Following one of my reader’s comments, I decided to look at the impact of South American deforestation on the global climate. A very recent review in ‘nature climate change’ by Lawrence and Vandecar (2015) is very informative on this issue, and I have presented one of their figures in my last post.
Global Circulation Models incorporate the hydrological cycle, and represent vegetative canopies with their effect on energy and water. “The atmosphere and biosphere form a coupled system whereby climate influences vegetation distribution and ecosystem function, which feed-back to affect climate” (Lawrence and Vandecar, 2015). Studies therefore use these to examine potential impacts of wide-scale land cover change on the global climate system.
The hypothetical situation of removing all tropical forests is analysed in several studies. The predicted increase in global mean temperatures ranges from 0.1–0.7 °C (of which the highest value is equivalent to doubling the warming cause by all GHG emissions since 1850!). This occurs as the cooling effect, which is a cause of the high rate of evapotranspiration in tropical forests, is lost through deforestation. Generally, the increase in temperatures is still greatest in the pantropical areas (due to the effects discussed in post 03/12/15).
Global mean precipitation is predicted to be left unchanged – however, Lawrence and Vandecar (2015) explain this to be primarily due to the opposing signals of local and regional impacts.
What I find interesting are the extra-tropical impacts of regional deforestation. The link that exists between regions of deforestation and regions of impact is a teleconnection (and arises by altering, for example, geopotential height, Rossby waves, Ferell and Hadley cell circulations).
Regional deforestation in the South American tropics can have severe effects on the climate over remote regions via teleconnections. I want to give some examples of this. Avissar and Werth (2005) show that tropical deforestation of the Amazon and tropical Africa have severe far-reaching impacts on regional precipitation in the U.S. Midwest. Here, precipitation is reduced dramatically especially in seasons in which it is most important to support local agricultural productivity. Gedney and Valdes (2000) predict Rossby wave propagation due to Amazon deforestation, directly affecting the winter climate over the Northern Atlantic and Europe with increasing rainfall.
We must not forget that a complete deforestation of all tropical forests is unlikely to happen, but assumed in many of the impact studies. Medvigy et al (2011) compare this with a ‘business as usual’ (incremental) deforestation of the Amazon. This new decadal-scale, meso-scale resolution modelling approach shows that precipitation changes are less than previously predicted in GCM models. While the general regional effects of deforestation are maintained in the ‘business as usual’ scenario, the degree of impact is much smaller. The extratropical impacts under full-deforestation scenarios (e.g. examples given in last paragraph) did not occur under the incremental deforestation scenario.
Lawrence and Vandecar (2015) examined various GCM studies that incorporated various levels of deforestation and suggest that “tropical forest clearing beyond ~30–50% may constitute a critical threshold for Amazonia”, beyond which the impacts on local climate (and ecosystem functioning) and remote climate will be dramatic.
Concluding thoughts:
Reviewing different model simulations, differing in their resolution and area/degree of deforestation, shows that the exact impacts of deforestation cannot yet be predicted with certainty, as these depend on the exact occurrence of land cover change and the nature of the teleconnections with other parts of the world. What we may conclude, however, is that the potential of climate alteration by deforestation far off the deforested areas is great. This finding supports the argument that deforestation is a global problem.
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