Convective-Scale Impacts of Deforestation on Amazonian Rainfall

The Amazon rainforest contains 40% of all remaining tropical rainforest in the world, but has seen rapid deforestation since the 1960s, and as much as 40% of the Brazilian Amazon could be deforested by 2050. Land-use change is an important man-made driver of climate change. We know that deforestation will generally make the atmosphere both warmer and drier, but how these changes will affect rainfall is more complex. Climate models mostly predict that deforestation will reduce rainfall, but the amount varies from 0 to 60% across different studies. Climate models use grid boxes of 10s to 100s km, which are much larger than a typical cloud. While cloud properties can be estimated from the conditions in the grid box, calculating the amount of rainfall is very uncertain, especially in the tropics. One solution is to run a model with much smaller grid boxes, but focusing on a small region, so that clouds and the detailed deforestation patterns found in the Amazon can be represented explicitly. These studies show that the surface patterns alter local weather patterns, increasing rainfall over the deforested patches, which contradicts climate models. However, because these studies focus on smaller regions, we do not know if these local effects are important for the water cycle of the entire Amazon.

This project will combine both approaches, using the increased computing power now available to simulate, for the first time, the entire Amazon basin while also explicitly representing clouds. This is a crucial improvement, because past studies have shown that resolving clouds leads to a complete change in model behaviour, greatly improving how tropical rainfall is represented, including climate extremes like flooding and droughts which have the most impact on local populations. We will use these simulations to investigate how increasing deforestation will affect rainfall over the Amazon, and how these changes compare to those caused by global climate change driven by increasing carbon dioxide levels.

The project is particularly exciting because it will provide a comprehensive understanding of how deforestation affects rainfall, simulating both changes in regional climate and the local weather patterns within it which directly affect people. Tropical rainfall is a key area of research in climate modelling, because although it is the most important climatic parameter to end users, it is also the most uncertain. For example rainfall drives a number of economic sectors such as agriculture and hydroelectric power, and while deforestation is used to clear land for agriculture, reductions in rainfall could reduce the yield per hectare, negating any economic gain from increasing the agricultural area. Patterns of deforestation can also affect where it rains, which could help planners identify ways to mitigate some of the negative effects on the remaining forest. This project will engage with stakeholders in the region through workshops to improve our physical understanding in a targeted way to address global challenges which have direct relevance to many people.