A new Met Office study has found that, if global greenhouse gas emissions are not reduced, we are likely to see large changes to the rainfall in tropical countries.

Scientists found that the size of these changes will be strongly determined by the total amount of greenhouse gases that are emitted.

Many people in tropical regions, which contain some of the least developed countries in the world, are already exceptionally vulnerable to variations in how much and how frequently it rains. Any large, long-term changes to rainfall amounts due to climate change could worsen this vulnerability, and test the ability of societies and wildlife to adapt to potentially unprecedented conditions.

To investigate possible changes in future rainfall patterns, Met Office scientists used a large number of climate change simulations of the 21st century, produced by research institutes across the world. All simulations run with high greenhouse gas emissions produced large changes in rainfall patterns across substantial areas of tropical land by the end of the century. On average around one quarter of all tropical land was affected – an area twice the size of Brazil. Simulations run with lower greenhouse gas emissions showed much smaller areas of land with large rainfall changes.

Which regions are most vulnerable to rainfall change?

Analysis of future climate simulations shows that under high greenhouse gas emissions, large rainfall changes are expected to occur over an even larger area of dry land than was affected during the Sahel drought. Of course the impacts of such changes would be heavily dependent on the resilience of the particular countries affected.

The long-term drought in the Sahel region of West Africa brought famine to hundreds of thousands of people and huge disruption to millions more in the 1970s and 80s. Although not clearly linked to greenhouse gas emissions, the Sahel drought provides a yardstick for the potential impacts of future climate change.

When and why will these changes happen?

Rainfall changes in tropical countries are expected by the end of the 21st century due to climate change affecting a number of the processes which determine where and how much it rains in different parts of the tropics. One of these is the pattern of surface temperatures across the tropical oceans. As rainfall tends to occur over the warmest parts of the oceans, any changes to these patterns can cause large changes to the regions where it rains – this is what happens during an El Niño event.

Exactly where will changes occur?

Exactly which countries will be affected by these future rainfall changes is much less certain, as climate simulations disagree on where the changes will occur. Regions thought to be most at risk of large decreases include southern Africa and Central America, while India and East Africa are among those most likely to experience large increases. However it should be emphasised that the location of changes is much less consistent among climate simulations than the fact that large changes occur.