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PRI's Environmental News Magazine

Feedback Loop

Air Date: Week of January 9, 1998

In global warming research a lot of attention is paid to a phenomenon that is not well understood called the feedback loop. Simply put, scientists worry that a rise in the earth's temperature could set off other reactions that would cause global warming to get much worse much faster. A study reported in the journal, Science, helps provide some details to one potential feedback loop. The study shows that warmer temperatures could cause much of the carbon dioxide now trapped in the world's northern forests to escape and hasten global warming. Laura Knoy spoke with Doctor Michael Goulden who is an assistant professor of earth sciences at the University of California at Irvine. Dr. Goulden headed the research on the feedback study.

Transcript

KNOY: In global warming research a lot of attention is paid to a phenomenon that is not well understood: the feedback loop. Simply put, scientists worry that a rise in the earth's temperature could set off other reactions that would cause global warming to get much worse much faster. A study reported in the journal Science helps provide some details to one potentially huge feedback loop. It shows that warmer temperatures could cause much of the carbon dioxide now trapped in the world's northern forests to escape and hasten global warming. Dr. Michael Goulden is an assistant professor of Earth sciences at the University of California at Irvine. He headed the research on feedback loops. Thanks for joining us.

GOULDEN: Thank you.

KNOY: Dr. Goulden, describe the role that the northern forests play in global warming.

GOULDEN: The northern forests, also referred to as arboreal forest or taiga, occur in a broad latitudinal band going around the Earth's surface about 1000 kilometers north to south. Through North America it sweeps in central Canada through Alaska and then a large, extensive northern forest in Siberia. When you first go to a typical northern forest or arboreal forest, it's not very impressive. The trees are maybe only 20, 30 feet tall. But the twist is that there's a tremendous amount of carbon stored in the soils of arboreal forest, and when you add together the amount of carbon stored in the soils plus that in the moss plus that in the trees, it's pretty much comparable to the amount of carbon in a tropical rainforest. And when you extrapolate this out over the entire Earth's surface, all the arboreal forest on the Earth, what you'll find is that the amount of carbon stored in northern forests is comparable to maybe two thirds of the CO2 in the atmosphere.

KNOY: In that research, you found that a warming of just a few degrees would cause the soil in the forest to give up carbon dioxide. Explain how that occurs.

GOULDEN: Well, warming in principle could have 2 effects. It could increase plant growth. It could stimulate the growth of plants and cause a net storage of carbon. Or alternatively, it could stimulate the decomposition of carbon in the soil or of litter sitting on the top of the soil. And we found in our research that it looks like the stimulation of decomposition would be much, much stronger than the stimulation of plant growth, and that would result in a large loss of CO2.

KNOY: How much carbon gas could be released?

GOULDEN: Well, that's one of the big questions that's still out there. We believe that it's a very sensitive lever that relatively small warming could cause a large increase in decomposition. But what we can't say as yet is how much of that soil carbon would ultimately decompose. If 80% of the carbon in the soils of arboreal forest were to decompose, it would have a major effect on the concentration of CO2 in the atmosphere. Now, there are still some uncertainties out there. For example, warming might cause deciduous forest to invade from the south, and that in principle could cause a large storage of carbon.

KNOY: Dr. Goulden, is there any evidence that this phenomenon we've talked about is happening already?

GOULDEN: Well, we have a suspicion. One of the main things that we measured during the study was the net exchange of CO2 by the site, and we actually measured that for 3 and a half years. And when we summed that up, we found that the site was actually losing a little bit of carbon. And that was a real surprise to us. Because we expected that this site should always be accumulating carbon. One possible explanation for that is that there's some evidence, though it's not conclusive, that the climate in that area has warmed over the last 20 to 30 years. In fact, the climate throughout much of the northern areas has warmed. And so, one interpretation is that we're already beginning to see an effect of climatic warming to cause this site to lose carbon. Though at this point at best it's just a correlation.

KNOY: Dr. Michael Goulden is an assistant professor of Earth sciences at the University of California at Irvine. He spoke to us from the studios of KLON in Long Beach. Thanks for joining us.

GOULDEN: Thank you very much.

 

 

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