Deforestation is believed to cause drier and warmer conditions because of a decrease in tree cover. However, a counterintuitive effect of increase in cloudiness and rain due to deforestation has been found by Khanna and Medvigy in Rondônia, a state in west central Brazil, which was once covered by over 50 million acres of Amazonian rainforests, half of which has been replaced by pasture.
The authors provide evidence that differences in vegetation height can influence cloudiness: Large patches of short pasture interspersed between large patches of tall trees give rise to different amounts of cloud cover because of faster wind speeds in the cleared areas.
The researchers investigated this atmospheric effect of deforestation in Rondônia using the recently developed Ocean-Land-Atmosphere Model. They modeled the cleared area, covered by grass approximately 30 centimeters tall, sandwiched between evergreen forests roughly 30 meters tall. This introduces a smooth landscape between tall rough forests and results in a speeding up of surface winds over the pasture.
This faster wind rises and forms clouds as it encounters an undisturbed forest patch. Conversely, a suppression of clouds is observed when the slow wind blowing over forests encounters a smooth patch of pasture. This response to variations in vegetation height gives rise to enhanced cloudiness in the downwind half of deforested regions and reduced cloudiness in the upwind half.
The authors also found that this effect of contemporary deforestation dominated other atmospheric effects of deforestation—such as thermal triggering of clouds found by previous studies. Thus, the findings also imply that an increase in deforestation will broadly shift the regional precipitation regime. If these results can be applied to other rainforests, they may help researchers glean some insight into the large-scale atmospheric impacts of deforestation around the world. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2014JD022278, 2014)
—Shannon Palus, Freelance Writer
Citation: Palus, S. (2015), Could Amazonian deforestation increase cloudiness and rain?, Eos, 96, doi:10.1029/2015EO029107. Published on 4 May 2015.