Biomass burning - the burning of rain forests, savannah grasslands, agricultural wastes, and other biological materials - is far more widespread than was previously thought, according to scientists attending the first international conference on biomass burning, held in Williamsburg, [Virginia]. It was sponsored by seven organisations, led by the American Geophysical Union and the National Aeronautics and Space Administration.

Biomass burning creates several greenhouse gases, as well as chemically active gases that cause acid rain and tropospheric ozone pollution. Researchers estimate that, in total, about 5% of Earth's surface is burned each year.

Just five years ago, the scientific community gave little attention to biomass burning, says conference organiser Joel S. Levine, a senior research scientist at NASA's Langley Research Centre. But recently it has become the subject of intensive research.

Scientific estimates of the amount of biomass burned in the world each year are greater than they were a decade ago. Paul J. Crutzen, director of the atmospheric chemistry department at the Max Planck Institute for Chemistry, Germany, says that some 3 billion to 5 billion metric tons of biomass carbon are burned annually, compared with 5 billion to 7 billion metric tons of fossil fuel carbon.

Meinrat O. Andreae, director of the biogeochemistry department at Max Planck Institute, calculates that the carbon dioxide from biomass burning is about 40% of that produced from all sources annually, and that the carbon dioxide from deforestation alone accounts for about 26%. Burning of the savannah in Africa is responsible for about one third of the global biomass burning.

Photographs taken by astronauts show that during the burning (dry) season, smoke areas in the Amazon have increased from scattered fires covering about 300,000 square km in 1973 to continental-scale palls over about 3 million square km in both 1985 and 1988, reports Charles A Wood, space scientist at NASA's Johnson Space Centre in Houston. "You had better look at Earth in the near future if you want to see it," he warns.

Biomass burning has a number of important impacts. The nitrogen oxides, hydrocarbons, and carbon monoxide released in the combustion process lead to the formation of tropospheric ozone. Also, the nitric oxides and formic and acetic acids created during combustion cause acid rain. In addition, biomass burning, by altering soil nutrients, increases the emission of methane and various oxides of nitrogen from the soil for many months after the burn.

Biomass burning affects global climate in several ways, especially by creating vast amounts of the greenhouse gases carbon dioxide and methane. The changes in land surface resulting from biomass burning also alter climate. Because deforested land is not nearly so efficient as a tropical rain forest at returning rainfall to the atmosphere, burning vast stretches of rain forest reduces precipitation in the tropics.

As a result of the enormous quantities of biomass burned each year in the Amazon and in Africa, ozone levels there in the burning season are close to those in the industrialised world. Ozone concentrations in Africa are almost high enough to be toxic to plants, Andreae reports.

During the burning season, rain over virgin rain forests in central Africa and South America is as acidic as that which falls in Europe and North America. The high acidity in the tropics is attributed to biomass burning.

Information about biomass burning is uncertain. Though experts agree that the amount of burning has accelerated geatly in the past few decades, the current global extent, the percentages of various ecosystems being burned, and the frequency of burn are known only roughly.

"We believe that more than 95% of global burning is human initiated and can be stopped once the world's leaders are aware of its detrimental environmental consequences," Levine says. "This is one area of science where we can affect a reversal of a dangerous trend by raising awareness."

Source: C&EN, 26 March 1990.