Vegetation Effects on Rainfall in West Africa (VERA)

Rainfall is the climatic parameter of greatest importance to the populations of the tropical continents. The arrival of monsoon rains drives a rapid transformation of the landscape, allowing crops to grow and river networks to refill. Yet predicting where and when rain will fall in the tropics is a notoriously difficult problem. Progress has been made in predicting how remote ocean conditions, such as El Nino, can affect rainfall in different parts of the tropics. However local factors such as vegetation also play a role. For example, when tropical forests are cut down for agriculture, we have evidence that this affects rainfall both locally, and across neighbouring countries. Indeed, climate scientists have to take into account future deforestation rates as well as greenhouse gas emissions when they assess how tropical climate will change in the 21st century.

Vegetation affects rainfall through the process of transpiration. When plants absorb carbon dioxide for photosynthesis, they lose water from their leaves. Trees are able to extract this water from several metres below the surface using their deep roots, allowing them to continue photosynthesising for months without rainfall. Crops and grasses on the other hand start to run out of soil water during dry spells, which reduces transpiration. Instead the solar radiation absorbed by the plant canopy raises the air temperature. Replacing forests with crops and grasslands changes the rates of moistening and heating of the atmosphere, particularly when the shallow-rooted species start to run out of soil water. These changes in turn affect the development of winds, cloud and rain.

The details of how the atmosphere responds to vegetation is an area of significant scientific debate. Firstly, there is evidence that clearing patches of forest may increase rainfall over the cleared area and reduce it over the remaining forest, depending on the particular weather patterns. On the other hand, new results have shown that as air masses cross the continent, they pick up additional moisture from forests, which then leads to more rain several hundred kilometres further downwind. Finally, by controlling the balance between heating and moistening of the atmosphere, the vegetation can affect the winds bringing moist air off the ocean, delaying or extending the rainy seasons which characterise tropical climate.

Although these 3 vegetation effects are each known to affect rainfall, we rely on computer models of the vegetation and atmosphere to understand how they might work in combination. Capturing the essential physical processes within a model is very challenging. In particular, there are large and long-standing uncertainties in the description of cumulonimbus storms (thunderstorms, which dominate the rainfall of many tropical regions) within the models. However through recent advances in computing power, we are now able to run these models for entire seasons with sufficient spatial detail to properly capture storms.

In this project we will use data from satellites and the latest weather and climate models to get to the heart of how vegetation affects rainfall. Focusing on West Africa, one of the most climatically sensitive regions of the world, we will examine cloud and vegetation observations from the last 30 years to detect where deforestation has changed rainfall, and how the rapid greening of the savannah each year affects the monsoon rains. We will perform new computer simulations, incorporating the detailed development of thousands of individual storms, and examine what happens when we artificially deforest a region in the model. These results will allow us to assess the performance of the somewhat cruder models used to forecast climate change globally. By focusing on specific processes in the climate system, our results will help to improve these models, and at the same time provide robust conclusions on deforestation to guide land managers.

The population of West Africa is forecast to double in the next 40 years, resulting in food security issues and increased pressure on forest resources. Ensuing land-use change will alter patterns of rainfall, further impacting food production across West Africa. Understanding the interactions between land use and precipitation is essential if we are to craft the best land-use decisions for the region, and to ensure that investment in development is robust, in a variable and changing climate.

Specifically for the UK, West Africa offers opportunities in terms of trade and investment (e.g. £4 billion bilateral trade between UK and Nigeria), but also threats through organised crime, extremism and terrorism ( http://www.publications.parliament.uk/pa/cm201314/cmselect/cmfaff/writev/extremism/m12.htm). Poverty is a key driver of these threats which is exacerbated by lack of rainfall across the region. To help address development concerns, the UK makes substantial financial contributions to West Africa (£600 million in 2012/2013). Sound knowledge of the underpinning science is required to better target future investments. With these concerns in mind, we have identified five stakeholder groups for whom our research will be of interest.

The primary group which will benefit from our research is the representatives of private companies, regional and national institutions in West African nations, and policy makers in West African nations, who are responsible for the sustainable development and protection of the environment, and who are concerned about impacts of climate and land use change on African water resources. The population of the region rely on the sustainable production of food and other crops for their livelihoods. Demonstrating how forest cover affects regional precipitation patterns will help raise awareness and should influence regional, national and international policies. Similarly, a reduction in the uncertainty of climate change predictions will support policy makers in their efforts to develop appropriate mitigation and adaptation strategies.

The work will benefit NGOs concerned with nature conservation, and specifically the protection of natural forests. This project will highlight the role that forests play in modulating regional precipitation patterns, and add to the evidence base which NGOs need to support their case for preserving, restoring and protecting forests in the tropics.

DFID is concerned with the impact of climate change on the lives and livelihoods of the people in developing countries. Recently, DFID has become aware that the effective use of its very large development budget for Africa is sensitive to climatic variability and change. The proposed work will directly contribute to one of DFID's aims, which is to improve our knowledge of weather and climate processes over Africa, to the benefit of African people, and so that development decisions (for instance, investment in projects which influence land-use) can be made on the basis of good climate science.

International research communities will benefit, through the assessment of a key process in climate dynamics: the iLEAPS community (integrated Land Ecosystem-Atmosphere Processes Study) directs research supporting the Intergovernmental Panel on Climate Change; and the GEWEX research community (Global Energy and Water cycle EXperiment) is core to the World Climate Research Programme (WCRP). The research in this project will further reduce uncertainty in the current climate change predictions for Africa.

Finally, the project should be beneficial to the scientific understanding of the general public, who are interested in the forests of our planet and the short- and long-term social, economic and climatic issues affecting the ecosystem services provided by forests. In basic terms, the control of rainfall by forest systems can be understood by high-school students, and we aim to clarify the processes involved in this interaction.