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Large rainfall changes expected in the tropics

Figure 1: Climate simulations of tropical land rainfall change and global temperature change over the 21st century under four different emissions scenarios.

September 2015 - A new study by Met Office scientists has found that climate change is likely to bring large changes to the amount of rainfall in many tropical countries, and that the size of these changes will be strongly determined by the total amount of greenhouse gases that are emitted globally.

Many people in tropical regions, which contain most of the poorest and least developed countries, are already exceptionally vulnerable to variations in how much and how frequently it rains. Floods or droughts often severely affect human life and health, food and water supplies, ecosystems and infrastructure. Such impacts are likely to be felt across the tropics in the coming months as El Niño exerts its influence on global weather patterns. 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. They found that 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. The central estimate gave changes (increases and decreases) of greater than 20% of current rainfall across 25% of tropical land, an area roughly twice the size of Brazil - the largest tropical country.

Simulations run with lower greenhouse gas emissions showed much smaller areas of land with large rainfall changes. The amount of land affected by large rainfall change is strongly tied to how much global temperatures rise (see Figure 1 above), itself largely controlled by the total amount of greenhouse gases that we emit now and in the future. Substantial changes are expected to occur by mid-century.

Which regions are most vulnerable to rainfall changes?

The countries most vulnerable to any future rainfall changes are likely to be those with low present-day rainfall, known as semi-arid regions. The potentially devastating effect of rainfall changes in such regions was seen during the long-term drought in the semi-arid Sahel region of West Africa in the 1970s and 80s, which brought famine to hundreds of thousands of people and huge disruption to millions more. Although not clearly linked to greenhouse gas emissions, the Sahel drought provides a yardstick for the potential impacts of future climate 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 semi-arid 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. Both decreases and increases in rainfall are seen across the tropics, and the impacts of climate change are not expected to be evenly spread.

Why will these changes happen?

Rainfall changes in tropical countries are expected 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. Another example is that climate change will enhance the temperature difference between land and ocean, one of the fundamental drivers of the Indian monsoon.Figure 2: Tropical regions at most risk of large future rainfall changes, under a high emissions scenario at the end of the 21st century. Colours show the percentage of climate simulations that show large increases or decreases at any location.

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. However it is possible to say which countries are at most risk of large rainfall changes, according to the range of climate simulations - this is shown in Figure 2. 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.

More Details

More details may be found in the article by R. Chadwick et al., "Large rainfall changes consistently projected over substantial areas of tropical land", recently published in Nature Climate Change (see 10.1038/nclimate2805).

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