Marshall fire in Boulder County crowns year of climate disasters


By Shuang Ye Wu, University of Dayton

Along with a lingering global pandemic, the year 2021 has been filled with climate catastrophes, some so intense that they have even surprised scientists studying them.

Extreme torrential rains turned into raging flash floods that swept through mountain towns in Europe, killing more than 200 people. Across Asia, excessive rainfall flooded large areas and inundated metro stations in China. Heat waves have broken records in the Pacific Northwest, Europe and the Arctic. Forest fires have swept through communities in California, Canada, Greece and Australia.

The area around Boulder was so abnormally dry on December 30, 2021, that a powerful windstorm started grass fires in the Superior and Louisville neighborhoods, burning hundreds of homes in a matter of hours. Officials said the winds were so strong that firefighters could do little but evacuate homes and businesses on the fires’ paths.


In the United States alone, damage from the biggest climate and weather disasters is expected to total well over $ 100 billion in 2021. Many of these extreme weather events have been linked to man-made climate change, and they offer an overview of what to expect in a rapidly warming world.

In the United States, something special stood out: a strong national split in precipitation, with one side of the country too wet, the other too dry.

As a climatologist, I study the impact of global warming on precipitation and the water cycle. Here’s what happened with precipitation in the United States in 2021 and why we’re likely to see similar scenarios in the future.

The east-west climate divide

The eastern United States suffered storm after storm in 2021. Record-breaking rainfall in Tennessee triggered deadly flash floods in August. The remnants of Hurricane Ida merged with another front a few days after the hurricane hit Louisiana and became so intense that they set rainfall records and inundated subway stations and apartments in sub- ground in New York and Pennsylvania, with devastating consequences. Severe storms hit several states with deadly tornadoes in December.

During this time, most of the West was in a phase of drought, helping to fuel the wildfires that swept through forests and towns.

This type of east-west climatic divide may be reinforced by La Niña, a periodic phenomenon fueled by temperatures in the Pacific Ocean that tends to leave the southwest drier than normal and the north and much of the wetter eastern half of United States.

But something else is happening: Global warming is fueling both drought and extreme rainfall.

3 impacts of global warming on precipitation

Three things in particular happen to precipitation when the planet heats up.

1) Global warming increases evaporation, resulting in increased drying of land and plants and also more overall precipitation.

A higher temperature increases evaporation from the Earth’s surface, drying out vegetation and soils, which can fuel forest fires. It also increases the ability of the atmosphere to retain moisture at a rate of about 7% per degree Celsius as the planet warms. With more moisture evaporation, global precipitation is expected to increase, but this increase is not uniform.

2) Global warming leads to more intense precipitation.

With higher temperature, more humidity is needed to reach the level of condensation to form precipitation. As a result, light precipitation will be less frequent. But with more humidity in the atmosphere, when thunderstorm systems develop, the increased humidity results in more precipitation.

Additionally, stormwater systems are powered by latent heat – the energy released into the atmosphere when water vapor condenses into liquid water. The increase in humidity in the atmosphere also increases the latent heat in thunderstorm systems, increasing their intensity.

Research shows that the frequency and intensity of heavy precipitation events have increased since the 1950s over most land areas.

3) Global warming tends to make humid places more humid and dry places drier.

Precipitation is not distributed evenly across the planet due to the circulation pattern of the global atmosphere. This global circulation brings humidity to places where winds congregate, such as the tropics where most of the world’s rainforests are found, and away from places where winds diverge, such as mid-latitudes where most of the rainforests are found. deserts of the world.

Assuming that there are no significant changes in wind pattern on a global scale, the increase in evaporation and humidity will mean that more moisture will be transported from the areas. dry to wetlands and in storm tracks at higher latitudes. Global warming could also alter the global circulation pattern, causing change in wet and dry regions of the world.

Mountains, humidity and east-west divide

These dynamics are also affected by local conditions, such as the shape of the land, the types of plants found there and the presence of large bodies of water.

The western United States, with the exception of the west coast, is dry in part because it is in the shade of the mountains. The west wind from the Pacific Ocean is pushed upwards by the mountain ranges to the west. As it rises, the air cools and precipitation forms on the windward side of the mountains. By the time the wind hits the leeward side of the mountains, the humidity has already rained. When the wind comes down from the mountains, the air heats up, further reducing the relative humidity.

A higher temperature in areas like these where humidity input is already limited means less humidity in the air, resulting in less rain. A higher temperature and less precipitation would also reduce snowfall accumulations in the mountains and cause earlier spring melt. All of these changes are likely to increase aridity in the West.

The eastern United States, on the other hand, receives abundant moisture from the North Atlantic and Gulf of Mexico carried by the easterly trade winds. With an abundant supply of humidity, the increase in temperature means more humidity in the atmosphere, resulting in more precipitation and more severe storms.

This is shown by the years of recorded precipitation and what is projected for future precipitation based on climate models. Both show a decrease in annual precipitation in the west, which likely means longer periods of drought, and an increase in the east with global warming.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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