Global warming changes precipitation in different parts of the world. Some regions with a lot of rain are drying out, but at the same time, a significant increase in precipitation is predicted for many dry areas.
The nature of the rains is also changing. The rains are becoming more seasonal, and the proportion of heavy rains is increasing.
The international climate panel IPCC has published estimates of changes in soil moisture when the climate has warmed by 2, 3 and 4 degrees compared to the time before climate change.
Soil moisture changes when rainfall increases or decreases, but it is also significantly affected by the increase in evaporation due to warming.
This article describes how precipitation changes in four regions: the Amazon rainforest region, the Sahel and the Arabian Peninsula region, the Mediterranean region, and Finland.
In accordance with the Paris Agreement, the countries of the world aim to limit global warming to 1.5 degrees compared to pre-industrial levels. Finnish Meteorological Institute according to that 1.5 degree limit has already been exceeded from time to time.
Researcher at the Department of Meteorology Kimmo Ruosteenoja estimates that during this century the climate will warm by 2 to 2.5 degrees compared to pre-industrial times. Notably even higher estimates has been presented.
The Amazon rainforest is drying up
The Amazon rainforest region, located near the equator, receives abundant rains, especially during the summer season of the southern hemisphere. However, with the warming, the situation is changing for several reasons, says Kimmo Ruosteenoja.
With warming, the El Niño weather phenomenon occurring in the Pacific Ocean becomes even more prevalent. As a result, rainfall is decreasing in the Amazon region. According to Ruosteenoja, the decrease in rainfall is not large in terms of percentages, but when at the same time warming increases evaporation, the soil dries out.
According to the researchers, the intensifying droughts have apparently already started a cycle in which the forest can no longer recover. Deforestation and conversion of the rainforest to agricultural use worsens the drought.
The water cycle is an integral part of the vitality of the Amazon rainforest. Coastal trees get water from the rain areas arriving from the Atlantic. Trees evaporate water through their leaves, which creates new water vapor, which in turn falls to the ground in the deeper rainforest area.
– One water molecule can circulate in the system 5 to 7 times before exiting elsewhere in the atmosphere or in the Amazon River, says a climate scientist at the University of São Paulo Carlos Nobre for National Geographic magazine.
When the forest is removed for one reason or another, the water cycle is interrupted, making the drought worse.
It is very difficult to make a rainforest grow again after it has been cut or burned once. The humus layer in the rainforest area is very thin, so it may disappear with the rains when the roots of the trees are not binding it. The soil is also in some places very poor in nutrients.
In the Amazon region, it can already be seen that in some areas the former rainforest area has turned into a savannah or even a wilderness.
The Sahel and the Arabian Peninsula may turn green
In Africa, the equatorial region receives a lot of rain. This is because when the sun strongly heats the equatorial region, the air heats up and rises. When the air is high enough, it cools and the moisture in the air condenses into rain.
In turn, the air that has risen high descends, creating a strong high pressure zone in the northern part of Africa. This means that the Sahel region north of the equator is a low-rainfall semi-desert, and the Sahara desert north of it is very dry.
When the climate warms, the conditions change so that the rainy area expands towards both poles, says Kimmo Ruosteenoja.
So the Sahel would already be getting significantly more rain in the coming decades. The same applies to the southern part of the Arabian Peninsula, which has a similar climate.
Potsdam Institute for Climate Research estimated in 2022 that the Sahel could already receive 50 percent more rain in 2040.
The change would be significant. An increase in vegetation would give local residents more opportunities for farming. From the point of view of curbing climate change, it would help with carbon sequestration.
At the same time, the change would be a difficult place for the region’s nature, which has adapted to scarce rainfall over millions of years. The poor countries of the region also face a tough adjustment, because, for example, the building stock has not been made to withstand the rains that are likely to come in torrential downpours. The increasing amount of water should be kept under control.
Last summer was perhaps a foretaste of what is to come, as the Sahel was plagued by long-term floods.
The drought of the Sahara spreads to the Mediterranean
When the rainy area spreads to the north, the climate zones that now prevail in the Sahel and Sahara will also move north due to climate change, says Ruosteenoja.
Central Europe all the way to northern Italy have so far received moderate rains in winter and less in summer. In the future, the amount of rain will decrease, as decreasing air currents will keep the weather dry.
Ruosteenoja also reminds that evaporation caused by warming affects the drying of the soil even more than the decrease in rainfall.
The rains are increasing in Finland
Researchers have estimated that the rains in the summer months in Central Europe decrease all the way to the 60th latitude, i.e. approximately to the level of the Gulf of Finland. North of it, especially in the winter season, the rains increase the further north you go, says Ruosteenoja.
He reminds, however, that the drying of the summer season spreading to Central Europe can also affect southern Finland from time to time.
Otherwise, the situation in Finland is not related to an actual new weather phenomenon. The increase in precipitation is due to the warming of the air. According to the laws of physics, warming of one degree increases the amount of water vapor in the air by about seven percent. Even now, when the climate has warmed by 1.5 degrees, the air humidity has increased by more than ten percent.
When approaching the polar regions, the warming is stronger than on average on Earth.
In Finland, the increase in precipitation increases the snow cover in the north, but in the south, the winter rains come more and more as water. As climate change continues, water rains in Lapland will also become more common.