The ability of the Amazon forest to soak up excess carbon dioxide is weakening over time, researchers reported last week. That finding suggests that limiting climate change could be more difficult than expected.

For decades, Earth’s forests and seas have been soaking up roughly half of the carbon pollution that people are pumping into the atmosphere. That has limited the planetary warming that would otherwise result from those emissions.

The forests and oceans have largely kept up even as emissions have skyrocketed. That surprised many scientists, but also prompted warnings that such a robust “carbon sink” could not be counted on to last forever.

In a vast study spanning 30 years and covering 189,000 trees distributed across 321 plots in the Amazon basin, researchers led by a group at the University of Leeds, in Britain, reported that the uptake of carbon dioxide in the Amazon peaked in the 1990s, at about 2 billion tons a year, and has since fallen by half.

Initially, the researchers postulated, the Amazon may have responded well to rising carbon dioxide levels, which are known to increase plant growth, but that response appears to be tapering off. Drought and other stresses could be playing a role, but the main factor seems to be that the initial acceleration of growth sped up the metabolism of the trees.

“With time, the growth stimulation feeds through the system, causing trees to live faster, and so die younger,” Oliver L. Phillips, a tropical ecologist at the University of Leeds and one of the leaders of the research, said in a statement.

Further research is needed, but the scientists say that climate forecasting models that assume a continuing, robust carbon sink in the Amazon could be overly optimistic.

At a global scale, studies suggest that forests are still absorbing far more carbon than they release into the atmosphere, even as stresses like fires and beetle attacks increase because of climate change. In essence, rising forces of growth have been outracing rising forces of death in the world’s forests.

Perhaps the big question now is whether that will flip. Will forests beyond the Amazon, such as the boreal forest that encircles the Northern Hemisphere, eventually follow the Amazon and weaken as carbon sinks?

That would mean, in effect, that human civilization would have less help from trees, and cuts in carbon emissions would need to be sharper than previously thought to limit global warming to tolerable levels.

“Forests are doing us a huge favor, but we can’t rely on them to solve the carbon problem,” Dr. Phillips said. “Instead, deeper cuts in emissions will be required to stabilize our climate.”

washingtonpost