Rising CO₂ levels are making deserts greener

Satellite observations from the past three decades have revealed that Earth’s deserts are getting greener. A collaboration between CSIRO and the Australian National University (ANU) has found that this increase in desert foliage is linked to an increase in atmospheric carbon dioxide (CO₂) through a process called CO₂ fertilisation.

CO₂ fertilisation is a process whereby plants grown under dry conditions are more efficient at using water under higher CO₂ levels than lower ones, as shown in greenhouse experiments. The elevated CO₂ enables a photosynthesising leaf to either extract more carbon from the air, lose less water to the air, or both. Thus, if elevated CO₂ causes the water use of individual leaves to drop, plants in arid environments will increase their total number of leaves.

According to CSIRO research scientist Dr Randall Donohue, “While a CO₂ effect on foliage response has long been speculated, until now it has been difficult to demonstrate.” But the research collaboration confirmed that CO₂ fertilisation correlated with an 11% increase in foliage cover from 1982-2010 across parts of the arid areas studied in Australia, North America, the Middle East and Africa.

Satellite data shows the per cent amount that foliage cover has changed around the world from 1982 to 2010.

“Our work was able to tease out the CO₂ fertilisation effect by using mathematical modelling together with satellite data adjusted to take out the observed effects of other influences such as precipitation, air temperature, the amount of light and land-use changes,” said Dr Donohue.

He said the effect on Australian vegetation was particularly prominent, with its strong ability to survive in arid environments making it particularly sensitive to CO₂ fertilisation. And although some deserts have actually decreased in levels of cover, “they are greener than they would otherwise be if CO₂ had remained constant.”

Dr Donohue said it has been suggested that increased foliage could be beneficial for forestry and agriculture in dry country. However, he stressed that there will also be “secondary effects that are likely to influence water availability, the carbon cycle, fire regimes and biodiversity;” for example, “possible decreases in surface water availability or the encroachment of woody vegetation into native pastures.” He also said the increase in vegetation is not enough to significantly offset the rise of carbon in the atmosphere.

Dr Donohue said research into the effects of elevated CO₂ in natural landscapes is an emerging area; thus, further research will be required to fully comprehend the consequences of the greening.

The study was published in the journal Geophysical Research Letters.