Along with a lingering global pandemic, the year 2021 was filled with climate disasters, some of which were so severe that even scientists who study them were taken aback.

Extreme rainstorms turned into raging flash floods in European mountain towns, killing over 200 people. Excessive rainfall inundated large areas across Asia, flooding subway stations in China. In the Pacific Northwest, Europe, and the Arctic, heat waves broke records. Towns in California, Canada, Greece, and Australia were destroyed by wildfires. And those were just a few examples of the extremes.

Damage from the worst climate and weather disasters is expected to exceed $100 billion in the United States alone by 2021.

Many of these extreme weather events have been linked to human-caused climate change, and they provide a preview of what is to come in a rapidly warming world. Something in particular stood out in the United States: a sharp national precipitation divide, with one side of the country overly wet and the other overly dry.

As a climate scientist, I investigate how global warming affects precipitation and the water cycle. Here’s what happened with precipitation in the United States in 2021, and why similar scenarios are likely in the future.

In 2021, the eastern United States was battered by storm after storm. In August, Tennessee experienced record rainfall, resulting in deadly flash flooding. Hurricane Ida’s remnants merged with another front days after the hurricane hit Louisiana, becoming so intense that they set rainfall records and flooded subway stations and basement apartments in New York and Pennsylvania, with disastrous results.

Evaporation from the Earth’s surface increases as temperature rises. It also increases the capacity of the atmosphere to hold moisture at a rate of about 7% per degree Celsius that the planet warms. Global precipitation is expected to increase as more moisture evaporates, but this increase will not be uniform.

As the temperature rises, more moisture is required to reach the condensation level and form precipitation. As a result, light precipitation will be rare. With more moisture in the atmosphere, however, when storm systems do form, the increased humidity causes heavier rainfall events.

Furthermore, storms are powered by latent heat, which is the energy released into the atmosphere when water vapor condenses to liquid water. Increased moisture in the atmosphere also increases latent heat in storm systems, causing them to become more intense. According to research, the frequency and intensity of heavy precipitation events have increased over most land areas since the 1950s.

Because of the global atmosphere circulation pattern, precipitation does not fall evenly across the globe. This global circulation transports moisture to areas where winds congregate, such as the tropical regions that contain the majority of the world’s rainforests, and away from areas where winds diverge, such as the midlatitudes, which contain the majority of the world’s deserts.

Assuming no significant changes in global wind patterns, increased evaporation and moisture will result in more moisture being transported from dry to wet areas and into storm tracks at higher latitudes. Global warming may also alter the global circulation pattern, causing a shift in the world’s wet and dry zones.

Local conditions, such as the shape of the land, the types of plants that grow on it, and the presence of major water bodies, all have an impact on these dynamics.

With the exception of the West Coast, the western United States is dry in part because it is in the rain shadow of mountains. The western mountain ranges force the westerly wind from the Pacific Ocean upward. The air cools as it rises, and precipitation forms on the windward side of the mountains. The moisture has already rained out by the time the wind reaches the leeward side of the mountains. As the wind descends the mountains, the air warms, lowering the relative humidity even more.

Higher temperatures in areas where the moisture supply is already limited result in less humidity in the air, which leads to less rain. Higher temperatures and less precipitation would also reduce snow packs in the mountains, causing spring melt to occur earlier. All of these changes are likely to exacerbate aridity in the Western United States.

The eastern United States, on the other hand, receives an abundance of moisture carried by the easterly trade wind from the North Atlantic and the Gulf of Mexico. With abundant moisture supply, increasing temperature means more moisture in the atmosphere, leading to more precipitation and stronger storms.