Human activity changes global water vapour patterns

Irrigation creates more water vapour flow in the atmosphere Copyright: IRD/Montoroi

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Future action taken to increase food production in sub-Saharan Africa could indirectly affect the capacity of South Asia to produce food, according to researchers based in Sweden.

Their suggestions are based on the results of their study published yesterday (9 May) on the website of the Proceedings of the National Academy of Sciences showing the extent to which human activity — 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 — or ‘deforestation’ — reduces the amount of water vapour released into the atmosphere.

Line Gordon, of the University of Stockholm, Sweden, and her colleagues, looked at how much water vapour is being produced around the planet and compared this to 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 four per cent. 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, Southeast Asia, southwestern and central Africa, the Amazon basin, northern South America and parts of the United States, have suffered more deforestation than other regions. As a result, they have become drier than they would be in the absence of human activities.

Distribution of areas where human activity has increased (red) and decreased (blue) water flow; click here to see a larger map
Photo: PNAS/Gordon

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 southeastern 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 to 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 [through its impact on the level of water vapour in the atmosphere] 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.

Reference: Proceedings of the National Academy of Sciences (online), DOI: 10.1073/pnas.0500208102
Proceedings of the National Academy of Sciences (print) 102, 7612 (2005)

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