Tropical forests become less effective as carbon sinks as the region warms, a new analysis of trends over the past 50 years has found.
“When they get warmer than average, forests in the tropics do not like it, and overall, they tend to put more CO2 into the atmosphere than they take out,” says Canadell.
Canadell says it is crucial to understand how natural CO2 sinks, such as tropical forests, will behave with a changing climate.
“A little more than 50 per cent of all anthropogenic carbon emissions are taken up by the oceans and vegetation on land — that’s 10 billion tonnes of CO2 per year,” he says.
“This is a big deal. It’s like a 50 per cent discount on climate change.”
Models predict that temperature and precipitation can affect CO2 uptake by plants, but Canadell and colleagues have now analysed actual data to explore the impact of such changes on tropical carbon sinks.
They looked at the relationship between fluctuations in global atmospheric CO2 between 1959 and 2011, and short-term fluctuations in temperatures in the tropics, independent from long-term warming trends.
Such ‘interannual variability’ is caused by El Niño, in which slowing trade winds lead to a build-up of heat in the tropics, and large volcanic eruptions that spew out Sun-shielding aerosol particles.
Using data from a global network that measures atmospheric carbon concentration, and changes in precipitation, they found that tropical temperatures were the main driver of global CO2 levels: the higher the temperatures, the higher global CO2 levels.
To investigate the cause of this, the researchers used a large model of the Earth’s systems that took weeks to run on a supercomputer.
This told them that at warmer temperatures photosynthesis of trees actually fell, while the respiration of microbes in the soil increased.
“You’re spewing more CO2 into the atmosphere from the soil and you’re taking less CO2 into the plants through photosynthesis.”
While this study focused on short-term natural climate variability, it will also help scientists design better models to predict how human influence may impact long-term climate trends because the same processes are involved over this time scale, says Canadell.
Such models will also have to take into account the ‘CO2 fertilisation effect’, in which increasing CO2 increases CO2 uptake by plants.
“What’s going to be the net effect into the future is a big question. We address one of the two questions,” says Canadell.
Read article at the ABC