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Climate change threshold in semi arid regions

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Dr Xuanlong Ma, Postdoctoral Fellow working in the Plant Functional Biology & Climate Change Cluster (C3) at the University of Technology Sydney, explains how remote sensing is being used to gather data about the impacts of climate change on Australia’s semi arid ecosystems. The global concentration of carbon dioxide in the atmosphere has increased from 280 parts per million (ppm) since the start of the Industrial Revolution to 397 PPM at this moment, representing a dramatic increase in this strong greenhouse-forcing agent. Plants play an important role in helping mitigate global warming, because they absorb on average a quarter of fossil fuel emission each year. Importantly, a recent study found that in 2011, more than half of the global land carbon sink anomaly was attributed to Australia’s semiarid ecosystems.

Drought frequency and drought severity in Southeastern Australia during the 2000-2013 time period. Various "hot-spots", affected by drought more than other areas during the early 21st-century, were identified, including almost all of southern Victoria and southwestern New South Wales. (Figure Source: Ma et al., 2015)    

Australia has an extremely variable climate compared to other continents. 2011 was the strongest La Niña year in past eight decades, meaning Australia had a very wet year and there was lots of flooding, a far cry from the big dry, a long-term drought that started in 2002 and extended into 2009. That is why he is working to study the impacts of such extreme climate variability on Australia’s ecosystems. Remote sensing can be used to objectively measure things on the land surface of an entire continent like vegetation development, surface temperature, precipitation, carbon dioxide concentration, soil moisture and ground water. Dr Ma’s recent study was focused on south-east Australia, an area significantly affected by the long-term drought during the early 21st Century (known as the “big dry"). It is also an important area for food production and home to the Murray-Darling Basin as well as being the most populated part of the country. The measurements looked at the development of vegetation in that area, tracking the phenology from the start of the year to the end of that year.

Map of global distribution of climatic zones. This maps is produced by Dr. Xuanlong Ma using aridity index data provided by CGIAR-CSI (link).     

In the northern hemisphere plants change when their growing season starts and finishes, a cycle that is primarily driven by seasonal variations in temperature and repeats. Australian plants have adapted to its environmental extremes, so do not follow that normal cycle as we expected based on our Northern Hemispheric experiences. During drought years the plants do almost nothing. It is so dry that plants do not take the risk to be active but when conditions do become favourable they flourish dramatically and thus compensate for poor growth during the dry period. Most Australian plants are also evergreen meaning the never shed their leaves. They do, however, have very deep root systems to access water from the deep soil. This makes the plants very drought resistant and resilient to its extreme climate. 


Australia’s unique native vegetation type Hummock grassland (Spinifex) in dry season (left) and wet season (right). Hummock grassland (Spinifex) covers ~20% of Australia’s dryland. These photos were taken at Ti Tree East OzFLUX tower site north of Alice Springs. Photo Credit: Dr. James Cleverly (UTS: School of Life Sciences).


On the other hand, agricultural plants are not native and being exotic are very vulnerable to drought and production is reduced dramatically. They also have shallow roots so they rely on rainfall or irrigation for growth. Having cleared a lot of native land for agriculture has made the semiarid ecosystems of south-east Australia the most vulnerable to drought and climate change. Semiarid areas will be most vulnerable in the future, especially from extreme climate impacts like mega droughts. Australia will never be free of drought based on historical meteorology records and climate models show that the potential for more frequent and severe droughts is very likely in the future. This raises concerns about how Australia’s ecosystems could support the population and continue to make south-east Australia a liveable place.


Right: Dr Xuanlong Ma recently presented some of these research findings at the 2015 International Congress on Phenology.
There is much to find out how semiarid areas perform as carbon sinks so these areas cannot be included in predictions of future climate change. The threshold for native plants to be drought resilient is also not yet known and whether they lose that resilience when that threshold is crossed is also unknown. Dr Ma and his colleagues are conducting researches to address these challenges Critical observational evidences gain from studying these questions would help us, not only scientists, but also stakeholders, land managers, and policy makers, better synthesise our knowledge to cope with the consequences of future climate change says Dr Ma. South-east Australia really does have a climate change threshold..
Dr Xuanlong Ma was interviewed for A Question of Balance by Ruby Vincent. All images provided by Dr Ma. Summary text by Victor Barry, December 2015.  

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