CO2 can influence climate for thousands of years after it is released.  Because the surface boundary conditions and ice sheets will change, there’s also no reason why such “self-correcting mechanisms” should bring us back to this climate or anything which humans will be adapted nicely to. 
• The paleoclimatic record, and even observations in the 20th century (like after the Pinatubo eruption) are incompatible with a very low sensitivity to change, and the geologic record provides a treasure chest of different climates.  Thus there is no basis for claims that further heating will “correct itself” or bring us back to a state like today.

3) Projections vs. Predictions: As noted above, relying on asteroid impacts or alien invasions or worldwide viruses to knock out predictions of future global change is not very worthwhile, and it’s also meaningless to discount current projections because those things “could happen.” 

The trajectory of future global change depends on climate sensitivity, but also on how human actions evolve in the future; the second one is purely up to us.  For instance, whether we decide to stop all emissions today, or steady out CO2 slowly, or do nothing for 50 years and then stabilize are all different scenarios that have different repercussions for climate. 

Therefore projection of future climate carries with it the assumption of various socio-economic scenarios (outlined in detail by SRES), something reasonable probably falls between the yellow line and the red line up there.  This depends on how humans decide to clean up their act, or if we choose to do so at all. This is a different kind of uncertainty than how the climate actually responds to a given change in atmsopheric chemistry, which is why “a doubling of CO2″ is often used as a better metric than “by 2100″ or other date. As described in this paper emissions continue to grow from economic expansion and use.  The world also doesn’t end in 2100, so choosing to do nothing will result in the high end of the above projections without stabilization at the turn of the next century, something a bit misleading with the green and purple line.

4) Attribution:

• How the past record ties into it: How the climate has changed before provides invaluable insight into how sensitive the system is to change, for testing our understanding of what forcing agents matters and on what timescales, etc.  However, it is not very meaningful for the attribution of 20th century warming or projections of future global change.  Humans are a new dimension in global climate, and thus our influence has to be evaluated accordingly.  Just because forest fires occurred naturally in the past doesn’t mean an arson can’t start one today.  Murderers don’t get off on the claim that “people always die anyway” and so humans shouldn’t get off the suspect list just because of how climate changed before, or because our measurements only go back a limited amount of time.
• Assessing spatio-temporal patterns of change: Attribution of climate change to a particular cause(s) is not a process-of-elimination approach.  We don’t pick out of a hat, nor do we take a vote.  Formal attribution doesn’t even depend on our ability to simulate the 20th century with models of greenhouse gas + natural forcing, and not natural forcings alone, and can even be accomplished when subtracting global mean trends (although this is interesting to stare at).   Usually formal attribution involves comparing spatio-temporal patterns to a set of forcings (through models or theory) and allowing the amplitudes of various forcings to vary.  Natural variations fail to account for the observed trends even with overinflated responses, something robust to various models, methods, or assumptions about internal variability.  A key point is that no physically plausible way exists to systematically increase carbon dioxide in the air and not expect a warmer world.
• Assessment of future global change is not based on the increase of temperature over the last century. 

The global mean temperature change is nearly 1 C over the last century.

This is not why we expect temperature to continue to rise…it has more to do with the fact that  we expect greenhouse gases to continue to rise, and we know that the climate still has to “catch up to us” so heating in the pipeline will be realized.  Even transient forcings like solar output going down or volcanic eruptions will eventually be outdone by the much longer-lived greenhouse gas influence.  

5) Sources

• Science is done by scientists, so reading what scientists have to say about the subject is probably a good idea, at least before opining on the topic.  Many people just don’t care all that much about the science (just as I know nothing about black holes or organic chemistry) and for such people the media or quick wikipedia searches will suffice, but then those people have to at least realize that they are not in a place to make judgment about the quality of research, just as I am in no posittion to disagree with an astrophysicist about black holes.  It makes no sense.  If you think  the scientists are bending the truth after a good amount of research is conducted, it’s your perogative (although much of the underlying data and physics can be validated for people really interested), but for those really interested there’s very good reports from the IPCC, from the recent Copenhagen climate conference as well as various pieces by the National Academies of Science, USCCP, as well as the hundreds of peer-reviewed documents available in journals, “.edu” resources, etc.  The studies and data coming out analyzing climate change are done by research groups at universities, organizations like NASA, NOAA, etc.  There is thus no need to rely on Al Gore (or Rush Limbaugh!!) for information, and such spokespeople do not directly contribute to the research in the field.  Some blogs, random “.com” websites, etc may be good, although you probably won’t be able to tell the difference if you don’t familiarize yourself with the original research first.
• I think many people are unaware of the breadth and depth of the scientific literature on climate change, and the overwhelming amount of research conducted by the scientific community.   The first estimates of the influence of CO2 change was done in 1896, and the field has evolved tremendouly over the last century, with many of the key aspects known well before any of the researchers today were born.   This is not one of those things which is open to public preference or opinion, and it involves a great deal of complexities which cannot be understood through quick google searches. 

Update– July 8: I’ve messed around with the wording of things for clarity.

Many readers who keep up with the blogs will be aware of the recent post at RealClimate concerning a recent review by Alan Carlin and John Davidson on the EPA Endangerment Finding. A certain blog, by one Thomas Fuller, discusses why the EPA should have listened to Carlin. Apparently the science in Carlin and Davison’s report is considered by Fuller and commenters to be “new evidence” that needs to be considered. Chris, feeling argumentive, decided to have a shot at the comments to see how this was so… I learned many new things in my visit, and I plan to document it in the best of scientific sources, a cutting edge research-database…

The blogosphere.

So here it goes…

I found that the “new evidence” was about the lacking hotspot in the troposphere, ARGO data showing a few years of cooling, the existence of a Medieval Warm Period and the Holocene Optimum. I also found that certain skeptics believe that the Urban Heat Island effect can explain much of the warming, and that certain groups who put out temperature products don’t account for any of this. I learned that climate sensitivity is an input put into computer models, where they automatically multiply 1 by 3.5 to get 3.5 C of warming per 2xCO2 (that’s why people get phD’s to model climate and study its behavior nowadays, take that gavin!!). When asked for peer-reviewed references, this was all supported, in total, by one paper on pan-evaporation.

As I said, this is all new evidence, apparently post-IPCC AR4. I also learned that the “warmists” don’t want to debate any of this new evidence. Oh, someone also mentioned something about a guy named Mann and Bristlecone pines, but I never heard of that.

In my vast blogging experience, I must conclude AGW is now a hoax. So I must ask those people who believe in things like radiative physics and “water vapor feedbacks” and who want to document things in journals…

Why do you believe in weird things?

A fundamental issue with changes in atmospheric chemistry is that there may be multiple, and potentially competing effects in terms of problems caused to ecosystems or human welfare.    For instance, aerosol declines in developed nations since the middle of the century result in less health and pollution issues, but also lead to global brightening which makes the more of the greenhouse gas influence show up.

The Montreal Protocol put limits on ozone-depleting substances such as Clorofluorocarbon’s (CFC’s).  HCFC’s and HFC’s usage have increased as a replacement for CFC’s, and the usage of HFC’s is expected to grow as the 21st century progresses.  HFC’s do not deplete the ozone layer, however a tradeoff exists: they are greenhouse gases and thus significant usage of them will contribute to radiative forcing that will add to the influence of CO2, CH4, etc in the future.  However, the future emission trajectories and impacts of increasing HFC’s are not well studied.  The incremental radiative forcing change with similar concentration changes in HFC’s are much larger than that of carbon dioxide, owing to where they block outgoing radiation and the low background concentration.  For instance, the radiative efficiency for a small change in CO2 from a background of 380 ppm is approximately .014 W m^-2 ppm^-1, whereas, say, HFC-134a is 160, HFC-23 is 190, and so forth (see IPCC AR4 Table 2.14).  The Global Warming Potential (which is proportional to radiative efficiency and changes with lifetime in the atmosphere) of HFC 134a is about 1,430 times that of CO2 given a time horizon of 100 years. 

A recent paper by Guus Velders et al have focused on this specific topic, presenting  consumption/emission scenarios as an update to SRES.  According to their results, in developing nations, use of HFC’s is expected to increase 800% more than in developed nations, and total GWP-weighted HFC emissions are about 6–9 Gt CO2-equivalent per year by 2050, several times larger than SRES projections. In 2050, the RF of global HFC’s is suggested to be the range of 0.25–0.40 Watts per Square meter, several times larger than SRES suggestions and a bit smaller than modern methane forcing. The authors express concern that HFC’s could become a very important contribution agent to climate forcing is unconstrained, and discuss mitigation options in the linked text.

First off, I apologize for my lack of posts recently… I’ve been busy and haven’t had much interesting to talk about. A hot topic this week has been the release of the Synthesis report from the discussions at the Copenhagen conference earlier this year. This report, in part, is to take off where the IPCC AR4 left off in discussing key developments that occurred after the deadline for AR4 references.

The general discussion is easy to follow (written like it is for newcomers to the topic) and not surprising to people who have studied climate change topics. It has all the usual stuff: The climate is warming, we have high confidence now of a significant anthropogenic signal in that trend, sea levels are rising, ocean heat content is going up, ice is melting, etc, etc. It is still worth noting that many climate observations are exceeding projections.

Combating climate change, and the language necessary to convey the threat of AGW to the public, has been the topic of a lot of discussion. A powerful paragraph issued in the synthesis report is

Past societies have reacted when they understood that their own
activities were causing deleterious environmental change by controlling or modifying the offending activities. The scientific evidence has now become overwhelming that human activities, especially the combustion of fossil fuels, are influencing the climate in ways that threaten the well-being and continued development of human society. If humanity is to learn from history and to limit these threats, the time has come for stronger control of the human activities that are changing the fundamental conditions for life on Earth.

Key finding #5 of the synthesis report is that “Inaction is Inexcusable.” Greenhouse gases will continue to rise substantially over the century unless widespread implementation of renewable energy, management of biological systems (e.g., reforestation), and wise policy action. There is no longer any reasonable doubt of the strong impact of GHG’s on global climate. The Copenhagen synthesis report uses a standard +2 C temperature anomaly as a guard-rail for dangerous anthropogenic interference, and warns of the prospect of not-well-understood “tipping points” and changes which are irreversible on timescales relevant to us and future generations. Postponing action could result in substantial distruption of ecosystems and infrastructure, and could result in a more costly transition into a “decarbonized” economy if humanity chooses to act in the more distant future.

This post has nothing particular to do with climate change, but I thought it might be worth pointing out a recent study analyzing the evolutionary history of Africans and African Americans.

Africa is the source of all modern humans, originating there a few hundred thousand years ago and spreading across the globe within the last 100,000 years. The authors analyze DNA from 113 populations of Africans from across the continent and find that they descend from 14 ancestral groups (with the highest within-population diversity worldwide) and find significant associations between genetic and geographic (as well as linguistic) distance in all regions of Africa. These groups later interact with each other to create the distinct populations that exist today. The study of African genetic diversity will be important for reconstructing African and African American population histories, as well as the genetic basis of diseases prevalent in Africa.

As another sidenote, there is a new blog called Darwinaia which will have focus on paleontology, history of science (particularly evolutionary related stuff), so if you’re into that, check it out.