Source: Ukkola, A. M., et al. October 19, 2015 Reduced streamflow in water-stressed climates consistent with CO2 effects on vegetation. Nature Climate Change 6, 75–78 (2016)

The carbon dioxide we’ve been pumping into the atmosphere is fertilizing plants and making them grow faster – but now those plants are sucking our streams dry.

Australia is already a parched country and will only become drier as the planet warms and rainfall decreases.

But now it turns out that Australia has lost about a quarter of its stream flow over the past 30 years as plants given an extra boost by our carbon emissions grow faster and use more water.

How that extra growth would affect water uptake has been a matter of debate. That’s because the extra CO2 has two opposing effects, says Anna Ukkola from Macquarie University in Sydney, Australia.

Plants have a waxy seal over their leaves that stops them from losing too much water to the air. To get access to CO2 in the air, which they need to photosynthesize, they have to open little pores in that seal. But they also lose water: CO2 goes in, water out.

Since there is a lot more carbon in the air than there used to be, plants can close their pores partially and still get the same amount of CO2 while losing less water, says Ukkola.  Early models concluded that stream flow would increase. If plants lose less water, they reasoned, then there will be more in the streams.

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 experimental plots. “But in the experiments, the changes in water use is varied – it’s all over the shop,” says Randal Donohue from the CSIRO in Canberra, Australia.

Boost plant cover

Donohue and colleagues were the first to show in 2013 that increased carbon dioxide 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 change.

Using similar methodology, Ukkola and colleagues repeated that analysis for Australia, and then compared the carbon-driven greening in 190 river basins with the changes in stream flow over that time.

After allowing for other factors like changes in rainfall, they found that a significant drop in stream flow was associated with the greening of the landscape. In areas that were greening more, stream flow was also diminishing more.

Overall, the CO2-induced greening was responsible for a stream flow reduction of between 24 and 28%, says Ukkola.

“The plants are growing more and bigger leaves,” he says, which evaporate even more water.

Ukkola says the results can probably be extrapolated to places with similar climates to Australia like the Mediterranean.