In the U.S., something in particular stood out: a sharp national precipitation divide, with one side of the country too wet, the other too dry.
As a climate scientist, I study the impact of global warming on precipitation and the water cycle. Here’s what happened with precipitation in the U.S. in 2021 and why we’re likely to see similar scenarios in the future.
This kind of east-west weather divide can be enhanced by La Niña, a periodical phenomenon fueled by Pacific Ocean temperatures that tends to leave the Southwest drier than normal and the North and much of the eastern half of the U.S. wetter.
But something else is going on: Global warming fuels both dryness and extreme rainfall.
3 impacts of global warming on rainfall
Three things in particular happen to precipitation when the planet warms.
1) Global warming leads to more overall precipitation.
Higher temperature increases evaporation from Earth’s surface. It also increases the atmosphere’s capacity to hold moisture at a rate of about 7% per degree Celsius that the planet warms. With more moisture evaporating, global precipitation is expected to increase, but this increase is not uniform.
2) Global warming leads to more intense precipitation.
With higher temperature, more moisture is needed to reach the condensation level to form precipitation. As a result, light precipitation will be less common. But with more moisture in the atmosphere, when storm systems do develop, the increased humidity leads to heavier rainfall events.
In addition, storm systems are fueled by latent heat – the energy released into the atmosphere when water vapor condenses to liquid water. Increased moisture in the atmosphere also enhances latent heat in storm systems, increasing their intensity.
3) Global warming tends to make wet places wetter and dry places drier.
Precipitation is not distributed evenly over the planet because of the global atmosphere circulation pattern. This global circulation brings moisture to places where winds come together, such as the tropical regions where we find most of the world’s rainforests, and away from places where winds diverge, such as the midlatitudes where most world’s deserts are located.
Assuming no significant changes in global wind patterns, increases in evaporation and moisture will mean more moisture is transported from dry areas to wet areas and into the storm tracks at higher latitudes. Global warming could also potentially change the global circulation pattern, causing a shift in the world’s wet and dry regions.
Mountains, moisture and the east-west divide
These dynamics are also affected by local conditions, such as the shape of the land, the types of plants on it and the presence of major water bodies.
The western U.S., with the exception of the West Coast, is dry in part because it lies in the rain shadow of mountains. The westerly wind from the Pacific Ocean is forced upward by the mountain ranges in the West. As it moves up, the air cools and precipitation forms on the windward side of the mountains. By the time the wind reaches the leeward side of the mountains, the moisture has already rained out. As the wind descends the mountains, the air warms up, further reducing the relative humidity.
Higher temperature in areas like these where the moisture supply is already limited means less humidity in the air, leading to less rain. Higher temperature and less precipitation would also reduce snow packs in the mountains and cause earlier melt in spring. All these changes are likely to increase aridity in the West.
The eastern U.S., on the other hand, receives abundant moisture from the North Atlantic and the Gulf of Mexico carried by the easterly trade wind. With abundant moisture supply, increasing temperature means more moisture in the atmosphere, leading to more precipitation and stronger storms.