Nearly 40% of the Mid-Atlantic's forests lost tree canopy cover, ranging from 10% to 15% between 2000 and 2010, according to a NASA study released this week. Other afflicted areas include southern Appalachia, the southeastern coast and to a lesser extent, the Rocky Mountains and the Sierra Nevada.

"There has been a series of summers — growing seasons for trees — that have been deficient in moisture. When you combine that with higher temperatures, it's stressing the trees," says author Christopher Potter, a research scientist at NASA's Ames Research Center in Moffett Field, Calif.

This double whammy is making trees, especially southern pines and the upper Midwest's hardwoods, more vulnerable to insects and new pathogens. "No tree is safe," Potter says.

Climate change is increasing the risk of forest death through wildfires, insect infestations, drought, and disease outbreaks, according to a 1,000-plus-page draft of the third National Climate Assessment, released by the U.S. government in January. The NASA study was done as part of that assessment.

If more trees die, the planet warms more. Trees absorb heat-trapping carbon dioxide emissions and thus can reduce the effects of climate change. In 2010, they absorbed 13% of U.S. emissions, the Environmental Protection Agency says.

Potter's study is based on monthly images from a new series of NASA satellites, launched in 2000. The series' continuous monitoring provides a more detailed picture of changes in forests, wetlands and grasslands over extended periods of time. It shows that in the western parts of Alaska, higher temperatures have helped by expanding the growing season for trees.