Just breathe

Published July 1, 2000

In the global warming research arena, even those things we are certain we know are things that cannot be confirmed with any certainty.

The latest big news you didn’t hear comes from two papers in a recent issue of the journal Nature regarding forest respiration.

In their overview of the two papers, the University of Edinburgh’s John Grace and Mark Rayment close their analysis with the following statement: “Does this [new research] mean that the doomsday view of runaway global warming now seems unlikely? We hope so.” Little wonder you didn’t hear about this one on the national news.

As you know, through photosynthesis, plants inhale carbon dioxide and exhale oxygen, the opposite of what humans do—which fortunately allows us to live with our deciduous friends in peaceful harmony. The carbon dioxide (CO2) that plants inhale is stored in woody tissue and in the soil. But that CO2 is eventually released to the atmosphere—through a process known as respiration. A primary contributor to the respiration of CO2 is the bacterial decomposition of organic matter in the soil.

At present, photosynthesis and respiration are not quite in balance, with photosynthesis in the lead. The result is that forests serve as net carbon sinks that remove CO2 from the atmosphere.

That’s good news. Forests are, in effect, keeping the greenhouse effect in check by holding in rather than releasing CO2.

But the future may not be so rosy. Global climate models predict a warmer future as atmospheric CO2 levels continue to rise. Even though these higher CO2 levels will serve as a plant fertilizer, enhancing photosynthesis and thus carbon storage, bacterial decomposition is expected to increase with higher temperatures. That will tip the balance in favor of respiration. This would be bad news, because respiration adds CO2 to the atmosphere, which would further increase the temperature, leading to still more CO2 release, and so on. This is known as a positive feedback loop and could lead to a runaway greenhouse effect.

But here may be one of those rare, paradigm-shifting moments in science. The assumption that respiration increases with temperature is based on fairly short-term laboratory studies. Two longer-term studies now question that fundamental belief.

In one study, Christian Giardina and Michael Ryan analyzed temperature and microbial decomposition rates in soils from 82 sites on five continents. They found that “decomposition rates were remarkably constant across a global-scale gradient in mean annual temperature, [that] decomposition rates for forest soils are not controlled by temperature limitations to microbial activity, and that increased temperature alone will not stimulate the decomposition of forest-derived carbon in mineral soil.”

Another study by R. Valentini and many other scientists examined respiration rates in European forests over a large latitudinal gradient. Their surprising finding is that respiration is a major component of the carbon balance in northern forests where the temperatures are lower; whereas photosynthesis is fairly constant as you move from south to north.

That finding counters previous thinking that respiration should be limited in northern forests by the lower temperatures there. Valentini wrote that:

“For single sites our data also show a significant relationship between temperature and ecosystem respiration for both short and annual time scales. However, when a plot of [respiration] versus temperature is drawn across all sites the relationship is not significant, indicating that mean annual air temperature may not be an important contributing factor to forest ecosystem respiration on a broader scale.”

One possible explanation is that microbes adapted to wetter, northern soils are active for a longer portion of the year despite the lower temperatures there.

To illustrate the importance of these findings, Grace and Rayment ran a simple ecosystem model for coniferous forests for two different cases. Case One is based on the prevailing wisdom that respiration will rise with increasing temperatures. Case Two is based on new research that respiration is insensitive to long-term temperature increases.

In Case One, forests become less of a CO2 sink over time, while in Case Two, forests become even more effective as a CO2 sink.

In other words, future forests may actually continue to store more and more carbon. In this way, plants will manage to keep their foot on the greenhouse brake, rather than pushing the accelerator.

Robert C. Balling Jr., Ph.D. is director of the Laboratory of Climatology at Arizona State University and coauthor of The Satanic Gases.


Grace, J. and M. Rayment, 2000. Respiration in the balance. Nature, 404, 819-820.

Giardina, C.P. and M.G. Ryan, 2000. Evidence that decomposition rates of organic carbon in mineral soil do not vary with temperature. Nature, 404, 858-861.

Valentini R., et al., 2000. Respiration as the main determinant of carbon balance in European forests. Nature, 404, 861-865.