VEGETATION on Earth could become saturated with carbon by the end of the century and stop acting as a brake on global warming, say scientists.
A four degrees celsius rise in global temperature, predicted by 2100, marks the threshold point after which terrestrial trees and plants will be unable to soak up any more carbon from the atmosphere.
Atmospheric carbon will then start to increase more rapidly, accelerating climate change, the researchers warn.
Vegetation acts as a "carbon sink", a natural system that takes carbon dioxide from the air and traps it. The Amazon rain forest and the vast belt of coniferous Boreal forest that rings the northern hemisphere both act as powerful carbon sinks.
Initially, higher levels of carbon dioxide in the atmosphere will stimulate more plant growth. But the impact of a warmer world will gradually counterbalance this trend until saturation point is reached, say the scientists.
Global warming of four degrees celsius will result in the Earth's land vegetation becoming dominated by negative effects, such as "moisture stress", caused by plant cells getting too little water.
At this point, further absorption of atmospheric carbon is likely to flatline, according to the research published in the journal Proceedings of the National Academy of Sciences.
Lead scientist Dr Andrew Friend, from Cambridge University, said: "Global vegetation contains large carbon reserves that are vulnerable to climate change, and so will determine future atmospheric carbon dioxide.
"The impacts of climate on vegetation will affect biodiversity and ecosystem status around the world."
The study used seven different computer models to simulate the effects of global warming on plant life.
"We use data to work out the mathematics of how the plant grows - how it photosynthesises, takes up carbon and nitrogen, competes with other plants, and is affected by soil nutrients and water - and we do this for different vegetation types," said Dr Friend.
"The whole of the land surface is understood in 2,500 square kilometre portions. We then input real climate data up to the present and look at what might happen every 30 minutes right up until 2099."
He pointed out that in heat waves, ecosystems can emit more carbon dioxide than they absorb.
This occurred during the 2003 European heat wave when temperatures rose six degrees above average.
"The amount of CO2 (carbon dioxide) produced was sufficient to reverse the effect of four years of net ecosystem carbon sequestration," Dr Friend added.
