Based on the amount of carbon dioxide and other greenhouse gases released into the atmosphere since the industrial era in the late 1800s, the Earth’s temperature should have risen by about 3.8 degrees. But scientists have only observed a 1.4 degree increase.
A forthcoming study in the Journal of Climate pinpoints two reasons why: 1) The Earth’s climate may be less sensitive to rising greenhouse gases than currently assumed; 2) The reflection of sunlight by atmospheric haze particles could be offsetting some of the expected warming.
“Because of present uncertainties in climate sensitivity and the enhanced reflectivity of haze particles,” says study lead author Stephen Schwartz of the Brookhaven National Laboratory, “it is impossible to accurately assign weights to the relative contributions of these two factors. This has major implications for the understanding of Earth’s climate and how the world will meet its future energy needs.”
Another possible reason for the lack of warming is the atmosphere’s slow response to the warming influence of heat-trapping gases.
Scientists are unsure how much additional carbon dioxide can be introduced into the atmosphere — beyond what is already there — without pushing the planet to dangerous tipping points. Study authors say much more research is needed to determine this level of CO2, especially as it relates to the influences of haze particles on the atmosphere.
Schwartz says 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.”