T.

Now I have a problem of understanding which I hope you can explain. I understand that the energy radiation balance is determined at the top of the atmosphere, and the surface temperature is determined by the lapse rate causing a temperature gradient between the surface and the top of the atmosphere.
Now comes the question. If the surface temperature rises, this should also increase the amount of water vapour. However, we know that increasing the water vapour content decreases the lapse rate, meaning that if the temperature of the top of the atmosphere is fixed, a reduced temperature gradient means that the surface temperature will drop. Doesn’t this amount to a negative feedback?

Response– Well remember that the height level of constant temperature is shifted to higher altitudes, so I’m not really sure that is a good argument. That said, it does turn out that you get a negative lapse rate feedback because of the moisture increases higher up, and the fact that the tropical atmosphere stays roughly close to a moist adiabat. This does act to partially offset the water vapor feedback. This is discussed in post here. It’s a relatively small effect, but it does matter (see the lapse rate is negative in the last diagram of that post).– chris

Most of the warming of the 80’s and 90’s can be directly attributed to the Pacific Decadal Oscillation. A persistent, multidecadal pheonomenon that can be shown through the temperature record. In warming mode it leads to frequent El Ninos. In cooling mode it leads to frequent La Ninas. From about 1880 to 1910 it cooled, then it warmed until about 1940, cooled until about 1980 warmed until about 2002. The PDO has recently switched back. Factoring in the PDO it looks unlikely that the total increase by 2100 would reach 1C even if CO2 were a factor. Feel free to look up on wikipedia or other sources.

Urbanization is another important factor, but not in the way one might think. There are four main “groups” dealing with global temperatures. The one most frequently used is GISS (thank you Dr. Hansen) which uses surface station data. Over time urbanization around these surface stations has lead to increasing bias. Many have gone from green pastures to asphalt parking lots. Some are even beside HVAC heat exchangers. It is likely that a great deal of the remaining warming is due to these issues. There also happen to be two satellites. At the time of their launch the satellites showed temperatures .2C lower. They’re now approaching .4C lower temperatures than GISS. The urbanization probably began after WWII but the affect was hidden by the cooling phase. An independent surface station audit is now being done http://www.surfacestations.org/ (you should check it out, the state of our surface station system is just…tragic)

The last important factor is the sun. The sun is the source of all of our heat in the first place and it’s activity seems to have been the reason for the warming after the little ice age. Indeed, graphing global temperatures v/s solar cycle length (a good indicator of the strength of the solar dynamo) the match is uncanny…and goes back hundreds (technically thousands) of years (not the pitiful 20 year period of “correlation” for CO2 that ended about 10 years ago).

While total output of the sun normally only varies by .1% over the highs and lows of a typical solar cycle the spectrum varies quite a bit. During the unprecedented activity of the 20th century the sun’s UV output varied by as much as 4-8%. Every bit of that extra energy (a significant part of the sun’s spectrum) was absorbed and turned into heat by the ozone layer. Now the sun has slumped into a level of inactivity unseen in the modern age. The solar wind and magnetic fields have dropped by 30% or more. The sun’s light output has also dropped more than usual..probably most of it coming from that UV part of the spectrum.

There’s also a suggestion that cosmic rays let in by the sun’s now weakened magnetic field may seed cloud formation, resulting in additional loss of heat from increased albedo. The science is anything but settled on “global warming”. Indeed, 2008 saw the largest temperature drop in recorded history. We should find out over the next year or two just how powerful the affects of the sun and PDO really are and also how (in)accurate our surface station data has been.

Response– You must be reading some very lousy sources. There has been no trend in solar activity or cosmic rays for over half a century now, and the PDO (by definition) cannot contribute to the long-term trend.

Response– The *greenhouse gas* forcing is 2.5 W/m**2 or something around there (From memory) and the negative forcings from aerosols push it back to 1.6 for the net effect. All of the forcings are relative to 1750 so a positive RF for the sun may not mean it’s been positive for the last few decades (it’s actually been slightly negative), but it was going up earlier in the 20th century.

Models need to be treated with a good amount of skepticism, and for some things they probably aren’t very reliable such as regional precipitation trends. In many cases (like Arctic sea ice melt) models have substantially underestimated the extent loss. In general, you’re right, the more different models from different groups agree the more robust the conclusion is. The global temperature response to given changes in external forcings is pretty good, but cloud feedbacks are responsible for much of the 2 to 4.5 C range you speak of…that’s a fairly big range but there’s a lot of evidence that the range is correct, and if you don’t like models, the paleoclimate record very clearly shows that the climate is not insensitive. The same people who claim it is are the same ones that shout about how much it changed in the past, or how negligible changes in the sun can have such a big influence.– chris

A couple of nitpicking points:
1. In paragraph 3, I think the units in your Boltzmann constant needs a / i.e. W/m^2T^4.
2. In the formula just below the Electromagnetic Spectrum graphic, should T be T^4? And I assume r=R.

Response– Thanks, the units are fine, but I fixed your point 2.– c

Response– Nope, the stratosphere is cooling, not the upper troposphere. I will have to do a post on that soon. The number you gave from the IPCC is the GHG forcing alone, the total *net* forcing is less because of the cooling effect of aerosols. The net forcing is ~1.6 W/m2. The rest of the gap is because that is an equilibrium number, not the instantaneous number. If everything remains constant and you let the oceans respond and the system to come back to equilibrium (which takes decades), then you have the IPCC number.– chris

I am a high school Earth Sciences teacher, and we will be discussing some of this topic in my class soon. I may use some of this material as a foundation, as it was instructive. One of my students who enjoys this topic and reads your site referenced me here, and I also learned some things. Keep up the good work.