Observations of increased tropical rainfall preceded by air passage over forests

There has been considerable controversy in recent years about the question of earth system tipping points in general and about the question of an Amazon dieback that is predicted by some of the global vegetation models in particular. A cornerstone for answering this question is to understand how tropical forests affect large scale precipitation patterns and the hydrological cycle. A new study by Dominick Spracklen and colleagues addresses this question by using

satellite remote-sensing data of tropical precipitation and vegetation, combined with simulated atmospheric transport patterns, to assess the pan-tropical effect of forests on tropical rainfall. We find that for more than 60 per cent of the tropical land surface (latitudes 30 degrees south to 30 degrees north), air that has passed over extensive vegetation in the preceding few days produces at least twice as much rain as air that has passed over little vegetation. We demonstrate that this empirical correlation is consistent with evapotranspiration maintaining atmospheric moisture in air that passes over extensive vegetation. We combine these empirical relationships with current trends of Amazonian deforestation to estimate reductions of 12 and 21 per cent in wet-season and dry-season precipitation respectively across the Amazon basin by 2050, due to less-efficient moisture recycling.


Figure: Simulated percentage change in precipitation due to 2000–2050 business-as-usual deforestation of the Amazon basin for wet season (a) and dry season (b). From Spracklen et al., copyright see publisher.

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