What Has Caused Global Warming Periods?
If carbon dioxide has not caused periods of global warming, what has? In our last post of this series on global warming, Global Warming Data, we concluded just on the basis of observed data from all five measured, globally averaged temperature datasets that global temperatures are currently falling not rising! The chart above displays the trends of all five datasets, calculated by Chip Knappenburger (See here and here and here for more on Mr. Knappenberger), which demonstrate current global cooling. The way Knappenberger obtained them was that he took the very noisy data of the temperature datasets, shown below, and performed what is called a least squares fit of a low order polynomial to them. This process finds a relatively smooth curve for each dataset that passes through the middle of the data points. It is a way of averaging out the fluctuations in the datasets to find the essential trends as a function of time. If you look at the units of the trends(deg C per year), it is clear what Knappenberger means by a trend are the slopes of the fitted temperature averages, i.e. their time rate of change. That means whenever a fitted trend curve falls below zero, the temperature in that series is falling. For readers who know a little bit about calculus, he took the first derivative of the fitted function and plotted that.
 The observed global cooling was then contrasted with the undeniable increases in measured atmospheric carbon dioxide, shown as a function of time in the chart below. The only way both charts can be true is if atmospheric temperature is not as sensitive to carbon dioxide as the global alarmists claim. You often hear the claim by believers in Anthropogenic Global Warming (AGW) that greenhouse gases trap infrared radiation so that it can not escape from the atmosphere. Nothing could be further from the truth, as we demonstrated on the basis of very simple physical theory in the post An Infrared Photon’s Life in the Atmosphere. What greenhouse gases do is to force infrared photons to diffuse to the top of the atmosphere by being scattered from greenhouse molecule to greenhouse molecule. Moreover, not all infrared photons are forced to diffuse out of the atmosphere in this way.
 Only those photons with energies that can boost a molecule from a lower energy state to a higher energy state can be absorbed and scattered by the molecule. This leaves the atmosphere totally transparent to all those infrared photons with energies that are different from differences in energy between the quantized molecular energy states. Taking all this into account for those photons that can be scattered, very simple physical theory can be used to estimate an upper limit on the amount of time that an infrared molecule is “trapped” in the atmosphere. Our result was  66 μsec (66×10-6 sec) for the very first photons to escape, so an estimate on the order of 100 μsec for all photons is appropriate, (When the phrase “on the order of” is used, it means that a multiple less than 10 of the estimate is a possibility.) Pretty much the same kind of argument can be made for other greenhouse gasses, leading to slightly different times of “trapping”.
These conclusions leave us with a very big problem. It is undeniable using any globally averaged temperature dataset that there was global warming between 1910 and 1940 and again from 1975 to 1999. Of course, one could note that the Little Ice Age has just ended approximately in the year 1860, and that temperatures will naturally rise from a period of lower temperatures. However, the global warming enthusiasts demand from us the answer to the following very reasonable question: If greenhouse gases in general and CO2 in particular were not the cause of those global warming periods, what was? There are at least two possible explanations, which are not mutually exclusive, that I know about. The first has to do with the physics of clouds, and the second is due to the variability of solar power output. In the next post in this series, I will take up the question of what was responsible for those periods of global warming.
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