The high temperature of 39℃+ is here again. It has become a "furnace" before the beginning of the dog days. Why is it so hot this summer?

This June, many areas in northern my country seem to be particularly hot. Take Beijing as an example. June has not yet ended, and Beijing's number of high temperature days has set a new record. As of June 27, the number of high temperature days in June at the Beijing South Suburb Observatory (the highest temperature reached or exceeded 35°C) has reached 13 days, exceeding the 12 days in 2000, 2018 and 2020.

Actual picture of the highest temperature in China in the past 30 days. Image source: China Meteorological Administration

Among them, on June 22, 23 and 24, the temperature at Beijing Nanjiao Observatory reached 41.1℃, 40.3℃ and 40.0℃ respectively, with the highest temperature exceeding 40℃ for three consecutive days, setting a new record of 40℃ for three consecutive days in the history of Beijing's meteorological observation. In contrast to the exciting new records in the Olympic arena, the "new records" of extreme weather do not make people excited at all, because they often represent disasters and crises.

The picture shows the high temperature red warning text message received by Beijing citizens on June 23. This is the first high temperature red warning issued by Beijing since 2015

At the same time, Beijing's precipitation in June also set a new record. Since the rain on May 27, Beijing has not had any decent precipitation. From June 1 to June 26, the cumulative precipitation at the Beijing Observatory was only 1.1 mm, a significant decrease of 98% compared with the normal value of 55 mm in the same period, and far less than 50.7 mm in the same period of 2022. 1.1 mm of precipitation is usually only seen in extremely dry winters, and it is extremely abnormal to occur in June in summer.

Whose fault is it that the weather is so abnormal?

1. Is El Niño a basket that can hold anything?

The occurrence of extreme high temperature weather is closely related to both short-term weather conditions and long-term climate anomalies.

In terms of short-term weather, during the high temperature period, Beijing, Tianjin and Hebei are controlled by warm air masses, and there is a broad high-pressure ridge in the northwest-Mongolia-North China area, which leads to long periods of sunny and hot weather. In addition, the longest daytime hours after the summer solstice are conducive to maintaining high temperatures. At the same time, during this period, southern my country is in the plum rain season, and the rain belt is maintained from Guangdong and Guangxi to the Yangtze River Basin for a long time, which blocks the transport of water vapor to the north, resulting in sunny weather in the north and rainy weather in the south, forming a unique period of the year in my country when the temperature in the north is higher than that in the south.

On a longer time scale, the most frequently mentioned concepts for climate anomalies are El Niño and global warming, especially El Niño. There is a joke in the industry that "El Niño is a basket that can hold anything." El Niño is the "naughty child" that stirs up global climate anomalies. This naughty child has been behind abnormal events such as the locust plague in Africa and the wildfires in Australia in the past few years.

Specifically in East Asia, when El Nino occurs, the East Asian monsoon is weak, and the rain belt is weak and faltering in its northward movement. Therefore, the plum rain season in the south of the Yangtze River and the middle and lower reaches of the Yangtze River started late this year, around June 18, nearly 10 days later than the annual average (June 9) in the south of the Yangtze River, and about 4 days later than the annual average (June 14) in the middle and lower reaches of the Yangtze River. The rain belt remains in the southern region, and for the north, it is mainly sunny, which naturally exacerbates the high temperature and drought conditions in the north.

However, the impact of El Niño on my country's climate is relatively complex. Taking extreme high temperatures as an example, the past three super El Niños occurred in 1982/83, 1997/98 and 2015/16. Whether it is the El Niño development year (1982, 1997, 2015) or the El Niño decay year (1983, 1998, 2016), taking Beijing as an example, the number of high temperature days in June and summer did not set a record, but only maintained at the average value (about 4 days in June and about 11 days in summer). In terms of rainfall, the above-mentioned El Niño development year and El Niño decay year did not experience extreme drought. In fact, the average precipitation of only 1.1 mm in June 2023 is the lowest value since Beijing has observed records. Its cause still needs to be analyzed in depth by scientific researchers. El Niño alone cannot explain this extreme situation.

2. The East Asian monsoon is like a child being jostled in a crowd, with a complex and ever-changing trajectory

East Asia spans the mid-latitudes. Climate fluctuations are affected not only by the thermal conditions of the oceans in the tropics, but also by high-latitude systems. In addition, East Asia's climate is also affected by monsoon anomalies and the tall Qinghai-Tibet Plateau. Therefore, in addition to the El Niño-Southern Oscillation (ENSO), the influencing factors include the thermal conditions of the northwest Pacific warm pool, the Indian Ocean sea temperature, the winter and spring snow cover of the Qinghai-Tibet Plateau, the Arctic sea ice, the North Atlantic sea temperature, the Eurasian circulation and other factors.

Affected by multiple factors, the East Asian monsoon is like a child being jostled in a crowd. It is very likely to deviate from its original direction, and its movement trajectory is complex and changeable. Therefore, the East Asian monsoon climate is like "love in novels, an eternal and fascinating topic", and the study of the mechanism of its abnormal changes has always been the core issue of my country's climate research.

In addition, just as "there are no two leaves in the world that are exactly the same", the factors that cause climate anomalies themselves also vary. Each event is not exactly the same, and their joint impact is diverse.

For example, even for the same El Niño event, the intensity and spatial distribution of each El Niño are different, and its impact is also different. In particular, El Niño occurs in the equatorial central and eastern Pacific Ocean, thousands of miles away from my country. The spatial range of the El Niño abnormal disturbance is often greater than the distance between the Yangtze River and Huaihe River basins in my country (about 200 kilometers), and even greater than the distance between the Yangtze River and the Yellow River basins (about 500 kilometers). The impact of each El Niño event on my country is also different.

In the past three years, the global climate has experienced a rare triple La Niña phenomenon. For three consecutive winters, 2020/21, 2021/22, and 2022/23, the equatorial central and eastern Pacific maintained a relatively cold sea temperature. However, the summer climate in my country varied greatly during these three years.

In 2020, my country's Yangtze River Basin suffered the strongest rainstorms and floods since 1998. The Wangjia Dam on the Huaihe River was opened to divert floodwaters, and the Xin'anjiang Reservoir released floodwaters through nine sluices. In 2022, the Yangtze River Basin experienced the most severe drought and high temperature since records began. Poyang Lake and Dongting Lake dried up during the flood season. The Sichuan and Chongqing regions were hit by drought, water shortages, and power shortages, and suffered from wildfires. There were no major disasters in the Yangtze River Basin in 2021, but the "720" torrential rain occurred in Zhengzhou, Henan, Beijing experienced the rainiest month in September, and Shanxi suffered severe autumn floods during the National Day holiday.

Therefore, the same climate influencing factors can cause very different results, and the El Niño "basket" is not easy to use.

It is worth noting that although the World Meteorological Organization (WMO) and meteorological departments at home and abroad have predicted that El Niño is about to occur, there is no final confirmation that El Niño has "occurred".

For example, the World Meteorological Organization released an assessment report on May 3, pointing out that the probability of transitioning from a neutral state to El Nino from May to July this year is 60%, and the probability will increase to about 70% from June to August, and to about 80% from July to September. The above are all possibilities, not declarations that El Nino has already occurred.

This is because the condition for the occurrence of the El Niño phenomenon is that the sea temperature anomaly in the equatorial central and eastern Pacific (commonly used NINO3.4 sea area) exceeds 0.5℃ for three consecutive months. Currently, only the data in May and June meet the standard of exceeding 0.5℃, and there is still time to go before it reaches three consecutive months. There are also cases of El Niño "false alarms" in historical records, that is, the sea temperature in the equatorial central and eastern Pacific is temporarily high in spring and summer, but returns to neutral in autumn and winter. For example, in 1980 and 1993, the sea temperature anomaly exceeded 0.5℃ at the turn of spring and summer, but returned to neutral in autumn and winter. These cannot be counted as El Niño events.

Even if the SST anomaly exceeds 0.5℃ for three consecutive months in May, June and July, and if the SST anomaly turns around in autumn and winter and does not exceed 0.5℃ at the end of the year, it cannot be regarded as an El Niño event. Therefore, those who swear that El Niño has occurred have a misunderstanding of the possibility stated by the WMO.

NINO3.4 sea temperature anomaly (solid line) and forecast (dashed line), from the FGOALS-f2 weather-climate dynamical ensemble forecast display platform of the Institute of Atmospheric Physics, Chinese Academy of Sciences (http://project.lasg.ac.cn/FGOALS_f2-S2S/index.php?Var=P-Enso)

3. Who tampered with the “dice” of extreme weather?

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The recurrence of record-breaking extreme heat is an inevitable consequence of global warming.

According to the annual climate report released by the World Meteorological Organization (WMO) on April 21, 2023, the global temperature in 2022 was 1.15°C higher than the pre-industrial period of 1850-1900, making the past eight years the hottest eight years in recorded history. "When the big rivers rise, the small rivers also fill up." As the global temperature rises, the temperature in the regions will naturally rise accordingly. If we subtract 1.15°C from the daily temperature in June this year, the temperature is not extreme in historical data.

In order to analyze the impact of global warming on a particular extreme weather event, over the past 20 years, the climate science community has developed a mature method of "extreme weather monitoring and attribution."

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. However, if you roll many times and the probability of getting a 6 is much greater than one in six, you can no longer attribute it to luck. It is very likely that the dice itself has been tampered with. The same is true for extreme weather. Simulate with and without global warming. If the probability of a certain extreme weather greatly increases in the simulation with global warming, it cannot be attributed to random (non-linear) weather processes, but rather that global warming has changed the probability of such extreme weather.

In 2004, Peter Stott and others from the Hadley Center of the UK Meteorological Office published an article in Nature, analyzing the causes of the extreme heat wave in Europe in 2003, which caused more than 70,000 deaths in Europe. Their analysis pointed out that due to global warming, the probability of such extreme heat wave in 2003 has doubled. Therefore, the "dice" of extreme heat wave has been changed by global warming. When such extreme heat wave occurs, it can no longer be regarded as a purely random weather process.

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 almost all kinds of extreme weather in the world can be traced back to global warming. For example, in August 2022, the highest temperature in London exceeded 40°C, and 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. 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. This is a deeply modified extreme high temperature "dice".

Global warming not only brings extreme high temperatures, but also a "full set" of disasters. Global warming brings about the important "wet to wet, dry to dry" effect, that is, originally humid areas will have more precipitation, and originally dry areas will become drier; in terms of seasons, floods are more serious in wet and rainy seasons, and droughts are more serious in dry and less rainy seasons.

The specific process and affected areas are very complicated. A simple explanation is that when the temperature rises, the atmosphere can hold more water vapor (saturated specific humidity increases). Therefore, when heavy rain occurs, there is more water vapor in the atmosphere, resulting in more intense precipitation. When no rain occurs, because the air can hold more water vapor, the air is less likely to be saturated, it is more likely to cause drought and high temperatures.

Many people have experienced being scalded by water vapor. They know that when water vapor condenses from a gas to a liquid, a large amount of heat (latent heat of condensation) is released. When it rains, because there is more water vapor in the atmosphere, more heat is released by condensation. Therefore, severe convective weather will be more violent, and corresponding tornadoes, hail, strong winds, lightning, heavy rains, floods and other phenomena will be more serious.

According to the United Nations Office for Disaster Prevention and Reduction's "Human Costs from Disasters 2000-2019", in the past 20 years, extreme heat events have increased by 232%, floods have increased by 134%, storms have increased by 97%, wildfires have increased by 46%, and droughts have increased by 29%. Global weather is becoming increasingly extreme.

According to the attribution analysis of the "720" heavy rainstorm in Zhengzhou, Henan in 2021 by the Institute of Atmospheric Physics of the Chinese Academy of Sciences, climate warming and wetter climate has doubled the probability of heavy rainstorms in Henan, 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 disaster.

By the end of this century, if estimated based on a medium emissions scenario, precipitation intensity will increase by another 21.9%, and the probability will increase by 4 times. Extreme rainstorms like the Zhengzhou rainstorm are also a "dice" that has been deeply transformed by global warming.

Attribution analysis of extreme weather has potential legal application prospects. Previously, personal encounters with extreme weather were considered "bad luck" or personal negligence, and failure to pay attention to weather forecasts and warnings. Now attribution analysis tells everyone that such extreme weather would not have occurred, but was caused by global warming caused by human greenhouse gas emissions. Global warming has greatly increased the probability and intensity of certain extreme weather. Therefore, individuals and organizations damaged by certain extreme weather can claim compensation from organizations and countries that caused global changes.

During the 2022 Sharm Sheikh Climate Conference (COP27) in Egypt, "Loss and Damage" was included in the official agenda for the first time to discuss whether wealthy countries with historical emissions responsibilities need to make climate compensation to vulnerable countries affected by global warming. The conference established a "Loss and Damage" fund, which is an important step towards global climate justice.

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. Huang Ronghui. 2010. Research progress on the variability of East Asian monsoon climate system and the mechanism of major climate disasters in China. Bulletin of the Chinese Academy of Sciences, 25 (02), 202-206.

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

3. Ling, S., Lu, R. 2022. Warmer SSTs in the Equatorial Eastern Pacific Delay the Seasonal March of the Asian–Pacific Summer Monsoon Onset. Journal of Climate, 35 (23), 7853-7870.

4. Palmer, PI, Wainwright, CM, Dong, B., Maidment, RI, Wheeler, KG, Gedney, N., Hickman, JE, Madani, N., Folwell, SS, Abdo, G., Allan, RP, Black, ECL, Feng, L., Gudoshava, M., Haines, K., Huntingford, C., Kilavi, M., Lunt, MF, Shaaban, A., Turner, AG 2023. Drivers and impacts of Eastern African rainfall variability. Nature Reviews Earth & Environment, 4 (4), 254-270.

5. 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.

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

7. Wang, J., Chen, Y., Nie, J., Yan, Z., Zhai, P., Feng, J. 2022. On the role of anthropogenic warming and wetting in the July 2021 Henan record-shattering rainfall. Science Bulletin.

8. 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 40oC in the UK would have been extremely unlikely.

9. Zhang, W., Mao, W., Jiang, F., Stuecker, MF, Jin, F.-F., Qi, L. 2021. Tropical Indo-Pacific Compounding Thermal Conditions Drive the 2019 Australian Extreme Drought. Geophysical Research Letters, 48 ​​(2), e2020GL090323.

Produced by : Science Popularization China

Author : Wei Ke, Institute of Atmospheric Physics, Chinese Academy of Sciences

Produced by : China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.