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Black carbon - element of uncertainty in climate prediction

The burning of fossil and renewable fuels affects our climate through the production of soot, which can act either warming or cooling. It is difficult for many reasons to take stock of black carbon particles, as shown in a recent article in the journal Science.

In recent months climate scientists have been surprised by a scientific publication. An American research team came to the conclusion that soot’s contribution to global warming is about twice as high as previously thought.

Picture: The smoke from a fire in extremely dry vegetation in the Great Victoria Desert, Australia on 17 January 2013 is nearly black. A bright cumulus cloud rises from the top of the plume. Picture: NASA/C. HADFIELD

Soot particles are formed from the burning of fossil fuels such as coal and oil and also from the use of biomass fuels such as wood. The chemist Tami Bond and her research team estimate that black carbon, the scientific term for soot particles, is more harmful to climate than the greenhouse gas methane.

How such large differences in the assessment of the effect of soot on climate were arrived at is described in a recent publication in the scientific journal Science. Meinrat O. Andreae, Director at the Max Planck Institute for Chemistry in Mainz, and his Californian colleague Veerabhadran Ramanathan explain the global and regional climate effects of the black particles. Soot is the biggest absorber of solar radiation in the earth’s atmosphere.

“Unlike greenhouse gases, black carbon is not a single chemically defined substance with constant physical properties. This alone makes it difficult to precisely determine the impact of black carbon on climate,” says the climate researcher from Mainz, Prof. Andreae. “One reason why there are such large discrepancies in the estimates is the existence of so-called brown carbon.” These brown carbon particles are produced from the burning of biomass and from chemical reactions in the atmosphere. Brown carbon absorbs light and heat exactly like black carbon - something that has been previously ignored in climate models. Instead, it was assumed that brown carbon cooled the climate because the particles reflected more sunlight back into the space than they absorbed in the atmosphere.

To budget the effects of soot on climate all mechanisms must be known

A further element of uncertainty lies in the interplay of the cooling and warming properties of soot. Soot particles not only absorb heat and release it to the earth’s atmosphere extremely well. They also serve as nuclei for the condensation of moisture, which leads to cloud formation. And, clouds in turn reflect the rays of the sun. To determine a correct climate budget for soot, it is necessary to know all the climate relevant mechanisms - cooling and warming.

The research team around the American scientist, Bond, has proposed just such a budget. They have synthesized all available model results and observations and calculated the amount of solar energy that the black particles absorb and therefore can release to the climate system. This so-called radiative forcing of 1.1 watt per cubic meter is twice as high as the value in a study from the United Nations Environment Program (UNEP) and the World Meteorological Organization (WMO), which has been used for predictions of climate warming. For Bond and her colleagues soot is more climate relevant than previously assumed.

How can this big difference to earlier calculations be explained? Andreae and his colleague Ramanathan compare different approaches to derive the radiative forcing resulting from the absorption of light by soot. The climate scientist Ramanathan determined this from absorption data from satellites and from 140 ground stations. The Bond study presents a substantially smaller value derived from emission inventories and atmospheric models, which they then scale up to get to the values from experimental data. From this, Andreae and Ramanathan conclude that in most climate predictions either an important source for soot is not taken into consideration, or that data from estimations of global emissions is not reliable enough.

Soot makes regional climate predictions more uncertain

The uncertainties due to the soot factor are even greater at the regional scale - for example in predictions of precipitation. This comes from the fact that soot particles can have opposing effects on cloud formation. In small clouds the precipitation efficiency is reduced, for large clouds it is increased. In the air over the Indian Ocean soot works like a sun umbrella, thus the sea water is warmed less and less water evaporates. As a result researchers fear that, for example, the monsoon rains in south Asia will weaken - a region in which biomass is increasingly burned.

In addition the absorption of solar radiation by soot causes heat gradients in the atmosphere which cause changes in air circulation. This appears to be responsible for the already observed northward shift of the tropical belt.

If the radiative absorption by soot in previous models is really so markedly underestimated as Bond’s American research team claim, then regional climate effects need to be reevaluated, according to Andreae and Ramanathan. This in turn would require accurate observations of the climate system, from satellite data to microchemical measurements of aerosols.

Yet, the question remains, whether all the effort to reduce the emission of soot is even worth the effort. The burning of fossil fuels releases compounds such as sulfur dioxide that cool our climate. A complete budget for our climate is therefore made even more difficult.

Why we should nevertheless make every attempt to reduce soot emissions as much as possible lies in how harmful it is for human health. Every year an estimated 3.5 million people die from air polluted with soot and related small aerosol particles in households, especially in developing and emerging countries where meals are often still cooked over open fires.

Original Publication:
“Climate’s Dark forcings”: Meinrat O. Andreae and Veerabhadran Ramanathan, Science, 19 April 2013: Vol. 340 no. 6130 pp. 280-281 DOI: 10.1126/science.1235731

Prof. Dr. Meinrat O. Andreae
Max Planck Institute for Chemistry, Mainz, Germany
Phone: + 49 (0)6131/ 305-6000

Dr. Susanne Benner | Max-Planck-Institut
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