Airborne particles from volcanoes and the burning of fossil fuels have reflected enough sunlight to offset about a third of the current climate warming caused by carbon dioxide over the past decade, says the NOAA.
The conclusion backs up a recent study from the University of Boston, which concluded that the soaring use of fossil fuels in China was temporarily offsetting the effects of CO2.
"Since the year 2000, stratospheric aerosols have caused a slower rate of climate warming than we would have seen without them,” says John Daniel, a physicist at the NOAA Earth System Research Laboratory.
Over the past decade, the amount of aerosol in the stratosphere has been in something of a 'background' state, lacking sharp upward spikes from very large volcanic eruptions. The authors analyzed measurements from several independent sources – satellites and several types of ground instruments – and found a definitive increase in stratospheric aerosol since 2000.
"Stratospheric aerosol increased surprisingly rapidly in that time, almost doubling during the decade," says Daniel.
"The increase in aerosols since 2000 implies a cooling effect of about 0.1 watts per square meter – enough to offset some of the 0.28 watts per square meter warming effect from the carbon dioxide increase during that same period."
The 10-year increase in stratospheric aerosols could be caused in part by natural sources – smaller volcanic eruptions – as well as human activities, which could have emitted gases such as sulfur dioxide, which react in the atmosphere to form reflective aerosol particles.
If levels continue to increase, temperatures will not rise as quickly as they would otherwise; but if they decrease, temperatures would increase faster.
"The 'background' stratospheric aerosols are more of a player than we thought," says Daniel. "The last decade has shown us that it doesn’t take an extremely large volcanic eruption for these aerosols to be important to climate."