As I discussed in a previous post (April 11), the people who deny anthropogenic global warming (AGW) have many similarities to creationists. Despite the fact that the reality of AGW is supported by a 95% or greater consensus of qualified climate scientists, the critics (mostly non-scientists, or scientists in fields that do not qualify them to assess climate science) keep on repeating the same false tropes over and over again, no matter how many times they are debunked. This is analogous to the shopworn old arguments of creationists, who invariably trot out fallacious arguments like “evolution contradicts the Second Law of Thermodynamics” even though it has been corrected hundreds of times by scientists. The creationists have such a strong denial filter to resolve their cognitive dissonance that they either don’t realize why their “Second Law” argument is invalid, or they are deliberately and deceptively using it over and over again because it impresses their scientifically illiterate following.
The same is true of the long-debunked example of cherry picking, “It hasn’t warmed since 1998″ (see my April 11 post). Another common false statement is “The planet warmed just as much during the Medieval Warm Period, but eventually it cooled down again.” They argue that if this warming preceded our modern injection of carbon dioxide into the atmosphere, then maybe our current global warming not caused by our burning of fossil fuels. Climate deniers repeat this old saw over and over again as if it’s somehow a devastating blow to the huge body of data about our recent climate changes. They often illustrate it with the anecdotes about how the Vikings could colonize Greenland for a while, then as climate cooled in the late Middle Ages, these colonies failed when Greenland became too cold again. The story about the fate of Viking colonies in Greenland is true—but the rest is not.
As climate scientists have long understood, there were numerous small fluctuations of warming and cooling over the last 10,000 years of the Holocene. Most are well understood to be consequences of the different orbital variations of the earth around the sun (the Croll-Milankovitch cycles), with some component of solar activity. But in the case of the Medieval Warm Period (about 950-1250 A.D.), the temperatures were only 1°C warmer in the Northern Hemisphere, much less than the temperature changes since the beginning of our current global warming (Fig. 1). The warmest years of the Medieval Warm Period are comparable to the mean annual temperatures recorded about 1960—and the earth has warmed dramatically in the past 50 years.
More importantly, the Medieval Warm Period was also only a local warming in the North Atlantic and northern Europe. If you look at the record of global temperatures over this interval, the mean temperature of the earth did not increase significantly, and actually cooled by more than 1°C. And in contrast to the current episode of global warming, which is caused by burning fossil fuels, the Medieval Warm Period was triggered by several well-known non-human causes: a long-term drop in volcanic activity (which can warm the earth by letting in more solar radiation) and an episode of unusually high solar activity. In addition, there may have been a strong long-term oceanographic effect, the North American Oscillation, that might explain why the warming was local to the North Atlantic and not global.
Likewise, the warmest period of the last 10,000 years prior to 1800 was the Holocene Climatic Optimum (5000-9000 B.C.) when warmer and wetter conditions in Eurasia caused the rise of the first great civilizations in Egypt, Mesopotamia, the Indus Valley, and China. This too was largely a Eurasian phenomenon, with 2-3°C warming in the Arctic and northern Europe. By contrast, there was almost no warming in the tropics, and cooling or no change in the Southern Hemisphere.
To the Eurocentric world, these warming events seemed important, but on a global scale the effect is negligible. In addition, none of these historic and prehistoric warming episodes is related to increasing greenhouse gases. The Holocene Climatic Optimum, in fact, is predicted by the Milankovitch cycles, since at that time the axial tilt of the earth was over 24°, its steepest value, meaning the poles got more solar radiation than normal. By contrast, not only is the warming observed in the last 200 years much greater than during these previous episodes, but it is also global and bipolar, so it is not a purely local effect. The warming that ended the “Little Ice Age” (from the mid-1700s to the late 1800s) was due to increased solar radiation prior to 1940. Since 1940, however, the amount of solar radiation has been dropping, so the only candidate for the post-1940 warming has to be carbon dioxide.
But you would never get this straight story that paleoclimatologists have known for years if you examine any of the climate-denier sites. Instead, they are loaded full of misinformation written by people who do not have formal training in, nor do they actively participate in, climate science research. Instead, like the creationists they keep repeating and recycling debunked and outdated ideas, unwilling or unable to read anything that does not get through their strong filter of confirmation bias. And, like the smug fundamentalist that I mentioned in my March 7 post, they are self-righteous and utterly sure that their arguments are sound, yet they never even bother to read or consider anything that might go against their ideological biases (any more than creationists will read or understand anything that undermines their biblical literalism). Such a behavior may be well understood in the world of psychology and neuroscience, especially as an example of confirmation bias—but it is indefensible in the scientific community, where peer review weeds out the false and bad ideas and data (like creationism and climate denialism) and scientists must learn to accept ideas which have become overwhelmingly supported by the evidence, such as climate change and evolution. Such ideas may not tell us what we want to hear, but that’s all the more reason to believe that they are probably real and not the result of wishful thinking.