As fires become more frequent and less controllable, they will impair the West's ability to absorb carbon and slow climate change, concludes a new analysis of the western United States' landscape.
The West's ecosystems sequester more than 90 million metric tons of carbon dioxide per year, about 5 percent of total U.S. carbon emissions and roughly enough to offset the carbon emissions of Arizona, according to the U.S. Geological Survey's report on carbon storage in natural landscapes in the West.
Although this rate would increase if future policies encourage the preservation and growth of forests, grasslands and other carbon-absorbing environments, the potential expansion of fires could increase carbon emissions by up to 56 percent by 2050, given future climate conditions. Under an extreme climate scenario, emissions could jump to 150 percent from the base-line annual average between 2001 and 2008.
"Almost all climate and fire models combined predict an increase," explained Bradley Reed, associate program director of USGS's Land Change Science program. "The rate of sequestration we expect will go down at a decreased rate."
The researchers covered 2.66 million square kilometers (1.03 million square miles) of lands. More than half were grasslands or shrublands. Forests took up 28.1 percent of the area, followed by agricultural lands at 6.1 percent, water at 1.5 percent and wetlands taking up less than 1 percent.
Watching the urban landscape grow
The potential for more sequestration is there, the report finds. The large and forest-rich Western Cordillera Mountains alone could hold 60 percent of projected future carbon storage. In the best-case scenario for maximum carbon storage, the West's lands could absorb 113.9 million tons per year up to 2050. If economics or policies drive changes that shrink carbon sinks, the West could be releasing a net 2.9 million tons of carbon per year.
Lakes and reservoirs in the West sequester up to 3.7 million metric tons of carbon in the sediment, while coastal waters absorb an average of 2 million metric tons of carbon. This is a highly variable number, given the high variability of coastlines.
The expansion of urban centers will become a growing challenge to the West's ability to store carbon in its ecosystems, one author said.
"It's strikingly clear that the urban landscape is going to be an important contributor to environmental change," said Ben Sleeter, the author of the report focused on emissions from land use change and a geographer with USGS. "That [4 percent urbanization] number is going to get much bigger."
Reed said the researchers on the report were unable to acquire reliable data for carbon emissions from the mountain pine beetle outbreak in the West but hope to study it in the future.
The report follows a similar analysis of the Great Plains, and is the second of a series of carbon sequestration reports for regions of the United States required by the 2007 Energy Independence and Security Act.
Given the report's mandate from Congress, Reed expects it will guide planning for the West to negate carbon emissions from man-made sources.
"It gives the scientific information to include in the package for counterbalancing emissions," he said.