Why is rain getting heavier on a warming planet?

Arabia Weather - Why does rainfall increase when temperatures rise? This may seem strange and unexpected, but the physics behind this phenomenon explains why it happens.

One of the strangest effects of climate change is its effect on rainfall patterns. While most people are dealing with the fallout from rising temperatures caused by global warming — considered the deadliest natural disaster in the United States and elsewhere — there is also an increasing risk of extreme precipitation rates.

On average, rainfall amounts on land will increase, and individual storms will become more intense.

Logically speaking, it may not make a lot of sense. But the physics are clear, and this is especially important, especially given the extent of destruction and deadly floods that already existed before climate change.

Think of rain as if the ground were sweating. When your body sweats and moisture evaporates from your skin, it takes heat with it. Likewise, evaporation of water from land and oceans takes heat away from those surfaces. (This cooling does about half the overall job of distributing heat from the planet's surface, keeping it in balance with incoming sunlight.) After the humidity rises, it condenses and falls as rain.

Greenhouse gases in the atmosphere are like a blanket that makes it difficult for Earth to release heat into space. The more greenhouse gases it contains, the “thicker” this blanket becomes. The Earth uses more evaporative cooling, just as you sweat more under a down comforter than a cotton sheet.

“It's a simple matter of energy balance,” says Liz Muir, an atmospheric scientist at the University of Chicago, who studies the impact of climate change on precipitation. "The same physics that gives us the greenhouse effect causes the planet to get rid of more of this energy through evaporation. And because everything that goes up must come down, that means more rain will fall, too."

Atmospheric scientists rely on the Clausius-Clapeyron equation, which states that for every 1 degree Celsius of warming, the air could contain 6% to 7% more water. If nothing else changed, we would expect the same increase in the amount of precipitation. resulting from a particular storm.

However, Muir warns, "The fact that a warmer atmosphere holds more moisture does not tell you how average precipitation will increase. This change is determined by different physics. You can even imagine an atmosphere holding more moisture but not having any." At average rainfall. In this case, you will have more intense storms, but rainfall will occur less often. In other words, more moisture may lead to more moisture without precipitation.

It has historically been difficult for scientists to separate natural rainfall fluctuations from the impact of climate change, says climate scientist Yoo Jeon-ham of Chonnam National University in South Korea (a country plagued by floods). Rainfall is by nature a very complex and variable phenomenon; It is normal for one year to be wetter or drier than the next, regardless of climate change. “Precipitation has a very high natural variability compared to other meteorological variables,” says Hamm. “Precipitation itself is a very difficult variable to detect signals of global warming.”

So, in a recent study, Hamm and his colleagues used a deep learning model to analyze rainfall data, extracting the signal of climate change in recent decades. “We have many instances of heavy rainfall, especially this year in East Asia and the eastern United States,” says Hamm. “We can conclude that this kind of increased occurrence of heavy rainfall is due to global warming.”

The West Coast of the United States will also be submerged. Here, the "atmospheric river" storms that penetrate it feed on moisture as they move across the Pacific Ocean. “When you warm the ocean surface by a degree or something like that, you actually increase the amount of water that comes into California via these atmospheric rivers,” says Rao Kottamarthi, a senior scientist at Argonne National Laboratory who studies precipitation and climate change. “The impact will be felt through additional heavy rainfall in California.”

Heavy rain becomes especially dangerous when the water is flowing quickly. The landscape simply does not have enough time to absorb the deluge, resulting in flash floods. If another storm follows, the soil may already be too wet to accept more water.

This type of risk becomes more serious in areas where there is a lot of snow, such as high altitudes. Earlier this year, one study found that extreme precipitation increases by 15% for every degree Celsius of warming in mountainous and high-latitude regions. This is more than twice what the Clausius-Clapeyron equation suggests.

“When we talk about extreme precipitation — and we look at its impact in terms of severe flooding and damage to infrastructure — it really matters whether the precipitation falls as rain or snow,” says Mohammed Ambadi, a climate scientist at Columbia University. Michigan State University and lead author of the paper said, "What we're seeing is that global warming is not only leading to increased precipitation because there is more water vapor in the atmosphere, but a higher percentage of that extreme precipitation is falling as rain rather than snow."

The risks multiply when there is more rain and less snow. Snow accumulates slowly and can take months to completely melt. Heavy rain releases all that water at once. In mountainous areas, rains can also trigger landslides, such as those that tore through the Himalayas in August. “Based on some preliminary data that people have collected, it appears that having a higher percentage of precipitation falling as rain rather than snow was actually a major factor leading to what happened last month,” Umbady says.

The current infrastructure is simply not built to handle this increasing deluge, and this would put people's lives at risk. In general, urban planners designed city drainage systems to drain rainwater as quickly as possible to avoid flooding. But as rainfall increases, canals and sewers cannot get the water out fast enough.

So the focus is shifting to making cities more “spongy,” with fewer impermeable surfaces where water can accumulate, such as concrete, and more green space so water can seep into underlying aquifers for later use. “We definitely need to change the way we design new infrastructure to be consistent with the change that global warming is bringing, and what will happen 10 years, 20 years, 30 years from now,” says Umbadi.

Source: wired