Credit: © ISTOCKPHOTO /MAC99

Climate change may cause monsoon rains in South America and West Africa to become lighter in spring and heavier in summer. A new study suggests that this is also true for monsoons in Southeast Asia, North America and Southern Africa.

Monsoon rain normally begins in spring, when warm, moist air rolls in from the sea. Anji Seth, at the University of Connecticut, and colleagues used multiple climate models to simulate how monsoon rainfall will change in the twenty-first century1. The results suggest that, if emissions are relatively high, five of the planet's seven monsoons could become drier in spring and wetter in summer. Dry seasons may also become drier: as climate change warms the troposphere, less rain may fall in the months leading up to spring.

As a result, it may take longer for the moisture-laden winds that blow inland in spring to build up enough humidity to trigger the monsoon — delaying its onset. When the rain begins, however, it will be heavier than it is today because warm air can hold more moisture.