In their recent study, “Effects of tropical deforestation on climate and agriculture,” Deborah Lawrence and Karen Vandecar review data from multiple sources, including general circulation models (GCMs), direct measurements and satellite data and imagery, to study the effect of tropical deforestation on worldwide temperature and precipitation. The effects are widespread, affecting agriculture in the US Midwest and Central America to monsoons in Southeast Asia and India.
They report that complete deforestation of the entire tropics – Amazonia, Central Africa and Southeast Asia – would “effectively double the observed warming since 1850.” Such deforestation would also decrease precipitation in the tropical regions. Amazonia would be the most affected, in terms of both warming and drying, with a temperature increase of up to 3.8°C and a precipitation decrease of around 15% of annual rainfall.
However, the effects of deforestation would not be localized to just the deforested regions. Lawrence and Vandecar find that the logger’s ax could hit as far away as Asia or the US, with reductions in rainfall over the US Midwest, Northwest and parts of the south but increases (especially during winter) over the US eastern seaboard and northeast Atlantic regions. Deforestation of Central Africa would reduce precipitation over areas as widespread as the Gulf of Mexico, the US Midwest and the US Northwest, Ukraine, Southern Europe, and southeastern Africa, while increasing it over the Arabian Peninsula, China, western Asia, and the center of southern Africa. The Asian monsoon circulation, both critical for agriculture and a source of dangerous natural disasters, would bear the greatest impact from deforestation of Southeast Asia. It is clear that these tropical forests are major players in the global water cycle, and removing them would have lasting and considerable effects the world over.
At what level of deforestation will we see these effects? Lawrence and Vandecar find that there is a critical threshold for deforestation of the Amazon between 30% and 50%. After this threshold is reached, the decline in precipitation leads to a “significant decline in ecosystem structure and function.”
Where are we now? “Business-as-usual” deforestation of the Amazon is expected to result in a loss of 40% of the forest by 2050. That is, if deforestation continues as it does now, we will hit the threshold within thirty-five years. What’s more, the critical threshold may be even lower, as location and pattern of deforestation are key factors. It is estimated that the critical patch size of intact forest is over 500 km per side – that is, on a continental scale of a half-deforested area, patches of forest without any loss due to deforestation must retain an area of 250,000 km2 in order to avoid significant decreases in rainfall. This is a difficult size to maintain as the tendrils of logging reach further and further into the old forest.
We are already seeing drying and warming of deforested areas. Direct measurements are showing that surface temperatures are increasing and evapotranspiration is decreasing in the Amazon when tropical forest is cut down and converted to pasture. These effects are exacerbated in the dry season, so already unfavorable conditions are worsening.
Lawrence and Vandecar present us with a ticking time bomb, the effects of which we can expect to feel within the next few decades unless serious, immediate and direct measures are taken to drastically reduce deforestation. Critical patch sizes must be maintained, and exploitation of the tropics, especially of the Amazon, must significantly slowed. Until these measures are taken, we are keeping ourselves on the path to a dramatically different climate – a dramatically different, harmful future.
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Source: Deborah Lawrence and Karen Vandecar. Effects of tropical deforestation on climate and agriculture. NATURE CLIMATE CHANGE | PUBLISHED ONLINE: 18 DECEMBER 2014 | DOI: 10.1038/NCLIMATE2430