Biogeochemist: "... perhaps most likely explanation is that increasing temperatures have increased rates of decomposition of soil organic matter, which has increased the flow of CO2. If true, this is an important finding: that a positive feedback to climate change is already occurring at a detectable level in soils."
One of the single greatest concerns of climate scientists is that human-caused warming will cause amplifying feedbacks in the carbon-cycle. Such positive feedbacks, whereby an initial warming releases carbon into the air that causes more warming, would increase both the speed and scale of climate change, greatly complicating both mitigation and adaptation.
The most worrisome amplifying feedback is the defrosting of the tundra (see “Science stunner: Vast East Siberian Arctic Shelf methane stores destabilizing and venting). Another major, related feedback now appears to be soil respiration, whereby plants and microbes in the soil give off more carbon dioxide as the planet warms.
As Nature reports (article here, study here, subs. req’d), a review of 439 studies around the world — including 306 performed from 1989 to 2008 — found “soil respiration had increased by about 0.1% per year between 1989 and 2008, the span when soil measurement techniques had become standardized.” Physorg.com interviewed the lead author, who said bluntly:
“There’s a big pulse of carbon dioxide coming off of the surface of the soil everywhere in the world,” said ecologist Ben Bond-Lamberty of the Department of Energy’s Pacific Northwest National Laboratory. “We weren’t sure if we’d be able to measure it going into this analysis, but we did find a response to temperature.”
The increase in carbon dioxide given off by soils — about 0.1 petagram (100 million metric tons) per year since 1989 — won’t contribute to the greenhouse effect unless it comes from carbon that had been locked away out of the system for a long time, such as in Arctic tundra. This analysis could not distinguish whether the carbon was coming from old stores or from vegetation growing faster due to a warmer climate. But other lines of evidence suggest warming is unlocking old carbon, said Bond-Lamberty, so it will be important to determine the sources of extra carbon.
Indeed the study itself concludes:
The available data are, however, consistent with an acceleration of the terrestrial carbon cycle in response to global climate change.
Moreover, a major study in the February issue of the journal Ecology by Finnish researchers, “Temperature sensitivity of soil carbon fractions in boreal forest soil,” has a similar conclusion. The Finnish Environment Institute, which led the study, explained the results in a release, “Soil contributes to climate warming more than expected – Finnish research shows a flaw in climate models“:
According to the results, the climatic warming will inevitably lead to smaller carbon storage in soil and to higher carbon dioxide emissions from forests. These emissions will further warm up the climate, and as a consequence the emissions will again increase. This interaction between the carbon dioxide emissions from soil and the warming of climate will accelerate the climate change.
The present climate models underestimate the increase of carbon dioxide emissions from soil in a warmer climate. Thereby they also underestimate the accelerating impact of the largest carbon storage in forests on the climate change. This result is also essential with respect to the climate policy measures concerning forests. The carbon storage of forests is, more than previously assumed, sensitive to climatic warming, and the carbon sink capacity of forests is endangered. To maintain the carbon storage, the accumulation of organic material in forests should increase. However, this is not compatible with the present bioenergy goals for forests and with the more and more intensive harvesting of biomass in forests.
Returning to the Nature study, the review was quite comprehensive:
They compiled data about how much carbon dioxide has leaked from plants and microbes in soil in an openly available database. To maintain consistency, they selected only data that scientists collected via the now-standard methods of gas chromatography and infrared gas analysis. The duo compared 1,434 soil carbon data points from the studies with temperature and precipitation data in the geographic regions from other climate research databases.
After subjecting their comparisons to statistical analysis, the researchers found that the total amount of carbon dioxide being emitted from soil in 2008 was more than in 1989. In addition, the rise in global temperatures correlated with the rise in global carbon flux.
And the study also confirmed worries about the unlocking of carbon in the permafrost:
Previous climate change research shows that Arctic zones have a lot more carbon locked away than other regions. Using the complete set of data collected from the studies, the team estimated that the carbon released in northern — also called boreal — and Arctic regions rose by about 7 percent; in temperate regions by about 2 percent; and in tropical regions by about 3 percent, showing a trend consistent with other work.
The researchers made clear that more research needs to be done to make definitive conclusions about exactly what is happening to soils around the world. Yet as the Nature story notes:
“There are a few plausible explanations for this trend, but the most tempting, and perhaps most likely explanation is that increasing temperatures have increased rates of decomposition of soil organic matter, which has increased the flow of CO2,” says Eric Davidson, a biogeochemist at the Woods Hole Research Center in Falmouth, Massachusetts. “If true, this is an important finding: that a positive feedback to climate change is already occurring at a detectable level in soils.”
As I noted in the methane post, the National Science Foundation press release (click here), warned “Release of even a fraction of the methane stored in the shelf could trigger abrupt climate warming.” The NSF is normally a very staid organization. If they are worried, everybody should be.
We are simply playing with nitroglycerin to risk crossing tipping points that could accelerate multiple amplifying feedbacks:
UPDATE: I would note that we’ve only warmed about 1°F over the past half-century (and indeed, far less than that over the time span of the 306 recent studies the form the basis of the primary conclusion). We are headed to 9°F warming on our current emissions path. The few studies that look at such emissions paths and attempt to model carbon cycle feedbacks including soil find they can add as much as 250 ppm and 2.7°F warming this century (see “Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model,” subs. req’d). Indeed, one very recent analysis of a high emissions, high feedback scenario finds impacts that are almost unimaginable by mid-century (see UK Met Office: Catastrophic climate change, 13-18°F over most of U.S. and 27°F in the Arctic, could happen in 50 years, but “we do have time to stop it if we cut greenhouse gas emissions soon”).
It is increasingly clear that if the world strays significantly above 450 ppm atmospheric concentrations of carbon dioxide for any length of time, we will find it unimaginably difficult to stop short of 800 to 1000 ppm, which would inflict on countless future generations Hell and High Water.