Using state-of-the-art climate models, a new study has found clear evidence of a discernible human influence on atmospheric temperature.
Specifically, Ben Santer of Lawrence Livermore National Laboratory and 21 colleagues found that while the troposphere — the lowest part of the atmosphere — has warmed over the past three decades, the stratosphere, which starts 5 to 12 miles above the ground, has cooled. This is exactly what you’d expect if greenhouse gases were trapping heat near the surface rather than letting it percolate upward. “This is not a new idea,” Santer said in an interview. “We did the first fingerprinting studies of the troposphere and stratosphere back in 1996.”
The problem back then, Santer said, was that only a couple of climate models were available for studies like this. Models are crucial in this kind of research because you can’t do controlled experiments with the planet the way doctors do when they test new pharmaceuticals. With medicines, you give some patients the drug and others a placebo, or sugar pill, and see the difference in how their illnesses respond.
With the climate system, by contrast, there’s only one patient, and it’s already been dosed with extra greenhouse gases such as carbon dioxide. So scientists like Santer do simulations of how the atmosphere should look both with and without those extra gases. Unlike in 1996, Santer and his co-authors had 20 different simulations to work with for this study, all of them state-of-the-art models developed for the upcoming major report of the Intergovernmental Panel on Climate Change, due out starting in 2014.
The obtained results mean, that the warming of the troposphere and cooling of the stratosphere can’t be explained in any other way than by the heat-trapping effects of human-generated greenhouse gases. “It was surprising to me how large the signal was,” Santer said
This is only one of the fingerprints scientists expect to see in a human–influenced climate, moreover. “In the past we’ve looked at ocean surface temperatures changes in hurricane-forming regions, patterns in atmospheric pressure; rainfall patterns, and changes in Arctic sea ice,” Santer said. All of these and more can be identified more easily and clearly with the new models. “I think these simulations are like a scientific gold mine,” Santer said. “Analysts will be exploiting them for many years to come.”