America's forests seem likely to scrub much less carbon dioxide out of the atmosphere in the future compared to the last few decades, according to a government report submitted last week to the United Nations.
Although the timing and extent of the shift is hard to pin down, the expected change could make it harder for the United States to meet its commitments to control CO2, the principal greenhouse gas that is warming the planet.
In recent years, the nation's forests have been growing. The density of their trees has increased as growth exceeded harvests, and there have been small annual increases in the area of forested land. But the beneficial trends are expected to slow, and ultimately to reverse, the report warned.
"In the long term, U.S. forest carbon stocks are likely to accumulate at a slower rate, and eventually may decline as a result of forestland conversion and changes in growth related to climate change and other disturbances," the report said.
"U.S. forests are unlikely to continue historical trends of sequestering additional carbon stocks in the future under current policy conditions," it added.
"These changes may already be starting," the report warned, although it will take years to collect the data to know for sure.
In a cover letter, Secretary of State John Kerry said the costs of inaction on climate change are "beyond anything that anyone with conscience or common sense should be willing to contemplate."
The report's overall conclusion was that the United States was "on track" to meet its 2020 goal for reducing CO2 and other greenhouse gases, but that it would not be easy.
The prediction about forest sequestration was just one element in a comprehensive report submitted formally on Jan. 1 by the State Department after several months of public comment on an earlier draft.
It cited several recent scientific papers addressing the complex interactions between land use and the climate, as well as data collected by the federal Forest Service.
It is well understood that trees breathe in CO2 through their leaves and store the carbon in their wood, a long-lasting form of sequestration that can be a significant factor in buffering greenhouse gas concentrations in the atmosphere. Forests account for the vast majority of carbon sequestration in U.S. crops and soil.
Forest sequestration grew from 431 million tons of CO2 removed from the air in 2000 to 800 million tons in 2005. It amounted to 758 million in 2010 and 762 million in 2011. The tally includes trees, roots, dead wood, leaves and other litter, and forest soil.
But with more pressure to convert forest lands to agricultural or urban uses, the study says this "carbon sink" is expected to start shrinking, perhaps to 623 million tons next year, to 445 million in 2020, and to less than 400 million in later years.
In recent years, overall U.S. emissions of CO2 have exceeded 5.5 billion tons a year. The U.S. has committed to reducing its emissions 17 percent below the 2005 levels by the year 2020, with much deeper reductions to follow in later decades. Since sequestration in forests and other land uses can be viewed as a "credit" in this kind of accounting, losing ground in the forests can make a real difference in whether these challenging goals can be reached.