Earth has warmed much less than expected during the industrial era based on current best estimates of Earth's "climate sensitivity"—the amount of global temperature increase expected in response to a given rise in atmospheric concentrations of carbon dioxide.
In a study published online on Jan. 19 in the Journal of Climate, Stephen Schwartz of Brookhaven National Laboratory, Robert Charlson of the Univ. of Washington and colleagues examine the reasons for this discrepancy.
According to current best estimates of climate sensitivity, the amount of carbon dioxide and other heat-trapping gases added to Earth's atmosphere since humanity began burning fossil fuels on a significant scale during the industrial period would be expected to result in a mean global temperature rise of 3.8 degrees Fahrenheit. That is well more than the 1.4 degrees F. increase that has been observed for this time span.
"The data show that either we have 40 years of emissions left before the atmosphere can't absorb any more carbon dioxide, or we're already past the point of no return. In other words, the uncertainty rate is unacceptably high," said Charlson, a UW atmospheric sciences professor who in the 1960s invented a device called the integrating nephelometer to measure atmospheric haze particles, producing data that is still used in climate models today.
The new analysis attributes the reasons for the discrepancy between projected and actual temperature increase to a possible mix of two major factors: Earth's climate may be less sensitive to rising greenhouse gases than currently assumed and/or reflection of sunlight by haze particles in the atmosphere may be offsetting some of the expected warming.
"Because of present uncertainties in climate sensitivity and the enhanced reflectivity of haze particles," said Schwartz, "it is impossible to accurately assign weights to the relative contributions of these two factors. This has major implications for understanding of Earth's climate and how the world will meet its future energy needs."
A third possible reason for the lower-than-expected increase of Earth's temperature over the industrial period is the slow response of temperature to the warming influence of heat-trapping gases.
"This is much like the lag time you experience when heating a pot of water on a stove," said Schwartz. Based on calculations using measurements of the increase in ocean heat content over the past 50 years, however, the present study found the role of so-called thermal lag to be minor.
A key question facing policymakers is how much additional carbon dioxide and other heat-trapping gases can be introduced into the atmosphere, beyond what is already present, without committing the planet to a dangerous level of human interference with the climate system. Many scientists and policymakers consider the threshold for such dangerous interference to be an increase in global temperature of 3.6 degrees F above the preindustrial level, although no single threshold would encompass all effects.
The paper describes three scenarios. If Earth's climate sensitivity is at the low end of current estimates as given by the Intergovernmental Panel on Climate Change, then the total maximum future emissions of heat-trapping gases so as not to exceed the 3.6-degree threshold would correspond to about 35 years of present annual emissions of carbon dioxide from fossil-fuel combustion. A climate sensitivity consistent with the present best estimate would mean that no more heat-trapping gases can be added to the atmosphere without committing the planet to exceeding the threshold. And if the sensitivity is at the high end of current estimates, present atmospheric concentrations of heat-trapping gases are such that the planet is already committed to warming that substantially exceeds the 3.6-degree threshold.
The authors emphasize the need to quantify the influences of haze particles to narrow the uncertainty in Earth's climate sensitivity. The task is much more difficult than quantifying the influences of heat-trapping gases, said Charlson, who likens the focus on heat-trapping gases to "looking for the lost key under the lamppost."
Schwartz observes that formulating energy policy with the present uncertainty in climate sensitivity is like navigating a large ship in perilous waters without charts. "We know we have to change the course of this ship, and we know the direction of the change, but we don't know how much we need to change the course or how soon we have to do it."
"These results do not in any way reduce or remove the need for solid action now to move toward a zero-carbon dioxide-emission economy. The results tell us that doing our utmost now might work very well if the most optimistic values of sensitivity are real, but that it is possible that nothing will work no matter how hard we try," Charlson said.
"If we do not reduce uncertainties, we will be in the same boat 10 or 20 years from now as we are today," he said.
Schwartz and Charlson coauthored the paper with Ralph Kahn, NASA Goddard Space Flight Center in Maryland; John Ogren, NOAA Earth System Research Laboratory in Colorado; and Henning Rodhe, Stockholm University.
The research was funded by the U.S. Department of Energy Office of Science.
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