The butterfly effect

Here’s an example of globalisation in action. More crops in sub-Saharan Africa could affect South Asia’s ability to produce food, according to researchers based in Sweden.

Their study shows the extent to which human behaviour—in particular deforestation and crop irrigation—is affecting the global circulation of water vapour.

Agricultural production of food causes water to evaporate into vapour. Large-scale irrigation of fields, for example, increases the amount of water vapour in the atmosphere.

Conversely, cutting down trees reduces the amount of water vapour released into the atmosphere.

Line Gordon and colleagues from the University of Stockholm in Sweden looked at how much water vapour is being produced around the planet and compared this with estimates of what would have been produced if human activities hadn’t modified land-use and vegetation.

Their study is the first to look at water vapour flows on a global scale.

The researchers found that, worldwide, deforestation has decreased the evaporation of water by 4%. Overall, this is almost exactly offset by the increase in the release of water vapour from irrigation.

But the authors warn that the balance at the global level hides strong regional differences.

For example, they point out, south-western and Central Africa has suffered more deforestation than other regions. As a result, these regions have become drier than they would be in the absence of human activities.

In contrast, other regions have had an increase in water vapour because of large-scale irrigation. These tend to be areas of intensive food production: particularly south-eastern China and on the plains around the Ganges in India, but also Central Asia, parts of the United States, southern Europe and Chile.

The combined effect, say that authors, is a substantial difference in the distribution of vapour at a global scale compared with what the distribution would have been without human deforestation and irrigation.

Studies in China have shown the changes to vapour flows within a region can affect the monsoon rains across the region. No one has yet studied the interaction between vapour flows and the climate on a global scale. The authors suggest the interaction could be large, and the implications for food security could be severe.

Both South Asia and Africa, for example, need to increase food production to meet the needs of their populations. In South Asia, this need is being met by high levels of irrigation, which is already increasing vapour flows; these will be raised further by increased agricultural efficiency.

In sub-Saharan Africa, however, the need for more food will probably be met by increasing the amount of agricultural land by cutting down forests. The resulting decrease in evaporation from the region, say the authors, will affect atmospheric conditions and could create a ring of highly modified vapour flows around the Indian Ocean, a key component of the Asian monsoon—which in turn is key to crop yields in Asia.

“Changing land cover in one place could affect weather patterns in other places and that could affect the preconditions for food production,” explains Gordon.

She underlines the need to start analysing the role of water-vapour flows in the global climate.

“We need to see how big an effect this can have on a global scale,” she says.

Gordon is currently working on combining her data on vapour flows with climate change models to do just that.—SciDev.Net

The research has been published by the Proceedings of the National Academy of Sciences, available via links on the Science and Development Network

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