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Australia’s Role in the 2011 Global Carbon Sink Anomaly

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Vegetation in the Southern Hemisphere was responsible for most of the 2011 carbon sink anomaly, with Australia’s arid zones accounting for about 60% of the dramatic increase in carbon uptake.

By Derek Eamus & James Cleverly

How did Australia’s vegetation cause a sudden and massive increase in uptake of atmospheric CO2 in 2011, and why did sea levels fall in the same year?

Although atmospheric CO2 concentrations are trending up at alarming rates, having risen by 80 ppm in the past 50 years, from year to year there is significant variation in the rate of this increase.

Much of the variability arises because the strength of terrestrial vegetation as a CO2 source or sink varies between years. Terrestrial vegetation can either be a net sink for carbon, when carbon uptake through photosynthesis exceeds loss of carbon through respiration, or a net source when respiration exceeds photosynthesis. Australia’s vegetation can be either a sink when biomass increases, such as the planting of pine forest or a crop of wheat, or a source during drought or bushfires.

In 2011 there was a large global anomaly in the carbon sink strength due to a dramatic increase in global uptake of CO2 from the atmosphere. The size of the sink in 2011 was estimated to be 54% higher than the preceding decade – equivalent to more than 40% of global emissions from the burning of fossil fuels. This is a lot of carbon suddenly absorbed by vegetation. To put this into context, the global increase was more than nine times Australia’s annual carbon emissions.

A surprising finding published in Nature last year (http://tinyurl.com/laza7b9) was that vegetation in the Southern Hemisphere was responsible for most of the 2011 anomaly. This is remarkable because the area of land in the Northern Hemisphere is about double that of the Southern Hemisphere.

What was even more surprising was that semi-arid regions accounted for the increase in carbon uptake. No one has ever predicted such an important role for arid zone vegetation because, by definition, arid zones have very little water for photosynthesis to occur at high rates. When there is too little water, stomatal pores in the leaves close and shut off photosynthesis.

Furthermore, high rainfall regions contain dense forests that have a very large leaf area above each square metre of ground. In contrast, arid regions have sparse canopies and a small leaf area.

The final surprise was that Australia’s arid zones accounted for about 60% of the dramatic increase in carbon uptake.

What Causes Increased Sink Strength?

Photosynthesis fixes carbon into sugars, respiration releases CO2, and the balance between these two processes is influenced by temperature, the availability of water in the soil, the amount of sunlight, and several other factors. Droughts reduce the strength of the terrestrial sink while wetter-than-average years increase the sink’s strength.

In 2010–11, Australia was in the grip of the largest La Niña period in 80 years. During La Niña years, eastern and central Australia receive increased rainfall, and in 2010 and 2011 Australia received about 55% more rain than the long-term average of 450 mm pa. In arid regions, such large increases in rainfall result in very large increases in vegetation cover, and total photosynthetic carbon uptake by the landscape increases. Consequently, in 2011 the sink strength of Australia’s landscapes increased and there was a huge gulp of atmospheric CO2.

Fortunately, our research team had an eddy covariance tower located in central Australia in 2011. Eddy covariance towers measure the rate of CO2 and water vapour fluxes above vegetation 10 times per second, all day, every day. From these measurements we were able to confirm that Mulga woodlands (Acacia species) did indeed soak up a lot of CO2 and were a very large sink during that year. Rainfall in central Australia was approximately double the long-term average, and net ecosystem productivity increased from values that were close to zero in 2010 to 259 grams of carbon fixed for every square metre of land per year in 2011 – a massive increase.

How Is This Linked to Sea Levels?

Global mean sea levels (GMSL) have been increasing by about 3 mm/year for the past 20 years, but something strange happened in 2011. In that year, GMSL fell by about 6 mm (Fig. 1).

Much of the variation in the Earth’s gravity field occurs because of changes in the Earth’s water cycle. Satellite data gathered by NASA’s Gravity Recovery and Climate Experiment (GRACE) revealed that there was a very large increase in water stored across Australia as well as in south-western Africa and north-eastern South America. The heavy rainfall associated with the strong La Niña effectively transported vast amounts of water from the oceans to the land surface, and this caused the drop in GMSL. The GRACE satellites estimated that about 1.8 trillion tonnes of water were lost from the oceans.

Australia has a unique surface hydrology. It has massive basins that lack drainage via rivers to oceans. Consequently, water remains within the basin and, if it is not evaporated directly back to the atmosphere from the soil surface, this water can be taken up by roots to support increased productivity, even in arid regions.

Between June 2010 and February 2011, much of Australia experienced an increase in water storage mass of about 100 kg/m2, or an increase of about 10 cm of water per square metre of land.

The importance of Australia to both the carbon sink anomaly and the GMSL anomaly of 2011 was profound. While north-eastern South America (the Amazon) also experienced increased rainfall, the surface hydrology of South America favours increased river run-off rather than increased storage. As a result, the increase in productivity of the vegetation was smaller there than we observed in central Australia.

How Long Did the Increase in Sink Strength Last in Australia?

The drop in GMSL in 2011 was short-lived, and now GMSL is back on-trend. But what of arid zone vegetation in central Australia? Did they spend all that extra water in just a year or did some carry over into subsequent years?

Rainfall in 2012–13 was about half of the long-term average for our eddy covariance site in central Australia. Net ecosystem productivity became substantially negative (–25 g C/m2/year): it lost more carbon than it gained.

However, these arid systems are highly variable in the amount of carbon they fix each year. An increase in rainfall of only 104 mm, from 191 mm to 295 mm (still less than the long-term average of 314 mm) between 2012–13 and 2013–14 turned these Mulga woodlands from a net carbon source to a net carbon sink.

Who would ever have thought that global changes in the strength of the terrestrial carbon sink and global mean sea level changes could be linked, and that both of these were directly influenced by Mulga woodlands in central Australia?

The challenge for future research lies in establishing the relative importance of vegetation in semi-arid and arid regions to the global carbon and water cycles in wet, dry and average rainfall years. Such regions are often located in countries with limited research infrastructure. Australia, being the driest of all forested continents but with a significant capacity for research, should be at the forefront of addressing this question.

Derek Eamus is Professor of Environmental Sciences at the University of Technology Sydney. James Cleverly is a Research Fellow in the Terrestrial Ecohydrology Research Group at UTS, and Associate Director of the TERN OZflux network of eddy covariance towers located across Australia and New Zealand.