Earth’s climate system is a complex beast. In order to discuss it, we often rely on simple measures like climate sensitivity—an estimate of how much warming we can expect to get from a given increase in greenhouse gas concentrations. Unfortunately, measuring the climate sensitivity isn’t that simple. Among other complications, you have to pick the timeframe you’re interested in. The climate system is sluggish in some ways, so the longer you give it to respond, the greater its response will be.

A few recent studies generated some hub-bub (partly through a popular article in The Economist) by coming up with low values for climate sensitivity based on analyzing the observed temperature change of the last few decades. Opponents of the scientific consensus on climate change seized on this to argue that carbon dioxide’s impact is small, and projections of future change are exaggerated.

Apart from the danger of reading too much into the slower surface warming of the last decade (which appears to mostly be the result of natural variability) this thinking suffers from another problem: those low values appear to be wrong.

The number most often talked about is “equilibrium climate sensitivity,” the change in temperature after waiting a few centuries for all the “fast” climate feedbacks to play out. That's generally estimated to be around three degrees Celsius for a doubling of CO 2 . But the IPCC reports provide a likely range; the most recent expanded that range to 1.5 to 4.5 degrees Celsius after having tightened it up to 2.0 to 4.5 degrees Celsius in the 2007 report.

A number that is arguably of more immediate relevance is the “transient climate response.” That’s defined as the warming experienced at the time a CO 2 concentration crosses the doubling line—no waiting. (Typically, the value assumes CO 2 concentrations are increasing by one percent each year.) The likely range for this in the latest IPCC report was 1.0 to 2.5 degrees Celsius.

The new estimates that were causing a fuss were focused on estimating values for the transient climate response. And by focusing on recent years, they generated a relatively low value.

NASA climate scientist Drew Shindell set out to examine a complicating factor in these estimates—the climatic effects of ozone and aerosol emissions. We normally think of ozone in terms of the stratospheric layer that helps us by blocking the harmful UV radiation that can cause skin cancer. But we also produce it in the lower atmosphere, where it adds to Earth’s greenhouse effect. Aerosols (which humans also produce), on the other hand, cool the Earth by reflecting sunlight back into space.

If you don’t properly account for the effect of these things when analyzing recent warming, your estimate of CO 2 ’s warming influence will be skewed. This particular type of estimate has relied on the assumption that all the climate’s control knobs are equivalent—that is, a unit of energy added by rising CO 2 will have an equal (but opposite) effect as the same unit of energy blocked by aerosols.

Surprisingly, this reasonable assumption has led the new estimates astray, because the people doing the estimating worked with simple, global averages for each parameter, rather than sweating geographic details.

Shindell learned this by analyzing climate model simulations run in several configurations. First, the models were run using all our best estimates of historical climate forcings like CO 2 and aerosol emissions. Then, they were run again with the greenhouse gases that mixed evenly throughout the atmosphere, but where aerosols and ozone were concentrated near their source. Finally, they were run with no anthropogenic emissions of any kind.

Aerosols and ozone don’t last very long in the atmosphere; since they're mainly emitted in the Northern Hemisphere, their effect is mainly felt in the Northern Hemisphere. And, because the land area is so much greater in the Northern Hemisphere, the models show that their effect is much greater than it would be if they were emitted in the Southern Hemisphere. Changes in ice or snow cover alter the amount of sunlight absorbed by the Earth’s surface, for example, and this makes the Northern Hemisphere a better amplifier of climate forcings (about 60 percent better, in fact).

All this means that, in reality, aerosols and ozone have had a stronger effect than you'd get by simply averaging their effect across the globe—stronger than the simple calculations of transient climate response gave them credit for. Ozone warming partly offsets the cooling from aerosols, but the net result is a larger cooling influence that masked some of the CO 2 -caused warming. Accounting for this raises the calculated transient climate response to CO 2 from 1.3 or 1.4 degrees Celsius (on the low end of the IPCC range of 1.0 to 2.5 degrees Celsius) to 1.7 degrees Celsius.

Beyond the difference between those best estimates, the differences in the lower and upper bounds were also significant. The 95 percent probability range ran from 1.0 to 2.1 degrees Celsius using the older method, but it shifts to 1.3 to 3.2 degrees Celsius. That makes the high end of the IPCC range much more likely than the low end.

So what does this mean for projections of future climate change? Nothing new. It does add an important wrinkle that improves simplified methods of estimating the climate’s sensitivity to CO 2 , however, and in doing so explains the mismatch between some of those estimates and others based on different kinds of analysis. It’s a good reminder that studies with surprising results have to be considered carefully in the context of previous work—the latest study isn’t always the most accurate one.

Nature Climate Change, 2014. DOI: 10.1038/NCLIMATE2136 (About DOIs).