Human activity changes global water vapour patterns
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
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
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.
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
"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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