Australasian Science: Australia's authority on science since 1938

Greener but Drier

Satellite observations published in Nature Climate Change have revealed that plants in Australia’s semi-arid and subtropical regions are becoming greener because of increased atmospheric carbon dioxide. However, the vigorous growth of plants requires more water, so less run-off now flows into many of Australia’s river basins.

“To compound matters, many important regions are projected to experience future declines in rainfall as a result of climate change,” said lead author Dr Anna Ukkola, who performed the research at Macquarie University but is now based at the ARC Centre of Excellence for Climate System Science.

“This means water resources for agriculture and environmental flows in places like the Murray-Darling Basin, inland Queensland and south-western Western Australia’s wheatbelt will be reduced even further.”

But there was also a positive side to the greening. Because plants use water more efficiently where there is more carbon dioxide, less water is required to produce the exact same amount of leafy vegetation under high CO2 conditions.

The satellite measurements indicated that areas of Australia with sufficient water resources were indeed showing an increase in vegetation. “This increased production is a boon for farmers where water resources are readily available, but the problem is for those regions that already experience water scarcity on a regular or semi-regular basis and where farmers rely on stream flow for irrigation needs,” Ukkola said.

As droughts regularly show us, once rainfall drops below a minimum level, plant growth slows and, if water scarcity is severe enough, they die. “The role of vegetation makes predicting future water resources difficult at best,” said co-author Dr Trevor Keenan. “We are, however, getting a better picture of likely possible futures for vegetation, water resources, and the impact of climate change on society in Australia.”

As part of gaining a clearer understanding, the researchers quantified the “precipitation threshold” at which there is enough rainfall to maintain a level of plant growth that is not restricted by a lack of water. Interestingly, observations from 1982–2010 show that as atmospheric carbon dioxide has increased over Australia and made plant growth easier, the precipitation threshold has declined.

During this time an average annual rainfall of around 900 mm was required for plants to grow without being restricted by a lack of water. However, the average amount dropped to around 750 mm/year from 2001–08.

“While there are clearly some positives for growth found by this research, it also shows us that some of our crucial agricultural areas will not see these benefits because of the future impacts of climate change on rainfall,” Ukkola said.