Natural aerosols confound climate predictions

Researchers can’t say how much cooling these floating particles cause

UNCERTAIN PREDICTIONS Aerosols like those that result from eruptions of volcanoes such as Iceland’s Eyjafjallajökull can cool climate, but scientists can’t say by how much. A new study suggests natural aerosols add more uncertainty to climate predictions than researchers realized.

U.S. Geological Survey and Oddur Sigurðsson/Iceland Meteorological Office

Airborne particles from natural sources add more uncertainty to global climate simulations than scientists previously realized. The finding could complicate scientists’ attempts to predict climate change.

Although scientists agree climate is warming, they cannot predict exactly how much temperatures will rise in a given time period. Among the main factors forcing researchers to hedge their forecasts are small floating particles called aerosols, which form either naturally or from human activity such as fossil fuel burning. Aerosols are thought to cool the planet by reflecting radiation from the sun and by seeding clouds (SN: 10/5/13, p. 26). Clouds tend to reflect solar radiation, but scientists are still struggling to understand precisely how strong aerosols’ cooling effect is.  

In the new study, atmospheric scientist Ken Carslaw of England’s University of Leeds and colleagues analyzed variables related to the amount and size of aerosols in the atmosphere and how they affect the rate at which clouds form. The researchers wanted to determine how much uncertainty each factor adds to predictions of temperature change. Emission rates of natural aerosols, produced mainly by volcanic eruptions and oceanic phytoplankton, accounted for nearly half the analysis’s variance, which is a measure of uncertainty, the team reports in the Nov. 7 Nature. The researchers urge scientists to find more accurate ways to represent natural aerosols in their simulations; however, they also say researchers may never fully understand how these particles affect climate.

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