Albedo changes in the Amazon due to deforestation

Deforestation rates across the Amazon remain high, despite large reductions since the start of the 21st century. Assessing the climate impact of this is difficult, due to the complex interactions between the atmosphere and the biosphere.

Some of these interactions are clear and well understood, such as deforestation releasing stored carbon and removing a carbon sink. However, others are much more complex; trees emit various atmospheric trace gases and aerosols which react in the atmosphere in a number of ways, forming climate-warming gases, such as O3, and aerosols, which have a cooling effect through interactions with incoming radiation and cloud formation.

Currently, the project is assessing one of the physical impacts of deforestation, namely the change in brightness of the surface. Forests tend to have a darker colour when compared to open land and thus reflect a lower percentage of incoming radiation; we can describe this as a low surface albedo. Therefore, the removal of trees produces a slight cooling effect when examining surface albedo in isolation.

This doesn’t mean we should cut down all the trees to cool the climate though! Instead, this cooling is merely one component of the change and so all the interactions must be combined before a clear view can be gained. For the tropics, when combined with the other interactions, there is still a clear warming effect overall from deforestation, due to the relative size of each (warming effects from the emission of CO2 and reduction in cooling from evapotranspiration dominate).

This albedo effect has been acknowledged in the past and studies have evaluated the differences in albedos of various surface types, however this project tracks direct albedo changes through historic deforestation events in the Amazon.

In order to do that, we have to look at the past and thus we turn to satellite imagery. The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard NASA’s Aqua and Terra satellites, provide snapshots of the planet’s surface albedo since 2000. Combining this with satellite imagery focussed on changes in tree cover, we can assess the changes in albedo following deforestation across a number of locations (Figure 1).

Figure 1: Surface albedo (left) and forest cover (right) satellite data taken from MODIS product MCD43A3 and processed Landsat data respectively. The regions with lower albedo correspond to a mixture of rivers and lakes, while the lighter regions correspond to a lack of forest, especially along the southern border of the images.

From these images, we are able to quantify the changes in albedo and explore the changes statistically, followed by assessing these values against previous studies.

By gaining a clearer picture of the changes in some of the lesser known interactions, the overall climatic impact of deforestation can be better understood, enabling both more informed policy decisions around land-use to tackle climate change and better understanding as to how such decisions will impact the climate.

Jamie Wilson is reading for a NERC and UBoC CASE funded PhD on the impacts of deforestation on the climate