Study shows tropics absorbing northern emissions

An international research team, including three CSIRO scientists, has thrown new light on the way that carbon dioxide produced by industrial activities in the Northern Hemisphere is absorbed by vegetation across the globe, particularly in tropical regions.

The research shows that plants in the mid to high latitudes of the Northern Hemisphere play a smaller role in the uptake of industrial carbon dioxide emissions, and that tropical plants play a larger role in that uptake than previously thought.

According to Dr Paul Steele, from CSIRO Marine and Atmospheric Research in Melbourne, the research is different from earlier studies because it uses most of the available data on the vertical distribution of carbon dioxide in the atmosphere.

"The concentration of carbon dioxide in the atmosphere isn't uniform - it varies spatially and temporally throughout the whole depth of the atmosphere," said Steele.

"When the vertical gradients from a range of different locations were properly included in model simulations, the carbon budget required a weaker uptake of carbon dioxide by land plants in northern temperate latitudes, and a weaker emission of carbon dioxide by plants in the tropics."

Steele explained that previous model studies have almost exclusively used time series of carbon dioxide measurements from only two dimensions of the atmosphere, latitude and longitude, near to the Earth's surface.

"By using the time series of measurements of the vertical distribution of carbon dioxide in the atmosphere at 12 sites around the world, we have arrived at this new result," Steele said.

One of the 12 sites is near Cape Grim, on the north-west tip of Tasmania, where a light aircraft was used to collect air samples from near the surface to high in the troposphere about once a month from 1991 to 2000. The Cape Grim station, managed by the Australian Bureau of Meteorology, monitors and studies global atmospheric composition in a program led by CSIRO and the Bureau.

Given the large carbon dioxide emissions caused by land clearing in the tropics, the research strongly implies that tropical ecosystems are showing strong uptake of carbon dioxide.

The result will improve estimates of global carbon cycling, including the fate of carbon emissions from human activity, which will be important for understanding and mitigating climate change.