Many scientists had hoped that serious forest damage would not set in before the middle of the 21st century, and that people would have time to get emissions of heat-trapping gases under control before then. Some of them have been shocked in recent years by what they are seeing.

“The amount of area burning now in Siberia is just startling — individual years with 30 million acres burned,” Dr. Swetnam said, describing an area the size of Pennsylvania. “The big fires that are occurring in the American Southwest are extraordinary in terms of their severity, on time scales of thousands of years. If we were to continue at this rate through the century, you’re looking at the loss of at least half the forest landscape of the Southwest.”

The Carbon Dioxide Mystery

In the 1950s, when a scientist named Charles David Keeling first obtained accurate measurements of carbon dioxide in the atmosphere, a mystery presented itself. Only about half the carbon that people were releasing into the sky seemed to be staying there. It took scientists decades to figure out where the rest was going. The most comprehensive estimates on the role of forests were published only a few weeks ago by an international team of scientists.

As best researchers can tell, the oceans are taking up about a quarter of the carbon emissions arising from human activities. That is causing the sea to become more acidic and is expected to damage marine life over the long run, perhaps catastrophically. But the chemistry is at least somewhat predictable, and scientists are reasonably confident the oceans will continue absorbing carbon for many decades.

Trees are taking up a similar amount of carbon, but whether this will continue is much less certain, as the recent forest damage illustrates.

Carbon dioxide is an essential part of the cycle of life on Earth, but geologic history suggests that too much can cause the climate to warm sharply. With enough time, the chemical cycles operating on the planet have a tendency to bury excess carbon.

In the 19th century, humans discovered the usefulness of some forms of buried carbon — coal, oil and natural gas — as a source of energy, and have been perturbing the natural order ever since. About 10 billion tons of carbon are pouring into the atmosphere every year from the combustion of fossil fuels and the destruction of forests.