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More moisture in the warm atmosphere encourages strong hurricanes and flooding

The summer of 2021 is a clear example of what destructive weather would look like in a warming world. In mid-July, the storm in western Germany and Belgium dropped 8 inches in two days. The flood tore the buildings apart and pushed them through the village streets. A week later, China's Henan Province had a year's rainfall in just three days-more than two feet. Thousands of people fled the river that burst its bank. In the capital city of Zhengzhou, commuters posted videos showing passengers trapped in a flooded subway car. They stretched their heads to the ceiling to touch the last trace of air on the rapidly rising water. In mid-August, the sharp turn of the rapids brought torrential rain to Tennessee, with 17 inches of rainfall in just 24 hours; the catastrophic flood caused at least 20 deaths. None of these storm systems are hurricanes or tropical depressions.

However, Hurricane Ida soon entered the Gulf of Mexico in a whirlpool. This was the ninth named tropical storm in the North Atlantic's busy season this year. On August 28, it was a first-degree storm with sustained wind speeds of 85 miles per hour. Less than 24 hours later, the Ada exploded to level 4, and its ascent rate was almost twice the speed used by the National Hurricane Center to define a rapidly intensifying storm. It hit the coast of Louisiana with winds of 150 miles per hour, causing more than 1 million people to lose power and more than 600,000 people to lose water for several days. Ada's anger continued to the northeast, New York City reached a record 3.15 inches of rainfall in one hour. The storm killed at least 80 people and destroyed large communities in the eastern United States

What all these destructive events have in common is water vapor—a lot. Water vapor (the gaseous form of H2O) plays a huge role in contributing to destructive storms and accelerating climate change. As the oceans and atmosphere warm, more water evaporates into the air. In turn, warm air can retain more steam before it condenses into cloud droplets, causing flooding. Since the mid-1990s, the amount of steam in the global atmosphere has increased by about 4%. This may not sound like much, but it is a big deal for the climate system. The juicy atmosphere provides additional energy and moisture for various storms, including summer thunderstorms, northeast winds on the east coast of the United States, hurricanes and even snowstorms. The extra vapor also helps tropical storms like Aida intensify faster, leaving security officials with little time to warn people in the crosshairs.

Scientists have long anticipated that climate change will produce more air-borne steam, contributing to so-called "steam storms," ​​which release more rain and snow than storms decades ago. The measurement results confirm that in the United States and the world, the impact of heavy precipitation events is increasing and the frequency of occurrence is also increasing. Since the late 1980s, approximately one-third (US$73 billion) of property losses caused by flooding in the United States have been attributed to increased heavy precipitation.

For example, in August 2017, Hurricane Harvey brought incredible 5 feet of rain to some communities in Houston during the five days that Hurricane Harvey lingered in the area, leaving even meteorologists with clear weather speechless. . Sometimes, the rain belt has an astonishing 6 inches of precipitation per hour. An analysis concluded that climate change has tripled the record rainfall and increased its intensity by 15%, especially the moisture-laden air from the unusually warm Gulf of Mexico.

Unlike most other atmospheric gases, water vapor is not uniformly distributed globally. In humid tropical regions across the equator, steam is abundant. From there, long moisture can extend along the path of the storm to the colder, drier poles, bathing the mid- and high-latitude regions in strong and long-lasting precipitation. These rivers of heat and moisture help balance the earth’s atmospheric energy distribution-they are generating intense steam storms along their path.

When we sweat under the scorching sun or boil the pot on the stove in the kitchen, we convert liquid water into steam. The necessary ingredient is calories. Similarly, the heat in the climate system causes water in moist soils, plants, oceans, lakes and streams to evaporate into the air. Steam carries a form of energy called latent heat. If the steam later condenses into a liquid-forming a cloud or dew on the lawn-then the heat will be released into the atmosphere. The resulting hot air bubble is lighter than the air around it, so it will rise. Since the temperature in higher altitude areas is usually lower, the bubbles will continue to rise and grow while condensing additional water vapor into cloud droplets and releasing more latent heat. If you take an airplane through a huge cauliflower-like cloud, you will feel the turbulence created by these rising air towers.

Latent heat is the main fuel that drives the normal onset of hurricanes, thunderstorms and severe weather. The energy contained in latent heat is huge; in a typical hurricane, the amount of heat released in a day is more than 200 times the amount of electricity generated every day in the world. A hurricane can release the explosive power of a 10 megaton nuclear bomb about every 20 minutes.

Perhaps the most worrying consequence of the increase in atmospheric water vapor is its role in the rapid strengthening of tropical storms. Meteorologists say that if the maximum wind speed increases by at least 30 knots (35 mph) in 24 hours, or the central atmospheric pressure of the storm drops by at least 42 millibars in 24 hours, the storm will intensify rapidly. In the past 40 years, the likelihood of a storm increasing rapidly in any given year has increased fivefold. In 2020 alone, 10 Atlantic hurricanes did this: Hannah, Laura, Sally, Teddy, Gamma, Delta, Epsilon, Zeta, ETA, and Aita. By 2021, five of the six Atlantic hurricanes formed as of mid-September have experienced rapid intensification, including Hurricane Ida and Hurricane Nicholas. Recent research is consistent with common sense in physics: as the ocean warms, more water is evaporated and more latent heat is provided to the atmosphere, and rapid strengthening becomes more and more possible. The ocean absorbs about 90% of the heat captured by the additional greenhouse gases emitted by humans. This heat raises the temperature of the water at the surface and deeper below; warm water is like a powerful battery from which storms can draw energy.

However, increasing water vapor is not the only impact of climate change on tropical storms. Reducing wind shear—the difference in wind speed or direction between wind near the ground and wind at high altitude—also benefits the development of storms, because rising air towers are less likely to be torn. Other variables currently being studied include changes in the amount of dust and pollutants in the air, and the difference in atmospheric warming between low and high altitudes, which will affect the rate at which these warm air bubbles rise.

For more than two decades, most parts of the tropical North Atlantic have been unusually warm, causing excessive evaporation and fostering strong hurricanes. Non-tropical storms are also devouring additional steam and energy in the atmosphere, leading to more heavy precipitation events and possibly even greater snowfall.

The threat of increased water vapor is not limited to storms. It also makes summer nights extremely hot and sultry--and more and more frequently, appearing in more and more places.

Since the mid-1990s, the minimum temperature at night in the global land area has been rising faster than the maximum temperature during the day. This is because steam is a greenhouse gas, and more means more warming: the heat that usually escapes into space at night is trapped, thereby preventing the cooling of the earth's surface. Unlike carbon dioxide, which spreads around the world wherever it is emitted, steam tends to stay locally.

More steam also makes hot nights dangerous. Higher humidity at night will prevent your sweat from evaporating-the body's natural cooling system-and make you overheat and affect sleep. One way to measure this discomfort is the thermal index, which combines the effects of temperature and humidity to represent the pressure that the body really feels. An index above about 100 degrees Fahrenheit (38 degrees Celsius) is considered dangerous; long-term exposure can be fatal, especially for the elderly and infants. High temperatures can also put pressure on livestock and pets. Wild animals are adapting, and if possible, they will move to higher latitudes or higher altitudes. Without a period of night cooling, heat will also accumulate in the soil, killing some plants and insects, while allowing other warm-loving species to flourish. According to the 2021 Climate Change and Health Declaration issued by a group of 32 health organizations in August, high temperatures at night will also increase the risk of exposure to insect-borne diseases and threaten humans, animals, and crops.

The dangers of nighttime high temperatures are increasing not only for tropical countries that are already very hot, but also for countries north and south of the equator. U.S. Gulf Coast cities have exceeded the unsafe threshold many times. Since 1970, the temperature in Houston has risen by more than 3.5 degrees Fahrenheit (2 degrees Celsius) because the city is close to the Gulf of Mexico and its expanding development has exacerbated the urban heat island effect. In July 2020, Houston's heat index exceeded 110 degrees Fahrenheit (43 degrees Celsius), far more than miserable.

If greenhouse gases continue to accumulate in the atmosphere, these conditions will soon become commonplace in many southern and mid-latitude cities such as Atlanta and Washington, DC. Before 2000, the U.S. capital experienced an average minimum temperature of over 80 degrees F (27 degrees Celsius) every five years. Since 2000, these nights have occurred roughly twice a year—a 10-fold increase in just 20 years.

However, certain countries in the tropics will suffer, and they are already the most suffering countries. In May 2015, a severe heat wave hit India and Pakistan. The daytime heat index exceeded 114.8 degrees Fahrenheit (46 degrees Celsius) for several days, and high humidity prevented night cooling; more than 3,500 people died from these suffocating conditions. Global warming increases by half a degree, and the number of people threatened by extreme high temperatures in the world will double to about 500 million.

If strong storms and sultry nights are not disturbing enough, water vapor can also make global warming worse. Although carbon dioxide has received most of the attention, water vapor is by far the most important greenhouse gas in the atmosphere. Compared with other greenhouse gases, it absorbs more infrared energy radiated upward from the earth's surface, thereby absorbing more heat. From this perspective, doubling the concentration of carbon dioxide in the atmosphere will warm the earth by approximately 1 degree Celsius. But the feedback loop-the vicious cycle-doubles the temperature. Similarly, although feedback such as the disappearance of sea ice has attracted a lot of attention, the water vapor feedback loop — warming causes evaporation, which absorbs heat and causes more warming — is the most powerful loop in the climate system.

Perhaps counter-intuitively, where water vapor is most abundant, water vapor feedback is the weakest. In humid areas, the infrared energy absorbed by water vapor is close to its physical limit, so adding a little extra moisture has little effect. However, in dry places such as polar regions and deserts, the absorbed infrared energy is far below its potential maximum, so any added steam will trap more heat in the lower atmosphere and raise the temperature.

The increase in the number and duration of Arctic heat waves is a clear symptom of more frequent and longer duration pulses of warm and humid air at low latitudes-these tendrils extend northward from the tropics. For example, in January 2021, the temperature in a large area of ​​the Arctic Ocean was 36 degrees Fahrenheit (20 degrees Celsius) higher than normal. The increase in Arctic heat waves, especially in winter, is slowing the annual freezing of sea ice and causing the loss of ice caps.

The endothermic effect of the additional vapor may be offset by the increase in cloud formation. Clouds reflect the sun's rays (causing a cooling effect), but they also absorb heat. Over the ocean, the cooling effect tends to dominate, but the effect of warming prevails in high latitudes. Recent studies have shown that, on average, the heating effect is greater on a global scale, creating another vicious cycle involving water vapor.

As human activities continue to produce more endothermic gases, the oceans and atmosphere will continue to warm, and more water will evaporate, leading to more frequent steam storms and debilitating steam waves. The strongest hurricanes will occur more frequently, and the rapidly increasing storms will also occur more frequently. Forecasting these quick starts will be a challenge for forecasters. When the storm intensified as it approached landfall, as with Hurricane Ida, officials had little time to issue an alarm, and people would only have a few hours to evacuate.

The main obstacle to predicting these super storms is the lack of temperature measurements below the sea surface. The deep temperature water layer contains more storm fuel than the shallow layer, but the satellite only measures the ocean surface temperature. Researchers are trying to devise methods to determine how much energy is contained in the upper hundreds of feet of water, because this is indeed the source of the storm. They are developing autonomous ocean gliders that can roam the upper ocean at different depths while sampling temperature and salinity. They also use satellite data that can detect changes in the height of the sea: the deeper warm water layer expands relative to the adjacent area, forming a hump on the sea that can be seen from space.

Satellite data is extremely valuable, but we also need instruments across the ocean to measure temperature, steam, and wind. We will continue to rely on "Hurricane Hunter" aircraft to fly into the storm and drop instruments in and around the storm. Researchers feed data from these flights into computer models, which can provide detailed information about the state of the atmosphere and storm intensity. Better data coverage, faster computers, and a deeper understanding of the process of storm formation can all help improve forecasts.

Vapor is produced from countless sources and affects many atmospheric processes. Scientists do not fully understand some of these interactions, and computer models are still difficult to fully predict the effects of water vapor in the changing climate system. The speed at which even seemingly simple water evaporates from the ocean or lake depends on many factors, such as the difference between water temperature and water temperature, how much steam is already in the air, and wind speed. On land, the calculations are more complicated and involve additional variables, such as the moisture content in the soil and the type of plants that are growing. Predicting what will happen when steam enters the atmosphere is another challenge. Will it condense into a cloud, trigger a storm, and then turn into rain or snow? Will it condense on the surface in the form of dew or frost? Will it travel hundreds or even thousands of miles from the tropics to high latitudes? Any errors in these calculations will affect the prediction of future temperature changes and weather patterns.

Increasing water vapor deserves more attention. Unfortunately, we cannot directly control its amount in the atmosphere. However, we can indirectly reduce carbon emissions by controlling warming, which is mainly caused by carbon dioxide and methane emissions, and by removing trees that help absorb carbon from the air. By reducing the heating rate, we can reduce the surge of steam. If we succeed, we can slow the intensification of future steam storms - and the damage they may cause.

This article was originally published in Scientific American 325, 5, 26-33 (November 2021), with the title "Steam Storm"

Changing rain bands. Julian P. Sachs and Conor L. Myhrvold; March 2011.

Jennifer A. Francis is a senior scientist and acting deputy director of the Woodwell Climate Research Center. She has conducted extensive research on the warming of the Arctic and atmospheric steam and energy. She is a member of the "Scientific American" advisory board. Credit: Nick Higgins

Nick Sobczyk and E&E News

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