As human activity continually pumps carbon dioxide into the atmosphere, the Earth is doing its damnedest to stash it all away, in the oceans and forests. But new research led by the University of Cambridge has found that forests may not be as useful a carbon sink as we thought. The team found that a warming climate makes trees grow faster and die younger, releasing their captured carbon back into the air sooner.

One of the most common arguments against the dangers of climate change is that more CO2 in the air is good for plants, which capture the gas and store it in their cells. That's true to an extent – young trees have been found to grow faster as temperatures warm up and there's more carbon dioxide in the air. It's been thought that greening the Earth with more trees could help offset some of the increases in CO2 emissions, but the new study shows that this might not work out as well as we'd have hoped.

"As the planet warms, it causes plants to grow faster, so the thinking is that planting more trees will lead to more carbon getting removed from the atmosphere," says Ulf Büntgen, lead author of the study. "But that's only half of the story. The other half is one that hasn't been considered: that these fast-growing trees are holding carbon for shorter periods of time."

To investigate, the researchers examined samples from 1,768 trees, both living and dead, from different forests. That includes over 1,100 mountain pines from the Spanish Pyrenees and 660 Siberian larch from the Russian Altai. Core samples were taken from the living trees, while whole discs could be obtained from dead ones, giving the researchers a window into how fast trees grew and how long they lived under different climate conditions over the past 2,000 years.

The team found trees grew slower under cold conditions, but that made them stronger, which in turn let them live much longer. On the other hand, trees that grew faster during their first 25 years of life tended to die much sooner. This association was found to hold true for trees both living and dead in both areas.

"We wanted to test the 'live fast, die young' hypothesis, and we've found that for trees in cold climates, it appears to be true," says Büntgen. "We're challenging some long-held assumptions in this area, which have implications for large-scale carbon cycle dynamics."

If this does turn out to be the case, it could add another feedback loop to a warming world that's already a complex network of vicious cycles. More CO2 in the air would mean forests grow faster and die younger, releasing their stored carbon back into the atmosphere to further warm the planet and make forests grow even faster still.

The research was published in the journal Nature Communications.