https://www.newscientist.com/article/mg ... bon-sinks/THE carbon dioxide we’ve been pumping into the atmosphere is fertilising plants, making them grow faster – but now those plants are sucking our streams dry.
Australia is already parched and will only become dryer as the planet warms and rainfall decreases. On top of this, the country has lost about a quarter of its streamflow over the past 30 years, as plants given an extra boost by our carbon emissions are growing faster and slurping more water.
A similar effect could be anticipated in other semi-arid subtropical regions in the Mediterranean, southern Africa and the Americas, where rainfall is also expected to decline as global temperatures increase.
Global greening is happening for a number of reasons. Most obviously, plants are able to grow in places that were previously too cold, for example, in the Arctic, which is warming faster than anywhere else. But extra CO2 is fertilising plants elsewhere, making them grow faster, especially in dry regions.
How that extra growth would affect water uptake has not been clear. That’s because the extra CO2 has two opposing effects, says Anna Ukkola at Macquarie University in Sydney, Australia.
Plants have a waxy seal over their leaves that stops them losing too much water to the air. To get access to the CO2 they need to photosynthesise, plants must open little pores in that seal. But as the CO2 goes in, water moves out.
Since there is a lot more carbon in the air than there used to be, plants can partially close their pores and still get the same amount of CO2 while retaining more water, says Ukkola. So early models concluded that if plants lose less water, then there should be more of it in the streams, so streamflow should increase.
But later models disagreed, showing that it depends on exactly how the plants’ growth is affected: if they become more leafy, then they will lose more water to the air.
Researchers have tried to sort this out by growing test plots. “But in the experiments, the changes in water use is varied – it’s all over the shop,” says Randal Donohue from the government’s research facility, the CSIRO, in Canberra, Australia. Donohue’s team was the first to show in 2013 that increased carbon levels were boosting plant cover around the world, by examining satellite images and removing the effects of other factors such as changes in rainfall and land use.
Using remote sensing data, Ukkola and colleagues have now repeated that analysis for Australia, and then compared the carbon-driven greening in 190 river basins with the changes in streamflow over that time. They found that greening of the landscape was associated with a significant drop in streamflow. In areas where the greening was greater, streamflow was also diminishing more (Nature Climate Change, doi.org/8jd).
Overall, the CO2-induced greening reduced streamflow between 24 and 28 per cent, says Ukkola. “That’s quite worrying for areas that are already water-scarce around Australia,” she says.
This paper shows that the increase in leafiness is more significant than the efficiency gain, says Andy Pitman at the University of New South Wales in Sydney. “Nobody has been able to do that before.” The plants are growing more and bigger leaves, he says, which evaporate even more water.
Ukkola says the results can probably be extrapolated to areas with similar climates to Australia like the Mediterranean. But to extend the conclusions to other places, the study would need to be repeated. That could be tricky, since you need to find river basins where land use hasn’t changed significantly – an easier task in relatively sparsely populated Australia, Ukkola says.
It’s unclear what will happen in the future, since plants might get saturated with CO2 at some point, and stop benefiting from even higher levels, says Ukkola. “But it’s worrying that precipitation is projected to decline in Australia anyway,” she says.
(I'm taking the unusual-for-me step of replacing the web headline with the print headline, as I have literally no idea what the web headline is on about).
