Global Warming and Decomposition Feedbacks

The Scientist Volume 22 | Issue 1 | Page 38


A Sensitive Reaction
Global warming could speed up decomposition, but how much might
decomposition speed up global warming?

By Kerry Grens

To understand what might happen in the sky as carbon increases and the atmosphere warms, Matthew Wallenstein at Colorado State University is looking to the ground. Beneath our feet and spread from Pole to Pole are countless numbers of microorganisms, which are decomposing organic matter and releasing many tons of carbon into the atmosphere each year.

Like any other chemical reaction, “we know that decomposition is sensitive to temperature,” says Eric Davidson, a senior scientist at the Woods Hole Research Center in Falmouth, Mass. As temperatures rise, decomposition speeds up, and more carbon gets released into the atmosphere. This additional carbon can then create a positive-feedback loop, raising temperatures higher and thus continuing to speed decomposition. The question Wallenstein wants to answer is: Will decomposition escalate global warming?

Wallenstein is trying to understand the mechanisms underlying an unexplained phenomenon that scientists have observed in a number of soil-warming experiments: Respiration rates from soil initially rise in response to elevated temperature, but then taper off.1 “In a sense there’s some kind of natural break in the system that would bring this positive feedback to a halt,” says Jerry Melillo at the Marine Biological Laboratory. For example, in a 10-year study Melillo led in the Harvard Forest, the response to warming, as measured in carbon flux, jumped an average of 28% in each of the first six years, but by the tenth year didn’t respond at all to warming.2 In other words, the researchers found that, with elevated temperatures, decomposition (and therefore carbon dioxide) rises, but then returns to normal with time, breaking down the positive-feedback loop. Why?

Wallenstein suspects that microbial communities are acclimating to long-term increases in temperature, which may favor microbes with less temperature-sensitive enzymes. “In the microbial world we understand very little how communities will change, how their functional characteristics will change,” says Josh Schimel at the University of California, Santa Barbara. To study acclimation over the long term, Wallenstein is pulling in proteomics to measure the relative abundances of enzymes from each organism over time. Additionally, he is examining enzymes’ temperature responses, using what he calls “crude” molecular biology techniques: He mixes up a batch of soil, fluorescently labels a particular molecule of interest, and observes how quickly the microbes break it down under different conditions. “We’re looking at the temperature sensitivity of some of these enzymes that degrade chitin, cellulose, and proteins in soil,” says Wallenstein

Another (and perhaps simpler) explanation for why the positive-feedback loop breaks down is that, as decomposition increases, microbes utilize all the easily accessible organic matter in the first few years of warming. Melillo’s hunch is that “once that relatively easy-to-decompose material is used up, the additional carbon loss [proceeds] at a slow rate.” Melillo and others have found that warming up soils at first increases the rate of decomposition, but over time it returns to normal, despite continued warming. Scientists want to know what’s responsible for the declining response.

Understanding why and when this feedback loop breaks down could make a big difference to climate models. There are two to four times as much carbon in soils (though mostly locked up in permafrost) as exists in the atmosphere; that’s a lot of carbon that could potentially getinto the atmosphere if microbes begin releasing it. A 2000 global warming model that included the feedback loop from increased decomposition, among other sources, estimated that by the year 2100 the temperature would increase 1.5 K more than without this feedback (5.5 K compared to 4 K).

However, this model has no acclimation built in, points out Melillo, and doing so might decrease warming estimates that include the feedback loop. “It’s just a very fundamental question that has not generally been answered by experimentation,” he says. The soil warming experiments at Harvard Forest are now in their 17th year, and Melillo expects to publish new data early this year. The long-term effects of warming on microbial communities remain a mystery, he says. “Only time will tell.”

Correction (posted January 7, 2008): When originally posted, this story had Matthew Wallenstein as Mark Wallenstein. The Scientist regrets the error.

1. Y. Luo et al., “Acclimatization of soil respiration to warming in tall grass prairie,” Nature, 413:622-5, 2001.
2. J.M. Melillo, “Soil warming and carbon-cycle feedbacks to the climate system,” Science, 298:2173-5, 2002.
3. P.M. Cox et al., “Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model,” Nature, 408:184-7, 2000.

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