What is the main idea for this PNAS paper?
The main idea is to understand the impacts of deforestation on the atmospheric circulation and rainfall in the monsoonal regions of the world. The effect of deforestation on atmospheric CO2 concentration and climate warning have been studied a lot and understood better. However, rainfall is a challenging climate variable because it is not only affected by the amount of water vapor in the atmosphere but also by converging circulations of moist air.
How does deforestation affect the climate system?
Deforestation releases of carbon dioxide into the atmosphere. This is called a biochemical effect, and its warming potential can be easily calculated. However, deforestation also changes the surface characteristics such as reflectivity and plant transpiration. These effects are difficult to quantify and their effects on climate are not accounted for in climate mitigation strategies such as afforestation and reforestation. These changes are called biophysical changes from deforestation and models can be used to quantify their effects. For example, deforestation can increase the reflectivity of the surface and hence could have a cooling effect. Similarly, a reduction in plant transpiration could lead to warming. All the individual effects due to deforestation are illustrated in the schematic. In our study, we have used a climate model to simulate the net effects of these biophysical changes on rainfall in the monsoon regions.
Schematic to illustrate the effects of deforestation: Any land cover change usually leads to both biochemical and biophysical changes in the climate system. Cooling (warming) effect is shown by the blue (red) boxes. Schematic is adapted from our publication in Plant Cell and Environment ( doi: 10.1111/pce.12488 )
What are the models/simulations used for this study?
We have used a state-of-the-art comprehensive 3-dimensional climate model for this work. The model can simulate the atmospheric circulation, land surface processes such as photosynthesis and transpiration, surface-ocean warming and ice melt. Models of such complexity are the latest trend. We wanted to get a basic understanding of the effects of large scale deforestation at different locations on monsoon rainfall. We performed 3 experiments: deforestation in the tropical, temperate and high-latitude areas. To increase the climate change signal from deforestation, our simulations removed all trees wherever deforestation took place.
What are the main findings from the study?
The main finding of the study is that deforestation can shift the location of moisture convergence in the tropics, the so-called inter-tropical convergence zone (ITCZ) in the tropical area. This has huge implication for tropical rainfall.
For deforestation in the temperate and high-latitudes, we find that the ITCZ shifts southward and hence all the monsoon regions in the northern hemisphere (NH) get reduced rainfall. The NH monsoon regions are in South Asia, North Africa, North America and East Asia. At the same time, the southern hemisphere monsoon regions such as Australia, South Africa and South America receive more rains.
The surprise is that the mid- and high-latitude deforestation displaces the ITCZ southward much more than tropical deforestation. This indicates that the remote effects on rainfall are stronger than local effects. This is exactly opposite to what we see in the case of temperature changes where local effects dominate.
What are the applications of this paper? Who would find these results most useful?
This study has huge implications for evaluating the climate benefits of afforestation and reforestation programs that are promoted for reducing climate warming. Our study shows that it is not sufficient to estimate only carbon sequestration benefits of these programs. We need to account for the biophysical effects as well since they also seem to have large effects. In the past, several studies indicated the importance of these biophysical effects on temperature. Those studies showed that in some cases the biophysical effects can overwhelm the carbon sequestration benefits. Now, our study brings in another factor in the form of biophysical effects on rainfall in the monsoon regions mediated via atmospheric circulation changes. Therefore, we may have to come out with an integrated assessment that takes carbon sequestration, and temperature and precipitation changes from biophysical effects into account to accurately characterize the effects of any land cover change.
Our work would be also useful to scientists who study the behavior of monsoons in past climates.
Where does this research lead in terms of future direction?
All assessments of deforestation/afforestation in the past have taken only temperature, not rainfall, changes into account. Such assessments may be adequate if rainfall changes are locally driven by local temperature changes alone. However, our study is showing that remote deforestation in mid- and high-latitudes can have a much larger effect on tropical rainfall than local tropical deforestation. The remote effects are transmitted by atmospheric circulation changes. In the future, therefore, we plan to improve the impact assessments of land cover change by formulating a metric that takes into account carbon sequestration as well as changes in both temperature and precipitation. This metric can be calculated for several regions of the planet. For example, we may want to answer questions like “What is the impact for India as estimated by this integrated metric for deforestation in 1 million hectares in the tropical or high-latitude regions?”