The Scientific Basis for Anthropogenic Climate Change
Note* The graphs that appear here can be clicked for enhanced versions
Climate Science can be a bit like detective work when dealing with an issue such as global warming. First of all, we have a “detection and attribution” process where we spot a problem, and then try to find a suspect. Like they do on CSI, you find a body, and then go and find evidence to put blame on someone or something. Right now, the problem is the globe is warming, and it appears that anthropogenic CO2 plays a large part in this trend.
(a) Temperatures have risen over the past 100 years by nearly 0.8°C. This rise may seem harmless at first glance, but averaged over the entire Earth, such a trend is rather significant. This number is not representative of “weather” like daily fluctuations where you live, but rather the global mean annual temperature which has been very steady over the Holocene (roughly 10,000 years from the last ice age), and is rather large when compared to global scale, decadal scale variability. Over 70% of the Earth is ocean, and water heats up slower than land, so in fact the “rise in global temperature” may be a bit misleading since you will see more effects on land, where we are. To put this rise it into another perspective, you only need to reduce pre-industrial global temperatures by ~4 C to get an ice age (this much increase is projected by 2100 under business as usual emission scenarios). Over the Holocene, global temperatures have not gone outside a range of + or – one degree C.
(b) The rise in temperature is unequivocal and is demonstrated in a variety ways: the instrumental record, which includes a global network of surface stations, weather balloons in the atmosphere, as well as satellites in space. Declines in snow cover, earlier springs, glacier loss, sea level rise, biological activity, etc are consistent with the warming trend.
Countless glaciers worldwide are retreating at astonishing rates, and the mass balance of most glaciers worldwide is negative (meaning that accumulation in the interior is not making up for the loss of ice on the edges, and the changes are generally consistent with warmer temperatures). In some instances, there is a bit more accumulation than ablation (or wastage on the edges) because you get more precipitation in a warmer climate, or perhaps other regional effects. Glaciers are complicated depending on where you go (e.g. in the tropics diurnal variations in temperature are more than seasonal changes, so wet-dry periods dominate over hot-cold periods, but temperature change is the key suspect now worldwide). More on glaciers here
Arctic sea ice has declined by more than 20 percent since 1979. This year, reports of a record low received big attention in the press. More alarming, is that melt rates and sea level projections for the future are turning out to be underestimated, mainly because of the phenomena of accelerated glacier flow from the outlets of ice sheets, which are not well understood and cannot yet be modeled by scientists. Researchers also found that the break up of the Larsen B ice shelf in 2002 (in Western Antarctica, a chuck of ice about the size of Rhode Island) led to a significant acceleration of glaciers flowing into the Weddell Sea. Glaciers flowing into the Amundsen Sea have also sped up considerably during the last decade (on the other side of West Antarctica). In East Antarctica, there is probably a bit more accumulation than ablation right now because of more precipitation that you get in a warmer climate, as wel las regional effects such as ozone depletion and circulation patterns.
(c)The ocean heat content change is an addition of 14.2 ± 2.4 × 10^22 J, J or 0.21 ± 0.04 W m–2 (data from Levitus et al., 2005) from 1961-2003. Currently, oceans are warming worldwide, and this influences sea level rise due to thermal expansion (warmer waters take up more room). This is also having noticeable impacts on coral reefs and other sensitive marine life. This is illustrated below:
(d) Estimates for the 20th century show that global average sea level rose at a rate of about 1.7 mm per year, and trends are accelerating (~3 mm per year since 1993). Current projections out to 2100 from IPCC, 2007 and others are also not taking into account accelerated glacier flow mentioned above. This poses problems especially for people and infrastructure on coasal areas and low-level islands. The Greenland ice sheet, and West Antarctica have enough ice on land to raise global sea levels 23 feet each. East Antarctica is by far the largest, and if that were added, sea level would rise 200 feet. This cannot happen in decades, and East Antarctica will not melt in the scenarios we are discussing, but West Antarctica and Greenland could possibly melt on timescales of centuries. The Greenand sea ice cannot directly raise sea level (because it is already in the ocean), however, it does become important for atmospheric circulation, as well as the influence on the planet’s albedo (that is, ice reflects more sunlight than ocean, so less sea ice means more solar absorption by the oceans).
Several factors can cause a climate change on scales of longer than a few years, notably a change in incoming or outgoing radiation, or a change in albedo of the planet. There are several natural ways this can be done, such as increases in solar irradiance, volcanic particles in the atmosphere, changes in plate tectonics on ong term scales, etc. There are also ways humans can contribute by altering the radiative balance at the top-of-atmosphere (such as adding infrared-absorbing gases which delay the escape of outgoing heat to space), by changing the albedo via deforestation and other land-use activities, and by anthropogenic sources of aerosols. Biology in general has the capability to alter climate, such as the big “oxygen revolution” when plants decided to make the atmosphere-ocean system toxic for anaerobic life.
There is now strong evidence that humans are playing a substantial part in the observed temperature increases, mainly from about 1950 onward, but a good amount of contribution since the industrial revolution. Let’s begin with the basics:
(e) The composition of the atmosphere has changed quite a bit in 150 years during industrial time, and the rate of change is unique when looking at geologic time. Before the Industrial Revolution, CO2 levels remained at ~ 280 parts per million by volume (ppmv), mainly because the CO2 contributed to the atmosphere was essentially equaled by the CO2 removed from the atmosphere. Today, the CO2 concentration is ~380 ppmv, the highest level it has been going back 850 ka and likely much longer (likely millions of years back). Figure 1 goes back 420,000 years, with Figure 2 a bit more narrowed down to our time:
The concentration of methane has increased from a pre-industrial value of about 715 ppb 1774 ppb in 2005. Here are these trends and others:
(f) The Earth currently has a greenhouse effect absorbing ~150 W/m^2 in our atmosphere (with clouds), with an energy balance (shortwave radiation in equaling longwave out) of ~240 W/m^2 at the top-of-atmosphere. CO2 contributes roughly one third of the greenhouse effect (in terms of IR absorption at the relevant areas for Earthike temperatures). The greenhouse effect itself keeps the planet ~33 K warmer (to support life) than it would be with no atmosphere. Changes in CO2 will alter this radiative balance (on the “going out” side of the equation). Energy comes from the sun in the form of solar radiation (on the visible part of the light spectrum) which is not absorbed by Greenhouse Gases, but is released from the surface back to space in the form of Infrared radiation, which is absorbed by Greenhouse Gases. Greenhouse Gases, therefore, do not block energy coming in, but they absorb some of it going out. This “blanket” of Greenhouse Gases delays the return of long-wave radiation back to space and further heats the surface. The imbalance created is our radiative forcing (with a positive forcing contributing to warming, negative to cooling). The total forcing from the trace greenhouse gases is currently ~2.5 W/m-2, and the net forcing is ~1.6 W/m-2 since the pre-industrial time (negative forcings with aerosol impacts included). A radiative forcing is not the same as the additional infrared radiation absorbed, since you must account for feedbacks (such as changes in water vapor) that will come with a “forcing” on the climate system. For more on Earth’s radiative balance and the impact of greenhouse gases, see my set of posts here, Part 2, and Part 3.
(g) The timing (with magnitude and rate) rules out some mechanisms which we know are associated with long-term climate changes- El Nino is too short, Milankovitch cycles and plate tectonics is too long, etc. Below is a simplistic version of different “forcing timescales.” (years plotted on Y axis)
(h) The “extra” CO2 in the atmosphere (around 100 ppmv from pre-industrial value) is surely from human sources. For one thing, no feedback mechanism or explanatory trend in any activityl ike volcanoes can come close to explaining the rapid rise, and rate of rise of CO2 emissions. Additional evidence that humans are responsible for increased CO2 is through carbon isotopes; a useful carbon isotope for determining the contribution of fossil fuels to atmospheric CO2 is radioactive C-14. Fossil fuels have very little C-14, since it has a half-life of 5,730 years and fossil fuels have been in the ground for millions of years. A decline in C-14 was first described by Hans Suess who first analyzed the C-14 content of wood. The reduced atmospheric C-14 as a consequence of fossil-fuel burning (now known as the Suess Effect), is not incredibly meaningful but is a clear “fingerprint” that the CO2 rise is ours. Later work has come to similar conclusions. Other trends exist such as a decline in C-13 compared to C-12 (since plants prefer C-12). Slight atmospheric oxygen decreases (as predicted by the combustion process) have also occurred.
(i) The increase in ocean heat content, as well as the radiative imbalance in the atmosphere (i.e. more solar coming than infrared going out), and timescales of forcing show that internal variability (like ocean circulation shifts) are not responsible for the observed warming, but rather an “external” forcing on the climate system. Internal variability, therefore, does not have explanatory power in suggesting what may be responsible for the warming over the last 100 years.
(j) Speculation of increased solar irradiance causing Global Warming has made its way into many internet venues, as controversy centers around an unusually bright sun. This does not explain current phenomena for a variety of reasons:
Since about 1950, there has been no secular trend in solar variability, and for later parts of the 20th century there has actually been slight decreases. Combined with data from volcanoes, ozone and aerosol trends, etc, since 1950 anthropogenic CO2 has provided the dominant forcing mechanism behind the observed increased CO2. The rapid warming observed, and the climate response, is consistent with the scientific understanding of how the climate should respond to an increase in greenhouse gases, and the warming is inconsistent with the scientific understanding of how the climate should respond to natural factors such as solar variability, volcanic activity, etc.
For example, a model of increased solar irradiance would have the stratosphere and troposphere warming (these are layers in the atmosphere), whereas a model of increased concentrations, and the effects of greenhouse gases would predict warming from the surface to troposphere, and cooling in the upper portions of the atmosphere (e.g. stratosphere) because you are keeping heat down and reducing infrared heating above. Observations show a consistent relationship, with strong warming at the troposphere and surface, and cooling in the upper regions of the atmosphere. In addition, we have also had a faster increase in the rate of night temperature warming, than day time, which is consistent with the greenhouse model but not increased solar variability.
I mentioned earlier that climate science can be like detective work. Generally when you find a blonde-hair sample of a suspect at the crime, the black-haired suspect would get a bit less of a glance. When a DNA analysis is done proving the samples to be different, that increases the confidence that he is innocent. When multiple people testify that he was at his beach house 200 miles away at the time of the crime, he can be ruled out. When we monitor the sun, and there is no change, when the climate response is inconsistent with what it should be given a predominant solar forcing, then it just won’t work in science.
(k) There has been an enormous amount of work in paleoclimatic study seeking CO2-temperature relations in the past. Work has been done from timescales of the last few hundred years, to the ice ages, to ancient climates while the dinosaurs roamed the world, and before that. There has been a significant relationship between CO2 and temperature, and obviously there are other factors so you can get cooling if CO2 is high (say, if the sun dims), or if aerosol concentrations rise dramatically (the 1940-1970 cooling trend observed on the first graphs), but CO2 produces a strong warming effect, and this relationship is seen now and in the past. This has repeatedly demonstrated to hold strong predictive and explanatory power, and the confidence is now very high there is a strong, detectable anthropogenic signal in today’s warming trend.
Observations and models in line up to mid-century, where we then have to include anthropogenic GHG’s and aerosols to explain current trends
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Intergovernmental Panel on Climate Change, Climate Change 2001: The Scientific Basis, Internet URL- http://www.ipcc.ch/pub/online.htm ; International Panel of Climate Change; Climate Change 2007 – The Physical Science Basis Internet URL: http://ipcc-wg1.ucar.edu/wg1/wg1-report.html
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Lockwood, Mike; Claus Fröhlich. “Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature“. Proceedings of the Royal Society. doi:10.1098/rspa.2007.1880
Rignot et al., “Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf.” Geophysical Research Letters, Vol. 31, No. 18, L18401
(September 22, 2004)
Stuiver, M., Burk, R. L. and Quay, P. D. 1984. 13C/12C ratios and the transfer of biospheric carbon to the atmosphere. J. Geophys. Res. 89, 1731–1748
Suess, H.E., “Secular variations of the cosmic-ray produced carbon-14 in the atmosphere and their interpretations,” Journal of Geophysical Research, 1965, V. 70, p. 5937-5952.
Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences. Thomas R. Karl, Susan J. Hassol, Christopher D. Miller, and William L. Murray, editors, 2006. A Report by the Climate Change Science Program and the Subcommittee on Global Change Research, Washington, DC. Internet URL: http://www.climatescience.gov/Library/sap/sap1-1/finalreport/default.htm
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Weart. Spencer; Letter Cause and Effect in Global Warming, Physics Today; Jan 2005
Graphs- I credit http://www.globalwarmingart.com , http://pewclimate.com , NASA Earth Observatory page, IPCC 2007, and Mauno Loa observatory data, and references within, for the above. The chart on “forcing times” is mine.