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.