Why hasn't earth warmed as much as expected?

Brookhaven National Lab News: Planet 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 (CO2). …Stephen Schwartz, of Brookhaven National Laboratory, and colleagues examine the reasons for this discrepancy.

According to current best estimates of climate sensitivity, the amount of CO2 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°F—well more than the 1.4°F increase that has been observed for this time span. Schwartz’s analysis attributes the reasons for this discrepancy to a possible mix of two major factors: 1) Earth’s climate may be less sensitive to rising greenhouse gases than currently assumed and/or 2) 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 fifty years, however, this present study found the role of so-called thermal lag to be minor.

…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.”…

Cirrostratus clouds, shot by Simon Eugster, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 2.5 Generic license