Volume of CO2 Caused by Forest Loss Poses a Puzzle
Scientists trying to understand Earth’s greenhouse warming are struggling with a basic frustration. They are convinced that heavy loss of forests is adding heat-trapping carbon dioxide (CO2) gas to the atmosphere. But they cannot make even a ballpark guess at the amount.
Statistics for Earth’s forest cover on which they base their calculations are turning out to be seriously in error. There is much less CO2 absorbing biomass per square meter on average in a forest than standard references report. This throws assessments of the climatic impact of deforestation--and, conversely, of the beneficial climatic effect of reforestation--into doubt.
Research by Daniel Botkin and Lloyd Simpson at UC Santa Barbara and by Sandra Brown at the University of Illinois, Urbana-Champaign, shows that widely used estimates of forest biomass overstate Earth’s inventory by 100% to 300%. The message, Brown said, is “Forget all those (old biomass) numbers. They’re just not valid anymore.”
Berrien Moore III at the University of New Hampshire’s Complex Systems Research Center calls this “one of the most frustrating things” for scientists trying to understand global climate change.
CO2 Produced by Decay
A growing tree takes CO2 out of the air and locks up the carbon in its tissues. The tree releases the carbon--generally as CO2--when it dies and decays or is burned. This is a key part of Earth’s carbon cycle. The burning of fossil fuels is the biggest source of new atmospheric CO2. But, with 70,000 to 110,000 square kilometers of land being deforested and put to other uses each year, scientists expect the loss of forest biomass to be a significant secondary CO2 source, Moore said.
He added that even taking account of deforestation along with other known CO2 sources and allowing for CO2 “sinks” such as removal by the sea, scientists have been unable to account fully for atmospheric CO2 concentrations. “It looks like we don’t understand the carbon cycle,” Moore said. The realization that scientists have been using the wrong numbers for forest biomass adds to this frustration.
Simpson said the numbers result from unwarranted extrapolation of narrowly limited surveys to global scales. He and Botkin have studied the northern (boreal) forest that stretches from Alaska to Newfoundland. He noted that previous estimates of the average biomass of that forest were based on sampling done near cities and universities. These are along the southern part of the forest where the tree cover is denser so they give an inflated estimate of the biomass average.
Botkin and Simpson applied a random-sampling statistical technique developed for crop surveys to the entire forest. They came up with a biomass average of 4.17 kilograms per square kilometer with an uncertainty of 24%. The averages in general use are two to three times higher.
Brown has found comparable overestimates for tropical forests. Her latest study for southern and Southeast Asia yields a biomass average of about 25 kilograms per square kilometer. Standard values are twice as high. “This suggests that deforestation is not as significant a source of CO2” as estimates since 1980 have implied, she said.
She added that this lays more of the blame for atmospheric CO2 buildup on fossil-fuel burning. But it does not justify deforestation. She points out that forests are valuable for many things besides their climatic role. These include genetic resources, water supply and products other than timber.
In fact, Charles Peters of the New York Botanical Garden, Alwyn Gentry of the Missouri Botanical Garden and Robert Mendelshon of Yale University published an analysis of the value of non-wood forest products last June 29 in Nature. They show that, in the Amazon forest, sustained harvest of such products as fruit, oils, latex, fibers and medicines can be two to three times more profitable than cutting the trees and trying to farm the land.
Botkin and Simpson are extending their research to North American temperate forests. Brown is moving ahead with a program to provide estimates of forest biomass within each square of a grid of 0.5-degree latitude/longitude squares for the entire forested globe. Both scientists emphasize much more research will be needed to give climate-change analysts a reliable biomass data base with which to work.
