Earth can be so touchy: Why climate sensitivity matters, and how scientists are trying to better understand it

In a perfect world, we'd have all the answers and solve the problems of humanity sometime before dinner. Sadly, in real life, dinner is often delayed, or put off altogether in favor of a microwave noodle pot.

On those long, metaphorical evenings, we turn to science as way of narrowing down the number of things we don't know, and helping us push through problems as best we can. Climate science is a great example of this process in action. We know some big, important facts about how Earth's climate works—how adding extra greenhouse gases to the atmosphere causes the planet to get hotter, for instance. Other things are more uncertain, such as exactly how sensitive our climate is to heating by greenhouse gases.

In an upcoming paper in the Journal of Climate, Stephen Schwartz, Ph.D., senior scientist at the Brookhaven National Laboratory, and his colleagues contribute to the ramen-fueled work of inspecting the things we don't know, and offering suggestions for how to get closer to the truth.

Climate sensitivity is an extremely important concept. Climate change tells you what's happening—that the gaseous detritus of modern life is accumulating in the atmosphere and causing the global average temperature to tick upwards. Climate sensitivity, on the other hand, is the information that you need to know in order to make decisions about how best to deal with climate change—how we should alter our behavior, and when.

The data on climate change is pretty unequivocal. Climate sensitivity, however, we're a little more fuzzy on.

"The basic issue is that the things that control climate are complex. We aren't idiots just going around looking at CO2, CO2, CO2," said Gavin Schmidt, Ph.D., a climate modeler at NASA's Goddard Institute for Space Studies in New York, and one of the brains behind RealClimate.org, a blog dedicated to explaining the intricacies of climate science.

"There are many different drivers of climate, including ozone and aerosols. We've only just started to relate these things to the policy choices that real policy makers are faced with," he said.

The effects of aerosols are one of the biggest sources of climate confusion. Tiny, submicroscopic particles suspended in the air, aerosols include things like dust, soot from burning furnaces and smog. Like greenhouse gases, they are produced both naturally, and by human activities. They've also been increasing since the Industrial Revolution.

The weird thing: Aerosols can both work to increase the global temperature—black carbon soot, for instance, traps heat the same way a black tar roof does—and also decrease it—other aerosols reflect sunlight away from Earth, cooling the planet's surface.

"There is good reason to think that aerosols are offsetting some of the warming that would otherwise have resulted from increases in greenhouse gases, but the amount of offset isn't well known," Stephen Schwartz said.

Without that information we don't have a clear picture of how sensitive our climate is to increases in greenhouse gases.

"If the aerosol cooling influence is a small fraction of the greenhouse gas influence then the observed warming is obtained with a rather low climate sensitivity. If the aerosol influence is offsetting a large fraction of the greenhouse gas influence, it implies a fairly high climate sensitivity," Schwartz said.

For you and me, that's the difference between having time to make changes that could limit climate change before we experience any major effects, and already being doomed to a seriously climate-altered future.

If the climate sensitivity is low—if the the global average temperature rises 1.5°C for every doubling of CO2 in the atmosphere—then we have about 80 years before we accumulate enough greenhouse gases to commit ourselves to an increase in the global average temperature of 2°C above pre-industrial levels. Many scientists and policy makers have agreed on 2°C as a cutoff point, based on the economic, environmental and societal impacts associated with that level of increase.

On the other hand, if climate sensitivity is high—say a 4.5°C increase for every doubling of CO2—then we already hit the point where the 2°C increase will be inevitable about 35 years ago.

The actual sensitivity isn't known. The IPCC says it's likely to be anywhere between 2° and 4.5°C, with 3°C as the most likely possibility.

Schwartz's paper is focused on increasing the accuracy in the way climate models address the unknown.

All climate models take aerosols into account, he and climate modeler Gavin Schmidt told me. But Schwartz thinks the models are going about it the wrong way. He says that most climate models match very well to observed, historical changes in climate, but that they do so with a wide range of climate sensitivities and aerosol forcing levels. Some of the models may be right for the right reasons, Schwartz said. But others are right for the wrong reasons—balancing out climate sensitivity and the impacts of climate forcings, like aerosols, in a way that doesn't match with what actually happened.

"They certainly can't all have the right sensitivity," he said.

His paper is meant to help narrow down the factors responsible for forcing the global temperature down while greenhouse gases force it up, and he thinks he's done that, pointing to aerosols as the primary perpetrator.

"If the community could constrain the total forcing, up and down, then we could constrain the modelers so they don't have all this latitude to get the right answer for the wrong reason," Schwartz said.

But the paper gives the impression that climate modelers don't realize how important aerosols are, which isn't true, Schmidt said.

"The models do have a range of sensitivities and a range of forcings. The pairs that match the real-world observed climate provide a set of plausible histories for what actually happened, but which pair is closer to reality remains to be seen," he said. "But there is nothing 'too wide' about the assumptions. They cover the ranges of the observational uncertainties."

As modelers around the world prepare for the next update to the IPCC report, due in 2014, they're already working on ways to better account for uncertainties and make the models more accurate, he said.

One method is out-of-sample tests—basically, taking a model that correctly "predicts" observed climate change in the 20th century, and seeing how well it does with other historical time periods. Out-of-sample tests can help weed out models that give right answers for the 20th century, but for wrong reasons.

Ultimately, to really get a grip on how aerosols influence climate, modelers need better observational data, Schmidt said. The best way to track aerosols is by satellite but, so far, no satellite has had the right sort of instruments. That could change late this year when Glory, a new satellite carrying a cutting-edge aerosol sensor, is set for launch.

"It may be that we never get good aerosol data for the 20th Century," Schmidt said. "But we may be better off in the 21st."

Image courtesy Flickr user John LeGear, via CC

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