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Massive CO2 sink down under

Carbon sponge Australian savannah was largely responsible for a massive spike in the amount of CO2 absorbed by land plants in 2011, new research has found.

The findings suggest that semi-arid areas could become key carbon sinks in the future, says Dr Pep Canadell of the Global Carbon Project and CSIRO.

But, he adds, because these areas hold carbon for a relatively short term compared to tropical forests, their implications for future climate change needs to be studied carefully.

Each year, land plants and the ocean absorb about half of the 10 billion tonnes of carbon emitted into the atmosphere by human activity.

On average, says Canadell, carbon sinks on land absorb around 2.6 billion tonnes of carbon, but in 2011 this figure spiked to about 4.1 billion, accompanied by a big drop in atmospheric CO2.

"The land had removed more anthropogenic CO2 than ever before recorded," he says.

Changing sinks

Traditionally, scientists believe that tropical forests like the Amazon are responsible for the bulk of the planet's yearly variability in atmospheric carbon.

"The exchanges are so big that if the Amazon gets upset because there's no rain or because it's too hot, it has a global effect."

But, Canadell and colleagues' study, published today in Nature suggests that this may be changing.

Their data shows the 2011 carbon sink anomaly was due to above average plant growth in the semi-arid areas of South America, Australia and Africa.

"We saw this incredible carbon sink in the southern hemisphere," says Canadell. "The semi-arid regions were playing the biggest role and particularly the grassy component."

"We never thought savannahs of the world could potentially have this effect."

Even more surprising, he says, was that 60 per cent of the extra plant growth was in Australia's semi-arid areas, north of Alice Springs.

Canadell says the massive semi-arid sink down under was apparently formed following the 2010-2011 La Niña.

It drenched the southern hemisphere and Australia got the lion's share of rainfall resulting in severe flooding in Queensland.

And the Australian savannah, parched from years of drought, soaked up the rain and turned it into new plant growth, says Canadell.

Canadell and colleagues relied on several independent lines of evidence for their findings, including satellites that use passive microwaves to detect the water content of plants.

The researchers also used models that link carbon sinks with atmospheric CO2 and fossil fuel emissions, and models that link vegetation growth with rainfall, temperature and solar radiation.

Canadell says it will be important to incorporate semi-arid carbon sinks in future global major climate models.

Short-term sink

While accumulating CO2 in semi-arid areas may sound like a good thing, Canadell warns that it is important to consider the likely short term nature of some of these sinks.

The IPCC predicts global warming will lead to more climatic extremes in semi-arid areas, including more extremely hot days, severe droughts and heavy rainfall.

On the one hand this could see greater plant growth in semi-arid areas, especially combined with the CO2 fertilisation effect, but the massive amounts of CO2 that could be quickly absorbed by this could just as rapidly be released.

Once the La Niña-boosted savannah growth in northern Australia dried out, the country saw fires near Alice Springs in 2012, says Canadell.

"Both drought and fire are the perfect vehicle to get the carbon back into the atmosphere very quickly."

Canadell says satellite data shows that overall Australia now has 6 per cent more plant cover than it had 30 years ago, but this has been mainly in semi-arid regions.

He says it will be important to carefully assess the risk of any plans to rely on semi-arid areas for carbon mitigation.

"The move of more carbon into semi-arid regions could be a liability to the future carbon cycle. It may come back to bite us later on," says Canadell.