Australasian Science: Australia's authority on science since 1938

Forests Feel the Heat

By Stephen Luntz

A temperature increase of 1°C causes tropical rainforests to release an additional 3.5 billion tonnes of carbon during El Niño events, with worrying implications as the world warms.

NASA Earth Exchange examined 50 years of temperature and rainfall patterns and carbon dioxide levels in the atmosphere. Once the upward trend from human emissions was removed, El Niño years could be seen to produce increases in atmospheric carbon while volcanic eruptions caused reductions through the cooling effects of aerosols.

"What we learned is that in spite of droughts, floods, volcano eruptions, El Niño and other events, the Earth system has been remarkably consistent in regulating the inter-annual variations in atmospheric carbon dioxide levels," says lead author Dr Weile Wang of NASA.

El Niño years are associated with higher temperatures in tropical regions and reductions in rainfall. However, co-author Dr Pep Canadell of CSIRO Marine and Atmospheric Research says: “The observed temperature changes are more important than changes in rainfall in influencing concentration of atmospheric CO2.

“Under moist conditions, microbes go ballistic if given more energy from heat,” Canadell says. “The tropics are usually wet, and more than any other biome have enough water to continue decomposition at higher temperatures.” The rapid breakdown of organic products in the soil leads to the release of carbon into the atmosphere.

Climate models already take into account that rising temperatures will increase carbon emissions from tropical forests, but Canadell says the work, published in Proceedings of the National Academy of Sciences, quantifies this for the first time. The observed figure for a single degree rise is one-third of human-induced emissions.

Canadell says the authors found that tropical factors dominated the natural release of carbon. “Despite reports of browning from major droughts we could not see any sign of it in the atmospheric record,” he says. However, he admits: “It is possible that trees killed by the drought take a long time to break down in temperate environments, and the carbon is only released later.”