Sun Shao, Associate Researcher, Chinese Academy of Meteorological Sciences: Every 1℃ rise in temperature, the saturated water vapor in the atmosphere increases by about 7%, resulting in a significant increase in the frequency, intensity and scope of extreme precipitation events. Global warming will also affect atmospheric circulation patterns, leading to complex changes in the climate system that will further increase the frequency and intensity of extreme precipitation events.

In addition, one direct effect of warming is the northward shift of typhoon tracks, "with warmer water providing an energy source for typhoons, and a weakening of the temperature gradient between the tropics and the temperate zone, meaning that tropical cyclones will be able to transport water vapor more efficiently further inland."

Freddy Otto, co-founder of World Weather Attribution: Climate change does not cause weather events because there are multiple causes behind all weather events, but climate change affects the probability and intensity of weather events. "If a heavy smoker gets lung cancer, we can't say that cigarettes cause cancer, but we can say that the damage they do to the body increases the risk of cancer."

In recent years, weather records around the world have been set more and more frequently, with extreme heat waves and heavy rain, wildfires and droughts lasting for months, and typhoons that are "extremely long standby". An answer is urgently needed - why are extreme weather events on the rise?

That's the question Frederike Otto, a British scientist, is trying to answer. She and a group of researchers from around the world, known as "weather detectives," conduct in-depth, rigorous statistical analysis of extreme weather events around the world and provide scientific explanations as quickly as possible.

On August 10, 2023, local time, a fire broke out on the island of Maui in Hawaii, the United States, and the fire swept through the entire island, and thousands of residents scrambled to flee their homes.

In the early 2000s, a group of scientists became concerned about the impact of climate change on extreme weather events. Today, this field of research has evolved into a separate Science, Climate Attribution Science. This kind of research aims to help the public understand the possible causes of extreme weather events and how human behavior exacerbates the impact of these events in a timely manner, so that the government and the whole society can adjust and improve disaster prevention and relief strategies in advance.

Attribution research offers a glimpse into the future of a warming world, makes the link between ongoing disasters and climate change clearer, and allows us to think in entirely new ways about the relationship between climate and the vulnerability of human societies. Because one of the causes of disaster is ourselves.

For her contributions to the field of climate attribution, Otto was named one of Nature's 10 people shaping science in 2021, and she was also named one of Time magazine's 100 most influential people in the same year.

Yet science has its limits. "We know there will be more extreme weather events in the future because humans are still warming the planet." "We live in a world where the best is for the few at the expense of the many, and so is climate change," Otto said.

Climate change is driving floods

On March 19, Sun Shao, an associate researcher at the Chinese Academy of Meteorological Sciences, visited Southern California. California is nearing the end of a long, months-long rainy season.

Between December 31, 2022 and March 25, 2023, California experienced at least 12 heavy rainfall events driven by Atmospheric rivers, resulting in flooding that killed at least 22 people. The so-called "atmospheric river" refers to a conveyor belt of water vapor formed by atmospheric flow, transferring a large amount of water vapor from tropical and subtropical seas to higher latitudes to form precipitation, which can carry more than 10 times the average flow of the Mississippi estuary, the largest river in the United States.

"The first two weeks I arrived in the United States were rainy and rainy, which was unusual for the country. It's been particularly rainy in California this winter, and I've seen some places outside where the grass has grown taller than people. And this winter, some parts of California have experienced near-zero temperatures. None of this has happened in the last 10 years." Sun Shao told the paper news.

A few months later, North China experienced its most intense rainfall event since 1964, from July 29 to August 1. To some extent, this is also an extreme rainfall event caused by long-distance water vapor transport by "atmospheric rivers".

The direct cause of this event was the combined influence of Typhoon Dusuri and subtropical high, which formed a north-south water vapor transport channel in eastern China. As the water vapor moved northward, it encountered the Taihang Mountains and Yanshan Mountains, forming heavy rainfall in the eastern part of the Taihang Mountains. At the same time, the western Pacific typhoon "Kanu" also sent a lot of water vapor to northern China under the guidance of the subtropical high. Water vapor from the two typhoons converged on the North China Plain, causing the record-breaking heavy rainfall event.

According to Sun Shao, the recent heavy rain in North China and the heavy rain in Henan two years ago have some similarities in origin. Both are due to the joint action of double typhoons and subtropical high, a huge amount of water vapor is continuously transported to the north. After the convergence of the two water vapor conveyor belts, they encounter the effect of topographic uplift, and the convergence and uplift in front of the mountains form a rainstorm.

On August 1, 2023, many places in Beijing fell heavy rain to heavy rain, and the river rose.

"This kind of weather system, as long as it lasts for a few hours to a whole day, is enough to trigger extreme rainfall processes." On the other hand, one of these conditions is indispensable, and if one is absent, the intensity of rainfall will be significantly reduced." "Sun Shao said.

In China, climate change is still a key word rarely mentioned after extreme weather events. However, in the international scientific community, it is a background that is repeatedly mentioned.

One of the current scientific consensus is that climate change can affect rainfall intensity through increased water vapor content in the atmosphere. This is because a warmer atmosphere "contains" more water as global warming causes sea surface temperatures to rise, making evaporation more efficient. For every 1°C increase in air temperature, the amount of water in the atmosphere can increase by 7%.

Therefore, in this case, precipitation will increase. This explains why climate change has led to a global increase in extreme rainfall. However, Sun cautions that an increase in water vapor content can lead to an increase in total rainfall, rainfall intensity and influence area, but there is currently no scientific basis to believe that the duration of rainfall will be prolonged.

The Sixth Report (2021) of the United Nations Intergovernmental Panel on Climate Change (IPCC), the most authoritative body in global climate science, endorsed these conclusions. Extreme rainfall has become more frequent and intense in most parts of the world since the 1950s, especially in Europe, much of Asia, central and eastern North America and parts of South America, Africa and Australia, as a result of blamed climate change, the report further notes. Floods in the above areas are also likely to become more frequent and severe, although there are other human factors that also play a role.

Second, a more complex link comes from the fact that climate change also affects the frequency of heavy rainfall conditions, such as storms and sudden storms, which in turn arise from complex weather phenomena and certain atmospheric circulation patterns.

For example, when talking about the connection between climate change and typhoons, Sun Shao on the one hand agrees that rising SST and increasing water vapor content will make it easier to form strong typhoons, and on the other hand points out that climate change cannot explain the double typhoon phenomenon, "In fact, with global warming, the number of typhoons generated in the past 30 years has not changed significantly.

The direct effect of climate warming is that typhoon tracks move northward; "Warmer waters provide an energy source for typhoons, and the weakening of the temperature gradient between the tropics and the extratropical zones means that tropical cyclones will be able to more efficiently transport water vapor further inland." "Sun Shao said.