A new research suggests that the western evergreen forests, which cover an area from southern Canada to northern Mexico, took up a lot less carbon from the atmosphere during the drought that lasted from 2000-2004.
Christopher Schwalm and his colleagues at Northern Arizona University's School of Earth Science and Environmental Sustainability, think that there is a good chance the drought could be the new normal. If that happens, a big carbon sink will be lost.
Researchers calculated that during the drought of 2000-2004, the amount of carbon the western forests took up dropped by between 30 million and 298 million metric tons per year. Ordinarily they would take up between 177 million and 623 million metric tons. By comparison, a 2011 study from the U.S. Forest Service estimated the global sink from forests is between 2 billion and 2.8 billion metric tons per year.
A lengthy drought will cause a big dieback of the evergreen forests that are familiar to hikers and skiers, bringing in vegetation that will likely more resemble a desert scrubland.
Climate change is the likely culprit for such a long drought, or "megadrought" that lasts for decades, say the researchers. As the climate warms, many areas that were dry become drier, and some that were wet become wetter.
“If people don't cut back emissions or mitigate the die-off somehow, the result will probably be an increase in the rate of carbon-dioxide accumulation in the atmosphere, leading to greater warming”, Schwalm told.
Schwalm and his team used several sources of data to get their estimates, such as Fluxnet, a network of sensors run by the Oak Ridge National Laboratory; the Center for Climatic Research at the University of Delaware; and the U.S. Department of Agriculture. Schwalm found that this most recent drought of 2000-2004 was as bad as any since about the year 1200.
Even that wouldn't be so bad for the forest, but he noted that it's important that the kind of forests that exist change after each of these drought cycles. The evergreen species we see now in the four corners region are probably different from those that were there 1,500 years ago. There will be some adaptation on the part of the plants.
The study was published in the journal Nature Geoscience.