Australasian Science: Australia's authority on science since 1938

Deserts Are Turning Green

By Stephen Luntz

Arguments that the world will benefit from increased plant growth arising from increased atmospheric carbon confirmed by an 11% increase in foliage cover from 1982–2010 in arid regions of Australia, North America and Africa.

Opponents of action to reduce carbon emissions have argued that the world will benefit from increased plant growth arising from increased atmospheric carbon. For the first time this has been quantified, with Dr Randall Donahue of CSIRO Land and Water revealing an 11% increase in foliage cover from 1982–2010 in arid regions of Australia, North America and Africa.

“In Australia, our native vegetation is superbly adapted to surviving in arid environments, and it consequently uses water very efficiently,” Donohue says. “Australian vegetation seems quite sensitive to CO2 fertilisation.”

Raised atmospheric carbon dioxide enables a leaf to grab more carbon for the same loss of water. “Fertilisation should be affecting plants across all environments, but doing so in different ways,” Donahue says. “We expect it

to affect foliage cover most in dry places. In places that are not water-limited, CO2 fertilisation is likely to affect photosynthesis rates per leaf area, which is impossible to detect by satellite and hard in other ways.”

Donahue is controlling for changes in rainfall. The many semi-desert areas that have become drier in recent times may actually have experienced a fall in foliage cover if loss of rainfall as a result of climate change outweighed the fertilisation effect. “We’re not saying which is larger,” says Donahue.

According to Donahue, “no one has done the numbers” to determine how much additional carbon is stored by the extra growth, since more storage occurs in the soil than the plants. However, he guesses that “the quantities involved would be very small compared to the increase in global atmospheric carbon”.

Donahue’s research sheds no light on the sort of plants that are benefiting, which he says is crucial to understanding the ecosystem impacts. “If it gives a boost to natural pastures it will make cows happy, but it could give a boost to trees overtaking grasslands. In some circumstances it could cause vegetation to use less water overall, so stream flows should increase, but the opposite can happen where trees use more water and stream flows decline.”