A new study warns photosynthesis could slow with global warning, with forests and plants switching from absorbing carbon to releasing it.
Professor Louis Schipper is a soil scientist and professor at University of Waikato and his research, in collaboration with Northern Arizona University, looked at the effect of heat on a plant’s ability to absorb CO2.
Analysing data sets from all over the world the research found that at 18 degrees air temperature, plants started to absorb less CO2.
Snaking vines Whirinaki Forest Photo: Kennedy Warne
However, the release of carbon, or respiration, was relatively unaffected by heat.
“Constantly during the day, they are drawing down CO2 out of the atmosphere, and there is a process that converts the captured carbon back to CO2 called respiration.
“So, we’ve got these two enormous processes going on in all of our forests around the world.
“Over the last several decades those processes have fixed carbon dioxide out of the atmosphere taking up about 30 percent of the CO2 that we put into the atmosphere through fossil fuels.”
This “invisible service” has been keeping things in balance but the research threw up worrying findings, he says.
“What it showed was photosynthesis has a temperature optimum for most plants of around about 18 degrees
“But the key point is also that respiration which is the conversion of that fixed carbon back to CO2 doesn’t seem to be constrained nearly as much by temperature.
“So, it continues to increase as temperature goes well beyond 18 degrees.”
What that means, when the temperature is above 18 degrees, is the rate of carbon sinking declines, he says.
“So, you are starting to spend more than you save. You’re still saving a bit, converting carbon into the atmosphere and storing it into biomass and starting it into soils, but the rate of saving is starting to decline, the service those eco-systems have been providing to us is starting to go away.”
This will then become a feedback process, he says.
“And the thing that really surprised me, when I first saw this data and we started extrapolating this around the world spatially, was how quickly this was going to happen.”
The data are suggesting this feedback loop could happen in the 20 to 30 years, he says
“If we go down a particular track of CO2 emissions and global warming we could see a halving of that sink of CO2.”
It is still vital, nevertheless, that we plant trees, he says.
“We need to do everything that we can to reduce the amount of greenhouse gases into the atmosphere so that is about reducing fossil fuel and planting more trees and looking after the trees we have, but also making sure we put the right trees in the right place.”
