Since 2000, the Earth’s average surface temperature has increased by 0.06° C; a fraction of what was predicted by the IPCC during the 1990s. This apparent plateau has been used as ammunition by climate deniers who accuse scientists of over-inflating results from climate models. Marotzke and Forster’s new paper analyzes the methodologies of climate models, revealing no inherent flaws in the models, even when they don’t match observations. They also conclude that this century’s slight increase in surface temperature, which deniers are labeling as a “pause,” is actually due to natural climate fluctuations. Many other metrics, including ocean temperature, show that the climate is indeed changing.
“The claim that climate models systematically overestimate global warming caused by rising greenhouse gas concentrations is wrong,” Marotzke said in a press release.
Marotzke and Forster analyzed 114 models by comparing their predictions of annual global surface temperatures in 15 year periods from 1900-2012 against the actual temperature recorded for that year. When these predicted numbers were compared to the actual temperature, they found that the models did a pretty good job. For the most part, the predictions were +/- 0.3° C of the observed temperature. This effectively absolved the models of having fundamental flaws that overestimate the climate's response to atmospheric carbon dioxide.
“On the whole, the simulated trends agree with the observations,” Marotzke continued. “In particular, the observed trends are not skewed in any discernible way compared to the simulations.”
Of course, that doesn’t mean every model is perfect; otherwise they would all match one another as well as observed data. The researchers then compared the models by examining the factors and values that the models considered or assumed, in search of an explanation of why the numbers weren’t aligning. They found that differing models used different degrees of sensitivity to solar radiation and had different assumptions about the amount of heat absorbed by the oceans, which would alter surface temperature predictions. However, even the models that were the most sensitive to carbon dioxide didn’t lead to a prediction that was drastically overestimated, as climate deniers have claimed.
“If excessive sensitivity of the models caused the models to calculate too great a temperature trend over the past 15 years, the models that assume a high sensitivity would calculate a greater temperature trend than the others,” Forster noted.
The researchers concluded that random variations, which cannot be accurately accounted for within computer simulations, are responsible for models and observation not matching up. It is also clear that the climate is definitely warming, with 2014 dubbed as the warmest year on record, and that nine out of the ten hottest years ever have occurred since 2000.