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Semi-arid lands expanding under climate change

201507-09

By Ana Bastos

Areas affected by water scarcity, or semi-arid regions have expanded by 13% (127 millions of hectares) since the early 20th century, scientists have found, with expansion expected to continue during this century. 

Under business-as-usual emissions, Amélie Rajaud from the Climate and Environment Sciences Laboratory has found that tropical parts of Central and South America as well as temperate regions in the Mediterranean, South-Africa, California and Australia are expected to convert to semi-arid regions by 2100. 

Drylands in general cover about 40% of the Earth's land surface, and they host almost one third of the world population. Water-limited, and therefore natural resources-limited, drylands have represented challenging regions for human societies. 

Their populations tend to show weaker wealth and health scores compared to the world population. Moreover, according to the UN Convention to Combat Desertification, 10–20% of drylands are already facing desertification.

“If these semi-arid regions continue to expand and get hotter and drier, we may see a decrease of their ability sustain the population, which may create or amplify poverty, migration and conflict problems,” said Rajaud.

The magnitude of future expansion depends on CO2 emissions trajectories. Under a low emissions scenario (RCP2.6), expansion will increase by 8% or 84 million hectares. If we continue business as usual emissions (RCP8.5), Rajaud predicts an increase of 30%, or 156 million hectares.

From the borders of rainforests to deserts, two billion people (around one third of global population) live in areas affected by water scarcity, or drylands. Within drylands, semi-arid regions are especially important because they are particularly sensitive to temperature and rainfall changes.

There are several ways that arid regions may develop and this affects the way scientists study aridity. 


Climate is the primary driver of arid regions


“At the planet scale, the tropical dry regions of the world are positioned by the atmospheric circulation: warm, humid air masses rise at the equator up to the upper atmosphere, where they lose moisture through precipitation, before subsiding in the subtropics. These subsidence regions are therefore dry, warm and cloud free."

A second major driver of aridity is ecological pressure. Semi arid regions are frontier areas where ecosystems might shift either towards fully arid (desert), or less water-limited (rainforests). For example, grazing and fire burn off in these boundary areas can cause once wet rainforest to morph into drier savannah ecosystems. 

Lastly, human pressure also contributes to the desertification of certain regions, for instance by deforestation.

While former studies have shown that over the past decades drylands have been expanding, Amélie Rajaud and her colleague Nathalie Noblet are analysing how warm semi-arid regions are responding to the global warming projected under scenarios of future anthropogenic emissions. 

Instead of defining aridity solely by the rainfall/evaporation ratio, their approach relies on a bioclimatic definition of semi-arid regions, which gives insight into the ecological impact.

"Ecologically significant thresholds are used that provide a useful link to land cover, thus, the resulting bioclimatic classes are meaningful to a wider range of users than purely climatic divisions are," Rajaud said.

Under both high and low climate scenarios, they expect the semi-arid expansion to occur in the tropics, but also under temperate latitudes. And the expansion of warm semi-arid regions implies an increase of regions with climatic conditions similar to those prevailing in dry savannas and grasslands.  

Anticipating such changes by improving land-management and forestry practices in these regions may be crucial for their populations, Rajaud said. 

“However, afforestation and reforestation projects may struggle to be successful in regions  expected to become more arid in 50 or 100 years. ” 

Moreover, as warm semi-regions are expected to expand also outside of the tropics, where they had been confined so far, land managers and development programs will need to consider whether they go on focusing on trying to preserve current ecosystems (for example through afforestation or reforestation projects that will help soils retain water) or adapt to future arid conditions by turning to specific land-management.


Ana Bastos has just completed a PhD in Geophysical Science at the University of Lisbon, Portugal and is currently working at the LSCE. She is interested in studying the relationship between climate variability and the land CO2 sink, and the impacts of extreme events and disturbances (fires) in ecological activity. She is a volunteer social reporter at the #CFCC15 conference.
 
This is part of a blog series profiling climate scientists, economists, social scientists and civil society members who are presenting and discussing innovative climate science at Our Common Future. For more follow @ClimatParis2015 and #CFCC15 on Twitter.

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