Thursday, 22 October 2015

Wildfires contribute to climate change positive feedback

I am reposting this to give it the prominence it needs.  Right across the globe, from the Amazon to the Alaskan boreal forests and, I suspect, the Indonesian rain forests forsts are losing their function as absorbers of carbon and are instead yet another source.

In this context I have extracted positive feedback #22 from Guy McPherson's Climate Change Summary and Update

22. Drought-induced mortality of trees contributes to increased decomposition of carbon dioxide into the atmosphere and decreased sequestration of atmospheric carbon dioxide. Such mortality has been documented throughout the world since at least November 2000 in Nature, with recent summaries in the February 2013 issue of Nature for the tropics, the August 2013 issue of Frontiers in Plant Science for temperate North America, and the 21 August 2015 issue of Science for boreal forests. The situation is exacerbated by pests and disease, as trees stressed by altered environmental conditions become increasingly susceptible to agents such as bark beetles and mistletoe (additional examples abound).

One extremely important example of this phenomenon is occurring in the Amazon, where drought in 2010 led to the release of more carbon than the United States that year (Science, February 2011). The calculation badly underestimates the carbon release. In addition, ongoing deforestation in the region is driving declines in precipitation at a rate much faster than long thought, as reported in the 19 July 2013 issue of Geophysical Research Letters. An overview of the phenomenon, focused on the Amazon, was provided by Climate News Network on 5 March 2014. “The observed decline of the Amazon sink diverges markedly from the recent increase in terrestrial carbon uptake at the global scale, and is contrary to expectations based on models,” according to a paper in the 19 March 2015 issue of Nature.

Tropical rain forests, long believed to represent the primary driver of atmospheric carbon dioxide, are on the verge of giving up that role. According to a 21 May 2014 paper published in Nature, “the higher turnover rates of carbon pools in semi-arid biomes are an increasingly important driver of global carbon cycle inter-annual variability,” indicating the emerging role of drylands in controlling environmental conditions. “Because of the deforestation of tropical rainforests in Brazil, significantly more carbon has been lost than was previously assumed.” In fact, “forest fragmentation results in up to a fifth more carbon dioxide being emitted by the vegetation.” These results come from the 7 October 2014 issue of Nature Communications.

Dennis Murray, a professor of ecology at Trent University in Peterborough, Ontario, thinks the dying moose of Minnesota and New Hampshire and elsewhere are one symptom of something far bigger – a giant forest ecosystem that is rapidly shrinking, dying, and otherwise changing. “The boreal forest is breaking apart,” he says. “The question is what will replace it?” **

Alaskan boreal forest fires release more carbon than the trees can absorb

19 October, 2015

A new analysis of fire activity in Alaska's Yukon Flats finds that so many forest fires are occurring there that the area has become a net exporter of carbon to the atmosphere. This is worrisome, the researchers say, because arctic and subarctic boreal forests like those of the Yukon Flats contain roughly one-third of the Earth's terrestrial carbon stores.

The research is reported in the journal Nature Climate Change.

Alaska fire records go back only to 1939, and scientists often assume that present-day fire activity mirrors that of the ancient past. The researchers on the new study instead used actual fire data from a previous study in which they analyzed charcoal fragments preserved in lake sediments in the Yukon Flats. In that study, they found that fire frequency in a 2,000-kilometer swath of the Yukon Flats is higher today than at any time in the last 10,000 years.

For the new analysis, the team plugged its fire data into a computer model ofcarbon cycling in the study area.

"Having these data allowed us to simulate not only recent decades, but the entire past millennium of carbon cycling," said Ryan Kelly, a postdoctoral researcher at the University of Illinois who conducted the study with Feng Sheng Hu, a U. of I. professor of plant biology and of geology.

"Our model confirms our hypothesis that the recent increase in fire frequency in our study region has caused massive carbon losses to the atmosphere. About 12 percent of the total stored carbon has been lost in the last half century," said Kelly, who now is a data scientist and modeler for Neptune and Company, Inc.

"Most studies of carbon cycling in boreal forests have been motivated by the fact that there's just an enormous amount of carbon in these high-latitude ecosystems," Hu said. "Up to 30 percent of the earth's terrestrial carbon is in that system. And, simultaneously, this region is warming up faster than any other parts of the world."

Study: Alaskan boreal forest fires release more carbon than the trees can absorb

U. of I. professor Feng Sheng Hu led a study of carbon cycling and forest fires in the boreal forests of the Yukon Flats in Alaska. Credit: L. Brian Stauffer
Increasing numbers of fires are unbalancing the cycle of carbon capture and release, the researchers report. More carbon dioxide in the atmosphere could enhance plant growth, but it also contributes to further climate warming in the higher latitudes, Kelly said.

"Such warming would likely be attended by increased wildfire activity, which would more than cancel out plants' carbon uptake and lead to a net increase in atmospheric carbon dioxide," he said.

The new findings challenge studies that assume that recent fire activityreflects the norm over thousands of years. Those assumptions would lead scientists to conclude that the region has been a net carbon sink in recent decades, the researchers said.

Replacing that assumption with actual fire data from the past millennium offers a starkly different picture of the carbon cycle in the Yukon Flats, they said.

"The effects of forest fires on the carbon cycle are very dramatic. Fires explain about 80 percent of the change in carbon storage over the past millennium, and a large amount of carbon has been lost from this ecosystem because of increasing forest fires," Hu said. "This area has burned more than any other place in the boreal forests of North America. We chose the area for this study because we thought it could be an early indicator of the future."

The researchers see a troubling trend, in which climate warming increases the number of fires, which release more carbon to the atmosphere and enhance warming.

"Boreal forests contain vast carbon stocks that make them inherently big players in the global carbon cycle," Kelly said. "And the main way that this stored carbon is eventually released is through fire."

More information: Paleodata-informed modeling of large carbon losses from recent burning of boreal forests, Nature Climate

Journal reference: Nature Climate Change search and more info website

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