Worldwide, plants are believed to produce about 60 billion tons of CO2 each year, around six times the amount humans produce through fossil fuel burning. Why? Plants, like animals, need to respire. They take in CO2 from the air for photosynthesis and emit roughly half of that amount during respiration. Like animals, plants also breathe harder as it gets hotter and, until now, there had been fears among scientists that respiration increases exponentially as temperatures climb, outweighing photosynthesis. In other words, plants were exhaling more CO2 than they were inhaling.
‘With this new model, we predict that some ecosystems are releasing a lot less CO2 through leaf respiration than we previously thought,’ says Kevin Griffin, plant physiologist at Columbia University’s Lamont-Doherty Earth Observatory and co-author of a new study on plant responses to global warming.
The study suggests that this effect has its limits and that rates of increased respiration actually slow as temperatures climb. ‘What we thought was a steep curve in some places is actually a little gentler,’ says Griffin. The global study looked at the respiration of 231 plants species from biomes all over the world and found that the biggest change in estimates are in the coldest regions, which have seen the most relative warming. They found that respiration increase was 28 per cent lower than estimated at the North Slopes of Alaska, and 15 per cent lower than estimated in Black Rock Forest, New York.
While the findings can tell us a lot about carbon storage methods in plants, they might not completely rewrite climate models to predict less warming in the future. ‘We now have a better way to estimate one process, but it’s only one process,’ says Griffin. ‘The whole system is quite complicated, and a small change in the balance between one part and another could produce a really big result. That’s the challenge we face when we think about the Earth as a whole.’