Monitoring atmospheric composition & climate
Aerosol radiative forcing products
Direct aerosol radiative shortwave forcing at the top
of the atmosphere

MACC II scientists have estimated the impact of man-made atmospheric aerosols on climate, finding that they offset up to a third of the warming due to greenhouse gases.

Aerosols are tiny particles emitted into the atmosphere by natural processes, such as sand storms and sea spray. Human activities, such as transport, power plants, industry, domestic heating, and agriculture, also emit aerosols, adding to natural levels. Man-made aerosols have competing effects on the Earth's climate. On the one hand, they cool the climate by reflecting more sunlight back to space and by leading to the formation of brighter clouds. On the other hand, aerosols made of soot warm the climate by absorbing more sunlight within the atmosphere. Climate change is the result of the competition between the warming caused by greenhouse gases and soot aerosols, and the cooling caused by the other aerosol species.

MACC II produces daily analyses and forecasts of atmospheric aerosols, creating an expanding record that starts in January 2003. MACC II scientists are using the MACC II record to routinely monitor the impact of man-made aerosols on climate. They estimate that man-made aerosols have reflected 1 Wm-2 of sunlight back to space, on a global average over the period 2003-2012. This represents about one third of the 2.6 Wm-2 change in the Earth's radiation balance caused by man-made increase in greenhouse gas concentrations.

There are large uncertainties in the amount of aerosols emitted into the atmosphere by human activities, their chemical composition, and their ability to make clouds brighter. Although the MACC II best estimate of the aerosol climate impact is 1 Wm-2, it could be as little as 0.5 Wm-2 and as much as 2 Wm-2. These estimates have fed into the IPCC fifth assessment report to be released in September 2013. Within MACC II, scientists continuously improve their analysis and methods to reduce those uncertainties and refine the best estimate.

These estimates have been used in a recent revision of current climate sensitivity and have fed into the IPCC fifth assessment report to be released in September 2013.

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Full list of variables [pdf]