Particles Multi-Task to Change Climate
Particulate, or aerosol, pollution has influenced climate in several ways during the past century. While the net effect is global-cooling, in opposition to the greenhouse gas warming, the aerosol effects include a combination of cooling and warming components. Probably the best understood effect is the direct scattering and absorbing of sunlight by aerosols suspended in the atmosphere, the "direct effect". Bright particles scatter incoming radiation. The most important scattering component is sulfate, which comes from combustion of coal and oil and from industrial activities. Darker particles absorb sunlight and warm the atmosphere where they are suspended. Black carbon is the most important absorbing particle; it comes from incomplete combustion of coal, wood and diesel. Since the scattering effect is larger than the absorbing effect, the overall direct effect over the century is cooling.
Pollution aerosols also cool climate by altering cloud properties, known as the "indirect" effect. Cloud droplets form around suspended aerosols. Pollution has increased the number of particles, which makes the cloud droplets smaller and more numerous. These polluted clouds last longer and are brighter. Overall, clouds cool Earth's climate by blocking the surface from incoming sunlight. So the increased cloudiness from pollution has cooled climate over the past century.
A third effect occurs when black carbon (BC) particles land on snow, and the tiny specks of dark material in the upper snow layers absorb heat from the sun and promote melting. This is called the "BC-snow-albedo" effect. Loss of snow or ice makes Earth's surface much darker, so that even more warming and melting occur. Thus the BC-snow albedo effect contributes to warming.
We recently conducted global climate model experiments to distinguish the ways these three aerosol effects have changed climate during the 20th century. The model was run for 1890 conditions and again for year 2000 conditions, and the resulting climates were compared. A first set of experiments made changes in aerosols only. In a second set we also changed greenhouse gases such as CO2 to determine whether the aerosol effects change if greenhouse gases simultaneously warm the climate.
In our study, the biggest aerosol effect on climate came from the effect of aerosol-cloud indirect effect. Over the century, it cooled the surface air temperatures -1°C, with more cooling in the northern hemisphere than in the south. Snow and ice cover increased 1% globally and 4% in the Arctic. Global cloud cover also increased by 0.5%.
The aerosol direct effect cooled the climate over the century by -0.2°C, also more in the north than the southern hemisphere. It also caused a small increase in cloud and snow/ice cover.
Warming from the BC-albedo effect was similar in magnitude to the cooling from the direct effect. The effect was largest in the Arctic, where snow/ice cover declined by 1%.
We found correlations among the aerosol impacts within regions, so that cooler (warmer) temperatures typically correlated with increased (decreased) snow/ice cover and increased (decreased) cloud cover.
If greenhouse gases increased together with aerosols over the century, the potency of the aerosol effects was reduced. One exception was the cloud changes from the indirect effect, which increased 0.5% with or without the greenhouse gas changes. Nevertheless the greenhouse gas warming reduced the indirect effect on surface air temperatures cooling by 20% and on snow/ice cover increase by 50%. Furthermore the greenhouse gas warming generally overwhelmed the changes from the BC-albedo effects.
Since the aerosol impacts were particularly great in the Arctic, we studied the seasonality of the changes there. Aerosols altered the surface air temperature changes most in winter, even though effects on snow/ice and cloud cover were greatest during summer. One explanation for the seasonal offset is that the large summertime snow/ice change alters ground temperatures, and these ground temperature changes are felt more at ground-level during winter when the surface atmospheric layer is most stable.
Koch, D., S. Menon, A. Del Genio, R. Ruedy, I. Aleinov, and G.A. Schmidt, 2009: Distinguishing aerosol impacts on climate over the past century. J. Climate, 22, 2659-2677, doi:10.1175/2008JCLI2573.1