Experts are scrambling to understand how forest losses from insects, wildfire and droughts might affect climate and the future habitability of the Earth. Scientists have found that forests have been absorbing more than a quarter of the carbon dioxide that people are putting into the air by burning fossil fuels and other activities. There is increasing worry that if forests were to die on a sufficient scale, they would not only stop absorbing carbon dioxide, they might also start to burn up or decay at such a rate that they would spew huge amounts of the gas back into the air — as is already happening in some regions.