It’s easy to show that global temperatures are rising. Tying climate change to human greenhouse-gas emissions — an area known as fingerprinting — is a lot harder. That’s what makes a new study in Proceedings of the National Academy of Sciences so important.

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It’s pretty easy to show that global temperatures are rising, or that spring is arriving earlier than it once did, but since climate has changed plenty of times in the past, that alone doesn’t prove anything. Tying climate change to human greenhouse-gas emissions — an area known as detection and attribution, or fingerprinting — is a lot harder.

That’s what makes a new study in Proceedings of the National Academy of Sciences so important. Using state-of-the-art climate models, Ben Santer of Lawrence Livermore National Laboratory and 21 colleagues have found what they call “some of the clearest evidence to date of a discernible influence on atmospheric temperature.”

Lower troposphere and lower stratosphere 1979-2011 temperature trend (°C/decade) and 12 months running mean global temperature time series with respect to 1979-1998. Credit: Wikimedia Commons

Specifically, they 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.

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