Why are we so sure that extreme weather is caused by global warming?

Global warming changes the probabilities of random weather processes.

Written by Wei Ke (Institute of Atmospheric Physics, Chinese Academy of Sciences)

Extreme weather occurs frequently around the world

The high temperatures in June 2023 are destined to go down in Beijing's extreme climate history. The number of high-temperature days (maximum temperature reaching or exceeding 35°C) recorded at the Beijing South Suburb Observatory that month reached 15 days, exceeding the 12 days in 2000, 2018 and 2020. High-temperature warnings have become the norm. In June, the Beijing Meteorological Bureau issued 1 high-temperature blue warning, 7 high-temperature yellow warnings, 3 high-temperature orange warnings and 1 high-temperature red warning; on June 22, 23 and 24, the highest temperature exceeded 40°C for three consecutive days, and the temperature at the Beijing South Suburb Observatory reached 41.1°C, 40.3°C and 40.0°C respectively, setting a new record of extreme high temperatures of 40°C for three consecutive days in the history of modern meteorological observations in Beijing.

High temperatures affect all countries equally. In mid-to-late June, many states in the United States experienced power shortages and even large-scale power outages. For example, the city of Jackson, Mississippi, was without power for nearly 100 hours. (Due to the raging wildfires in Canada, large areas of the Midwest and Eastern United States are suffering from severe air pollution.) High temperatures occurred simultaneously in many places around the world. On June 21, the highest temperature in Nuevo Laredo, Mexico exceeded 45°C for three consecutive days, and the highest temperature in Monclova exceeded 46°C for three consecutive days; the highest temperature in Nokundi, Pakistan reached 49°C; the highest temperature in Zabol, Iran reached 50.8°C, becoming the highest in the world that day.

Extreme heat is just the tip of the iceberg of extreme weather disasters around the world. The "disaster package" brought by global warming also includes heavy rains, floods, droughts, wildfires, stronger typhoons, ocean acidification, rising sea levels, etc. Since June, heavy rains in Haiti have caused at least 42 deaths and displaced about 19,000 people; Esmeraldas Province in Ecuador received a month's worth of rainfall in 12 hours, causing floods and affecting tens of thousands of people; wildfires in Canada spread, burning an area of ​​more than 77,000 square kilometers; Cyclone "Billjoy" landed on the coast of India, killing 2 and injuring 23; Vladikavkaz, the capital of the Republic of North Ossetia-Alania in Russia, declared a state of emergency due to heavy rains.

Why is extreme weather occurring because of global warming?

Why are we so sure that extreme weather and global warming are inseparable? This is because in the past few decades, the climate field has developed relatively mature methods for extreme weather detection and attribution analysis. The idea is not complicated. If you roll an ordinary dice, the probability of getting the maximum value of 6 is one in six; if you roll a 6 once, you can attribute it to random luck, but if you roll it many times, the probability of a 6 is much greater than one in six. At this time, you can’t attribute it to luck. It is very likely that the dice itself has been tampered with. The same is true for extreme weather. Simulations are performed with and without global warming: if the probability of a certain extreme weather occurring in the simulation with global warming is greatly increased, it cannot be attributed to random (non-linear) weather processes, but rather that the increasing global warming has changed the probability of such extreme weather occurring.

In 2004, Peter Stott and others from the Hadley Center of the UK Meteorological Office published an article in Nature [1], which analyzed the causes of the extreme heat wave in Europe in 2003. This extreme heat wave caused more than 70,000 deaths in Europe. Their analysis pointed out that due to global warming, the probability of such extreme heat waves in 2003 has doubled. Therefore, the "dice" of extreme heat waves has been changed by global warming. When such extreme heat waves occur repeatedly, they can no longer be regarded as the result of purely random weather processes.

In the past few years, the International Weather Attribution Team (WWA) has conducted attribution analysis on global extreme droughts, high temperatures, heavy rains, cold waves and storms, and found that global warming can be found behind almost all kinds of extreme weather in the world. For example, in August 2022, the highest temperature in London exceeded 40°C; at the end of June 2021, the highest temperature in Lytton, Canada reached 49.6°C. Analysis shows that such events are almost impossible to occur without global warming [2, 3]. In March 2022, the extreme high temperatures in India and Pakistan in South Asia broke the 122-year historical record. Simulation analysis shows that climate change has increased the probability of such events by 30 times [4]. If we still use the metaphor of rolling "dice" to describe the weather process, this aspect of extreme high temperatures has been deeply transformed by global warming.

According to the attribution analysis of the "720" heavy rainstorm in Zhengzhou, Henan Province in 2021 by the Institute of Atmospheric Physics of the Chinese Academy of Sciences, climate warming and wetting have doubled the probability of heavy rainstorms in Henan Province, and the rainfall intensity has increased by about 7.5%. Don't underestimate this 7.5%, as it may be the excess precipitation that causes the most serious disasters. By the end of this century, if estimated according to the medium emission scenario, the precipitation intensity will increase by another 21.9% compared with the recent period, and the probability will increase by another 4 times. Extreme rainstorms such as the Zhengzhou rainstorm are also a "dice" that has been deeply transformed by global warming.

The reason why extreme weather occurs frequently is that when global temperatures rise, more heat enters the climate system, especially the atmosphere can accommodate more water vapor. Therefore, when rainfall occurs, because there is more water vapor, it is easy to cause heavy rain and floods. For arid areas, it is more difficult for the air to reach saturation, so droughts will become more severe. This is the "wet to wetter, dry to drier" effect brought about by global warming [5]. In addition, more latent heat is released during the condensation of water vapor, which will make storms stronger, resulting in more severe severe convective weather such as squall lines, downbursts, hail, and tornadoes. According to the United Nations Office for Disaster Prevention and Reduction's "Human Costs Caused by Disasters 2000-2019" report, in the past 20 years, the number of extreme high temperature events has increased significantly by 232%, floods have increased by 134%, storms have increased by 97%, wildfires have increased by 46%, and drought events have increased by 29% - we have fully entered a new stage of frequent disasters.

With the arrival of summer in the northern hemisphere, extreme high temperatures will become the norm, and the World Meteorological Organization calls on countries to warn early and take early action. For governments and management departments at all levels, in addition to providing weather warnings and forecasts, they also need to pay more attention to the rights and interests of vulnerable groups, outdoor workers and workers in hot weather, and provide public heat-avoidance centers, especially during orange and red high-temperature warnings, open public activity centers, government departments, libraries, etc., so that outdoor workers can avoid the hottest weather at noon. For the public, they need to pay attention to various forecasts and warning information, and update the latest forecasts and warnings in a timely manner to reduce the risk of heat stroke.

References

[1] Stott, PA, Stone, DA, Allen, MR 2004. Human contribution to the European heatwave of 2003. Nature, 432, 610-614.

[2] Philip, SY, Kew, SF, van Oldenborgh, GJ, Anslow, FS, Seneviratne, SI, Vautard, R., Coumou, D., Ebi, KL, Arrighi, J., Singh, R., van Aalst, M., Pereira Marghidan, C., Wehner, M., Yang, W., Li, S., Schumacher, DL, Hauser, M., Bonnet, R., Luu, LN, Lehner, F., Gillett, N., Tradowsky, JS, Vecchi, GA, Rodell, C., Stull, RB, Howard, R., Otto, FEL 2022. Rapid attribution analysis of the extraordinary heat wave on the Pacific coast of the US and Canada in June 2021. Earth Syst. Dynam., 13(4), 1689-1713.

[3] Zachariah, M., Vautard, R., Schumacher, DL, Vahlberg, M., Heinrich, D., Raju, E., Thalheimer, L., Arrighi, J., Singh, R., Li, S., Sun, J., Vecchi, G., Yang, W., Seneviratne, SI, Tett, SFB, Harrington, LJ, Wolski, P., Lott, FC, McCarthy, M., Tradowsky, JS, Otto, FEL 2022. Without human-caused climate change temperatures of 40℃ in the UK would have been extremely unlikely.

[4] https://www.worldweatherattribution.org/climate-change-made-devastating-early-heat-in-india-and-pakistan-30-times-more-likely/

[5] Held, IM, Soden, BJ 2006. Robust responses of the hydrological cycle to global warming. Journal of Climate, 19(21), 5686–5699.

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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