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Author: L. Cormack
Source: The Age
Date: 20th October 2015

Increased atmospheric carbon dioxide is making plants in Australia’s semi-arid and subtropical regions bigger and greener. It’s also making them a hell of a lot thirstier.

It is a peculiar paradox that sees plants “greening” and growing better as a result of climate change, while water supply across Australia’s grazing and annual cropping land is suffering.

“What we are seeing is very much consistent with [the effect of] increasing carbon dioxide, the main ingredient of photosynthesis. So higher concentrations allow plants to grow bigger, thus they consume more water,” said lead author Dr Anna Ukkola​, who performed the research at the Department of Biological Sciences at Macquarie University.

She said while there has been research on the impact of increased atmospheric carbon dioxide in the last 10 years, this is the first attempt to show its effect based on satellite and stream flow observations.  

“The greening has been shown by other studies, but the effect on the water resources hasn’t really been shown from observations before,” said Dr Ukkola, who is now based at the ARC Centre of Excellence for Climate System Science.

Over the past 30 years Dr Ukkola estimates greening has led to a significant reduction in river stream flows, of around 24-28 per cent.

The year-long study found that while water resources in places like the Murray Darling Basin, inland Queensland and the wheat belt of south-western Western Australia will be reduced, the greening also had its benefits.

It revealed that less water was needed to produce the exact same amount of leafy vegetations, because of increased atmospheric carbon dioxide.

Co-author of the report Professor Albert Van Dijk​ from the Australian National University said there are a few ways in which the benefits could be revealed.

“If you can imagine I am a plant, if I can make more biomass I may be able to extend my roots deeper into the soil, I might be able to mine the soil water deeper,” he said, adding that the study did not look at this specifically, however this was one possible response.

“This confirms what we expected, that vegetation grows better under carbon dioxide. It also shows us it doesn’t translate into less water use, actually more water…which has big impacts when trying to work out future climate change scenarios.”

According to the study, satellite measurements suggested parts of Australia with sufficient water resources were showing an increase in vegetation.

Dr Ukkola said while increased production was “a boon for farmers where water resources are readily available,” it could have a negative impact on regions already experiencing water scarcity on a regular or semi-regular basis.

In order to determine how much rainfall is is needed to maintain a level of plant growth unrestricted by a lack of water, Dr Ukkola and her team came up with a “precipitation threshold.”

Above that threshold, there is no lack of water to restrict growth. Below that threshold, the lack of water impacts plant growth.

From 1982-2010 observation data shows increased atmospheric carbon dioxide made plant growth easier, thus the precipitation threshold has declined.

“Water is the liquid gold that powers our economy, agriculture and daily lives now and into the future. At its core this research shows we must focus on how to preserve our water resources as our world changes,” Dr Ukkola said.

Read the article at The Age