Journal Article

Observations of increased tropical rainfall preceded by air passage over forests

Taylor, C. M., Arnold, S. R., Spracklen, D. V.

2012

Overview

Remote sensing and simulated atmospheric transport patterns are used to show that air passage over tropical forests produces about twice as much rain as passage over sparse vegetation; in an idealized Amazonian deforestation scenario, a reduction in seasonal precipitation of approximately 12–21% is estimated. Tropical rain follows air passage over forests This global observational analysis demonstrates that forests exert a strong control on rainfall hundreds of kilometres downwind through a water-cycle feedback. When precipitation occurs, some of the water returns to the atmosphere through transpiration and evaporation. In the tropics, this process has long been thought to be an important part of the overall precipitation budget. But most evidence has come from modelling studies, which remain inconclusive. Dominick Spracklen and colleagues use remote sensing and atmospheric back-trajectory modelling to show that air passage over dense forests produces about twice as much rain as passage over sparse vegetation. They estimate a 12–21% reduction in seasonal precipitation if Amazon deforestation continues at the current rate, and conclude that efforts to curb deforestation are vital if drastic impacts on regional rainfall are to be avoided. Vegetation affects precipitation patterns by mediating moisture, energy and trace-gas fluxes between the surface and atmosphere 1 . When forests are replaced by pasture or crops, evapotranspiration of moisture from soil and vegetation is often diminished, leading to reduced atmospheric humidity and potentially suppressing precipitation 2 , 3 . Climate models predict that large-scale tropical deforestation causes reduced regional precipitation 4 , 5 , 6 , 7 , 8 , 9 , 10 , although the magnitude of the effect is model 9 , 11 and resolution 8 dependent. In contrast, observational studies have linked deforestation to increased precipitation locally 12 , 13 , 14 but have been unable to explore the impact of large-scale deforestation. Here we use 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. Our observation-based results complement similar estimates from climate models 4 , 5 , 6 , 7 , 8 , 9 , 10 , in which the physical mechanisms and feedbacks at work could be explored in more detail.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 704/106 / 704/158/2450 / Air - analysis / Animal and plant ecology / Animal, plant and microbial ecology / Atmosphere - chemistry / Biological and medical sciences / Brazil / Climate models / Deforestation / Earth, ocean, space / Environmental aspects / Evapotranspiration / Exact sciences and technology / External geophysics / Feedback / Forest ecology / Forestry / Fundamental and applied biological sciences. Psychology / Humanities and Social Sciences / Humidity / letter / Meteorology / multidisciplinary / Observational studies / Pasture / Precipitation / Precipitation (Meteorology) / Rain / Remote sensing / Science / Science (multidisciplinary) / Seasons / Steam - analysis / Synecology / Terrestrial ecosystems / Trees - growth & development / Trees - metabolism / Tropical Climate / United States / Vegetation / Water in the atmosphere (humidity, clouds, evaporation, precipitation)