Warming Since A.D.1850

Plausible Causes
To review: there are at least three plausible reasons for the warming after 1850: (1) the system was rebounding from an anomaly caused by low solar input, (2) the system was rebounding from an anomaly caused by high volcanic activity and (3) the system was not rebounding at all, but was forced toward a warmer climate by the increase in greenhouse gases. An addition, and plausible, solution is some combination of these three causes.

A simple explanation of the warming after 1850 is that the system was responding only to the addition of trace gases to the atmosphere, mainly carbon dioxide and methane. Using only the rise in carbon dioxide, and the assumption that a doubling of carbon dioxide produces a global temperature increase of 2°C, we actually obtain a remarkable fit to the overall trend of the temperature rise. The coefficient of regression is very good for this fit: R = 0.75. It is much better, R = 0.89, if we use something called a “gliding decadal average” for describing the warming. The assumed response to doubling of carbon dioxide is on the conservative side of the range of estimates in the literature (about 1.5°C to 4.5°C). This is prudent, because these estimates refer to purely equilibrium conditions, while one would expect that the real warming would be retarded due to the heat uptake by the ocean.

On the other hand, if the Sun did increase its output and contribute to some of the warming of the climate since 1850, our simple warming model above cannot be correct. In this case, the trace-gas effect would have to be less than anticipated, or the ocean must have taken up much of this increase in solar heat. Also, there is much evidence that minute particles created by human activities, called aerosols (created by industry and the burning of firewood), block sunlight and could have offset the increased heat from the sun. A simple way to explain the good fit seen between trace gas release and warming is that the increase in solar brightness was largely compensated for by the cooling which is expected from oceanic heat uptake and from air pollution. As discussed earlier, work by Judith Lean and colleagues indicated that solar forcing may have contributed about half of the observed 0.55°C surface warming since 1860 and one third of the warming since 1970, with the rest of the warming attributable to increased greenhouse gases.