The ability of plants to counteract rising carbon dioxide emissions has declined over the past few decades, a study published Friday in the academic journal Science concludes.

A higher concentration of carbon dioxide in the atmosphere causes plants to absorb more carbon — the so-called carbon dioxide fertilization effect, responsible for an estimated 70% of the accelerated plant growth observed by researchers. Through this process, plants help mitigate human-made climate change.

But according to the new study, led by researchers at Nanjing University in eastern China, the carbon dioxide fertilization effect has seen a “robust decrease” in strength worldwide between 1982 and 2015.

From satellite data and on-the-ground observations, the international team, which included scientists from France and Spain, found that vegetation has become less sensitive to the increase of atmospheric carbon dioxide. The decline is more evident in plants growing in cold climates than those found in tropical areas.

It’s unclear why this happens. But the study hypothesizes that a lack of water and soil nutrients — two factors that previous research showed can constrain the carbon fertilization effect — might also limit plants’ ability to photosynthesize, the mechanism by which they convert carbon dioxide to oxygen.

The findings present yet another challenge to mitigating climate change, but they don’t necessarily mean that the Earth’s land-based ecosystems are sequestering less carbon. According to the U.N.’s Intergovernmental Panel on Climate Change, an estimated 30% of greenhouse gases are absorbed by land-based ecosystems, mostly through photosynthesis.

To Richard Houghton, an ecologist and senior scientist at the Woodwell Climate Research Center in Massachusetts, the new findings emphasize the need for researchers to better understand the different ways in which land acts as a carbon sink. Carbon dioxide fertilization is considered the Earth’s main carbon storage mechanism, “so it’s a bit surprising that it is declining,” he told Sixth Tone.

Houghton, who wasn’t involved in the Science study, said his earlier research using data from the Global Carbon Project, a Canberra-based organization that quantifies greenhouse gas emissions, shows that the carbon taken up by land and oceans is still increasing.

“Basically, there is still good news out there,” said Houghton. But he’s afraid that nature’s other carbon storage mechanisms, which are poorly understood, will also slow down under the effects of climate change. “I would think that as the Earth warms, in fact, those things would diminish and decline,” he said.

Editor: Kevin Schoenmakers.

(Header image: Alberto Agnoletto/People Visual)