The scale of land cover change impact on the climate system: a complex matter

The paper by Hoffmann and Jackson (2000) explores how the conversion of tropical savannah to grassland influences climate and finds a similar pattern of climate evolution to what I have outlined in my previous post (concerning the conversion of tropical forest to grassland). I want to take this, slightly different example, to explore the complexity revolving around scale and location of impact.
Despite a common pattern of change (increased temperature, lower seasonal rainfall, lower ET), the study shows strong regional variations in the climatic effect of an assumed universal conversion of land cover.

Within each of the world regions examined, some areas existed where precipitation is expected to decline by considerably more than 10% (see the dark red shading in Figure 1). While the authors point to the stochastic nature of the model used as a potential reason for this prediction, it very likely represents the real variability to be expected. Notably, with uneven land cover change over a tropical biome (as realistically not ALL of the land will be converted) it is even less predictable where the effects will hit most strongly, posing challenges to risk management.

Figure 1: click for source

Another complicating factor to the location of climatic impact is that through deforestation, roughness length of the vegetation is decreased, having significant effects on circulation patterns and thus making the rainfall pattern in an area more uncertain.

What this paper touches upon too, but Pires and Costa (2013) examine in more detail, is the concurrent land cover changes happening on the South American continent, with accumulating and interacting effects on the climate and biomes. They show that inner Amazon forest regions keep their rainforest biome characteristics, but outer forest regions may cross a forest-savanna bioclimatic threshold even at low deforestation levels. Different regions vary in their resilience to be pushed to the next “climate-vegetation equilibrium state” (Oyama and Nobre, 2003). Due to the climate-vegetation feedbacks induced by the warming, drying climate response to deforestation locally and as spread effects of adjacent land degradation in the cerrado savannah, the bioclimatic boundary between rainforest and cerrado is predicted to move 500-1000km north into the Amazon basin. Hirota et al (2011) have a more methodological perspective on how to model the climate effects of land cover change – but highlight just as effectively that deforestation in the tropics in particular may have local, meso and even global scale effects, which interact with the climatic effects of other region’s land cover change. Thus, observed and also modelled climatic changes must be evaluated upon the imminent local drivers, as well as on the relative impacts of more far-reaching changes.