US forests could become a "substantial carbon source" by 2070, suggesting that forests could worsen global warming instead of easing it, a new Agriculture Department report says.

US forests currently absorb 11 percent of US carbon emissions (释放), equal to the combined emissions from 40 coal power plants. The report predicts that the ability of forests to absorb carbon will start declining after 2025 and that forests could emit up to 100 million metric tons of carbon a year as their emissions from decaying (腐烂) trees go beyond their carbon absorption.

The prediction suggests that this will require the US to cut emissions more rapidly to reach net zero, according to Lynn Riley, a senior manager of climate science. "Eleven percent of our domestic emissions. That is a really significant portion," Riley said. "As we work to decarbonize... forests are one of the greatest tools available. If we were to lose that, it means the US will contribute that much more" in emissions. The report also assesses and predicts the extent of renewable resources provided by the nation's forests and undeveloped landscapes, including farmlands, wetlands and grasslands.

According to the report, the loss of carbon absorption is driven in part by natural disasters which are increasing in frequency and strength as global temperatures rise. Development in forested areas, which the report predicts will continue to increase, is having the same effect as people increasingly move to the so-called wildland urban areas. Aging forests also contribute as older, mature trees absorb less carbon than younger trees of the same species, and the US forests are rapidly aging.

More aggressive forest management can help by cutting down a small portion of aging forests to make ways for younger trees that absorb more carbon. A thorough study of each forest should be done before removing older trees, Riley said.