Sulfate aerosols implicated in global warming slowdown

The rise of global mean surface temperatures slowed in the first years of this century. Scientists have pinpointed variation in Pacific Ocean surface temperatures that occur over timescales of decades as the primary cause of this warming slowdown.

But the shift of this so-called ­Pacific Decadal Oscillation (PDO) to a cooler phase may not have been an all-natural event. Instead, changes in fossil-fuel emissions may have driven the swing, according to a ­climate modeling study by a team from the U.K. government weather and climate office (Nat. Clim. Change 2016, DOI: 10.1038/nclimate3058).

Sulfur dioxide released by fossil-­fuel combustion leads to the formation of sulfate aerosols, which reflect sunlight and consequently have a local cooling effect. In China, an increase in emissions and aerosols resulted in cooling during the time period studied. In North America and Europe, decreasing emissions and aerosols resulted in warming.

The consequence was a strengthening of the Pacific trade winds, which acted to bury heat below the water’s surface, says Doug M. Smith, who led the work. The modeling cannot say for certain whether the effect pushed the PDO into a cooler phase or strengthened what was already happening naturally.

The modeling study further indicates that as China reduces emissions to improve air quality, the PDO may move into a warmer phase and drive global surface temperatures to increase faster. “There are some signs that this could be happening now, but we need more years to know for sure,” Smith says.