As you may know, Craig Loehle caused quite a stir last year when he released a reconstruction of Global Temperatures that showed a very large Medieval Warm Period, which was novel, in my opinion, becuase it excluded trees and it used only precalibrated proxies. Well, Craig came under fire for some mistakes (sort of like Mann et al) but he has responded quickly and released a correction. Read about it here:
The new data extend up to 1935 instead of 1980. But using the instrumental data (GISS no less), you can confidently say, as the corrected paper does:
“The warmest tridecade of the MWP was warmer than
the most recent tridecade, but not significantly so.”
Which means that if you apply a 29 year smooth to minimize the effects of the dating errors (a pitfall of not using trees) and apply the same to the recent record, and “splice” them together (not necessarily good practice) to show that the peak of the MWP was higher than the most recent temperatures. I have done just that, but used HADCRUT and UAH Satellite data, produced this graph, which includes the 95% confidence intervals (compare to the original, also).
If we take this at face value, what does it mean? It means that, at the very least, you can no longer say that recent temperatures are “very likely” unprecedented. Indeed that is one possibility, but it is not the most likely one. But what does that mean? Well, it means that there are natural forcings on Millennial timescales that are as large as recent forcings, alledgedly mostly due to man. Which means that there is a mechanism by which large climate shifts can take place without our help, and the recent warm period could be an example of such a thing, slightly enhanced or damped by human effects. But what could these forcings be, and is there any reason to believe that they are here presently? One suggestion is solar activity.
Given this reconstruction of solar activity:
The sun seems as could a culprit to “blame” as any. This strongly implies that the Sun is responsible for at least some of recent warming, to. In case you are a solar change denier who thinks that the effect of solar activity can’t explain things becuase there is a such thing as the “solar constant” I suggest that you look at this graph of measurements of Total Solar Irradiance:
Which means that the Sun has contributed to warming in the last thirty year, if only a little (and becuase if the strong T^4 dependence or something (gosh, I sound smart when I say that!) the direct effect would be small.) It has also contributed over the long term, to:
If by this point your thinking: So what? The effect has been calculated to be to small. This would be the case, if there were no direct effects and that only makes sense if solar activity hasn’t effected climate substantially in the past! The trouble with that last part is that it has. Obviously the MWP appears to be a good example. There are other examples of correlations, to. Like this one:
Ján Veizer wrote a paper which shows many correlations on virtually all timescales:
The point is that these would all seem to indicate that there is some “amplifier” to the effect. Various ideas have been proposed (and given the correlations, one certainly hopes some mechanism exists. Otherwise you’d have a hard time explaining how they came to match so well!) but the best suggestion of all has been the modulation of Cosmic Rays by the Sun. Nir Shaviv explains how this works here:
The idea is that Cosmic Rays effect cloud cover. Experimental results support the link, and of course empirical correlations between the Cosmic Ray Flux over various periods of warming and cooling show that the effect should be large (Cosmic Rays vary over the long term due to the movement through the Galaxy as well as variations in solar output). So the explanation is a good one, at least for most 20th Century warming. Which means that the rest won’t be very interesting (a Climate Sensitivity of around 1.3±0.4° Celsius implied by Nir’s work wouldn’t be catastrophic, as opposed to the computer model sensitivities which range from about 2 to 5° Celsius.) becuase the feedbacks mostly cancel out. For a good explanation of Climate Sensitivity and numerous reasons to think it is smaller than the IPCC thinks see these:
The point of this being that the effect of CO2 is over estimated due to certain feedbacks being included, others possibly excluded. The trouble in particular is that the crucial water vapor feedback appears to a fiction:
The point here being that these are natural factors that can’t be ignored.
Now about Temperature data. As you may know, RSS recently released a correction to there data set which removed a cool bias from recent data. Interestingly the error was found by none other than Roy Spencer and John Christy. Read Lubos’s post here:
For those who think you can’t “trust” the skeptics, note that they actually helped fix their competitions data so that it warmed more and matched better with their UAH data. Spencer and Christy are serious scientist, and their opinions deserve more respect than some would like to give them. If you want to know Roy’s take read this:
I remain skeptical of Catastrophic Anthropogenic Global Warming, becuase I know that not enough is known to say what effect we have on the Earth’s climate quantitatively, perhaps even qualitatively. I also know that the Doomsday scenarios depend on several assumptions, several of which appear to be bad. And of course I also know that many of the resulting disasters as suggested by Algore certainly aren’t and won’t happen regardless of whether we are causing catastrophic warming.