What Ballooning Carbon Emissions Will Do to Trees

Apart from the experts, few people realize that climate change could be worse. Every year, trees, shrubs, and every other kind of plant absorb 9 billion tons of CO₂—one quarter of what we let loose from our tailpipes and smokestacks—and help slow the gas’s accumulation in the atmosphere. If not for the world’s photosynthesizers, the concentration of CO₂ in the air, along with Earth’s temperature, would be rising much faster than it already is.

Our terrestrial plants do us “a fantastic favor” sponging up all that CO₂, says Scott Denning, an atmospheric scientist at Colorado State University. That’s especially true in the tropics: By some estimates, the band of jungle that hugs the equator sucks up about half of the carbon absorbed on land. But in the coming years, tropical regions are projected to see steeply rising temperatures and, in some areas, increased drought. That will create less and less hospitable conditions for these crucial equatorial jungles. Nonetheless, published forecasts of the future of climate change, including the marquee results of the Intergovernmental Panel on Climate Change, take it for granted that the world’s forests will continue acting as a terrestrial carbon sink—an assumption that may be disastrously overoptimistic.

There is one slender thread of hope that tropical jungles will keep buffering the amount of CObuilding up in the air. Scientists have long known that plants grow better in CO-enriched air, and some hypothesize that steadily rising amounts of atmospheric carbon might protect forests’ health despite the otherwise deteriorating circumstances. Most likely, such carbon fertilization has already boosted the productivity of forests all over the world. But