Concerns about human effects on climate have centred largely on carbon dioxide released by industry, vehicles and burning of forests. But huge amounts of 'greenhouse gas' carbon dioxide are also cycled between ecosystems on land (the 'terrestrial biosphere') and the atmosphere. So predictions of future climate change need to take into account this natural 'carbon cycling', and how it might change in a warmer world. Two new studies now reveal that carbon cycling processes are subtle and surprising. This new work should help to make climate models more accurate.

Climate scientists measure the inputs of carbon dioxide to the atmosphere in terms of 'carbon fluxes' -- how much carbon, in the form of carbon dioxide gas, is added. In the 1990s, humans burning gas, coal and oil injected around six billion tonnes of carbon into the atmosphere each year.

But the terrestrial biosphere serves as a 'carbon sponge', absorbing large amounts of carbon dioxide from the atmosphere. Some estimate that this 'carbon sink' can take up as much as two and a half billion tonnes of carbon annually. Even this, however, obscures the true magnitude of carbon cycling between the terrestrial biosphere and the atmosphere. The size of the 'sink' represents just the net balance between an enormous uptake and an almost-as-enormous release of carbon, each of which accounts for about 60 billion tonnes of carbon per year.

So even a small shift in the balance of carbon cycling in the terrestrial biosphere could have a large impact on climate. Understanding how vegetation responds to changes in climate is critical to future predictions.

In the first report, an international team of researchers from the EUROFLUX study of 15 European forests between 1996 and 1998 say in Nature1 that European forest ecosystems can show considerable variation. Some forests are carbon sinks, others sources.

Forest ecosystems remove carbon dioxide from the air through photosynthesis and release it through two processes: respiration and decomposition. Plants release carbon dioxide through a respiratory process analogous to that of our own breathing. Microbes decompose dead plants breaking down the carbon-rich matter to carbon dioxide.

The EUROFLUX results show that the rate of carbon uptake by forests does not vary significantly between forests at high latitudes (in Iceland or Sweden, say) and those at low latitudes (in Italy). But low-latitude forests seem to act as a relatively larger carbon sink -- so they might release less carbon through respiration than do the northernmost ecosystems.

The second study analysed decomposition. Christian Giardina of the University of Hawaii and Michael Ryan of the US Department of Agriculture Forest Service in Colorado suggest that decomposition processes in forest soils don't depend on temperature. This is in line with what the EUROFLUX team proposes -- they think that the decreased rate of carbon dioxide release in southern forests is due mostly to lack of water. Giardina and Ryan's findings contrast with an earlier suggestion that global warming might increase the release of carbon from soils by speeding up microbial activity.

This is good news for climate change: it means that the response of forests to warming will not obviously make matters worse. On the other hand, the results show that we still have a lot to learn about the finer points of the carbon cycle, one of the principal natural 'engines' of climate change.