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New Evidence of Human Fingerprints on Global Warming

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.
Click image to enlarge.
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.

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 extra modeling power was crucial because the climate system is so complex no single model can truly capture it. They’re all approximations at best, so each has some uncertainty built in. But if you compare a number of models against each other and see the same fingerprint in most of them, you can be pretty confident that you’re seeing something real.

“After removing all global mean signals,” the authors write, referring to natural changes like volcanic eruptions and changes in the brightness of the sun, “model fingerprints remain identifiable in 70 percent of the tests involving tropospheric temperature changes.”

In plain English, that simply means 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.”

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By Lewis Cleverdon (Wales)
on December 4th, 2012

If we don’t establish something a hell of a lot more productive and free of governmental censorship than the IPCC, with its exclusion of a proper account of even the permafrost feedback in AR5 now publicized by UNEP at Doha, then I think Dr Santer is dead wrong to say:
“I think these simulations are like a scientific gold mine. Analysts will be exploiting them for many years to come.”

Many years hence the vast majority of analysts would be out of a job and starving, just like the rest of us.



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By Dave (Basking Ridge, NJ 07920)
on December 4th, 2012

With respect I note the following errors:
1. The PNAS link should be:
This includes supporting information.
2. “5 to 12 above the ground” should presumably be “5 to 12 km… or miles”

As I read your report it seems to me to give an initial impression that the computer simulations are the “new evidence” referred to whereas of course it is the degree of correspondence between those simulations (which include anthropogenic forcings) and satellite measurements, in this case, that is of interest.

Additionally, the connection between stratospheric temperature decline and gobal warming is not an obvious one for most non scientist readers. Hence and personally, I think the piece could be improved by including a bit more of an explanation such as global warming can only occur if less heat is radiated out into space. And that can only occur if the earth’s temperature as seen from space declines. Hence by themselves such observations are indicators of surface warming and so on…


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By Randy Coots (Oroville, CA 95966)
on December 4th, 2012

Quote: “In plain English, that simply means 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”

I don’t object to the research, nor the findings that there is warming of the troposphere cause by green house gasses. What I do object to is the assumption that all of the heat is a result of the effects of human-generated greenhouse gasses.

We simply do not know exactly how much of the heat is cause naturally vs the amount caused by human activity. I do believe that human activity has resulted in warming, but some of it is a natural occurrence. I wish these statements would be less sensational, and actually state facts, instead of statements that are far reaching and general in nature to achieve some affect politically.

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By Lewis Cleverdon
on December 5th, 2012

Randy - consider what is meant by the scientist saying “After removing all global mean signals . .”
If you’re well enough educated, you’ll realize this addresses the straw-man issue you try to raise in toto.

Your comment implies that you want scientists, who are by profession reticent and shy of making inflated claims
(since their reputation is critical to getting a hearing from other scientists)
to provide statements that are even more qualified and nuanced to emphasize the last particle of uncertainty,
and thereby to try to reduce the rate at which they convince the shrinking band of flukers
(those who still view rampant climate destabilization as being just thousands of flukes in a row)
that our pollution of the atmosphere is actually destabilizing the climate - to our great peril ?

Or is it just coincidence that your proposal would have that censoring effect
- if anyone took any notice of it that is.

I have to say that I’m unable to distinguish what you say from what a paid denier would predictably say
once he’s realized that with current extreme weather events, and widespread discussion of the science,
it’s just no longer credible to deny AGW outright in a growing number of discussion sites.

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