Why is this year’s heatwave so wild?

  According to the World Meteorological Organization (WMO) and the World Health Organization, heat waves and extreme heat are becoming more frequent and will become the norm in the future, and we will see even more frightening extreme weather. In addition to seriously affecting human health, they can also damage agricultural production, cause water shortages, and threaten human survival.
  In early July this year, German scholars published an article in the Nature sub-published saying that
  Europe has become a “hot spot” for heat waves. In the past 42 years, the frequency of heat waves in Europe has increased at a rate of 3 to 4 times that of the mid-latitude regions of the northern hemisphere such as the United States and Canada.
  This is largely related to the increased frequency and duration of the “dual jets” over Eurasia; 35% of the temperature change in Western Europe can be attributed to the “dual jets”.
  A jet stream refers to a strong and narrow airflow band in the atmosphere with a maximum wind speed of more than 30 meters per second in the center, which is an important feature of atmospheric circulation. Under the influence of certain factors, the jet stream over the European continent is sometimes split in two, forming a double jet stream, which contributes to the production of extreme heat.
  Due to latitude, South Asian countries such as India, Bangladesh, and Pakistan started hot summers in March; after entering April and May, these regions experienced hell-level high temperatures. India experienced its hottest March on record, its hottest April in 122 years, and a scorching experience of nearly 50°C. April in Pakistan was the hottest in 61 years, with a high temperature of 50°C.
  The heatwave also reportedly had serious knock-on effects. The high temperature has caused a surge in demand for electricity from local residents, which in turn overwhelmed the grid: two-thirds of Indian households were without power, and Pakistan was without power for up to 12 hours. Without electricity, many households are without water. The hot weather also raised dust and ozone levels, causing a surge in air pollution in major cities in the region. In addition, mountain glaciers are melting faster due to heat waves, triggering flash floods in Pakistan.
What is a heat wave How does a heat wave come

  Heat waves are actually a relative concept that adapts to local conditions.
  Generally speaking, if a place lasts for 3 or more high temperature days, we can determine that the place has experienced a heat wave, and then delineate the intensity of the heat wave according to the duration. So how is this hot day defined?
  The China Meteorological Administration takes the daily maximum temperature ≥35 °C as the threshold for determining high temperature days, and stipulates that provinces, autonomous regions and municipalities can set their own thresholds according to local weather and climate characteristics.
  The IPCC (United Nations Intergovernmental Panel on Climate Change) standard is based on the historical temperature data of the past few decades, with the 90th percentile as the high temperature day threshold – if the highest temperature in a certain place on a certain day exceeds the historical record for the same period in that place The 90th percentile, which is higher than 90% of the historical data, is a high temperature day.
  Generally speaking, 3 to 5 consecutive high temperature days are weak high temperature heat waves, 5 to 7 days are moderate to strong heat waves, and more than 7 days are strong heat waves.
  The direct driver of the heat wave is the anticyclone.
  As the name suggests, an anticyclone is a reverse cyclone/cyclone. The cyclone is called cyclone in English, and anticyclone is anticyclone; the former is characterized by a low pressure center and high surrounding, and the airflow near the ground converges from the surrounding to the center, and the center airflow rises; the latter is just the opposite, with high center pressure and airflow from The ground subsides upwards, and the airflow near the ground diverges to all directions. Therefore, the anticyclone is also called a high-pressure system.
  Anticyclones are divided into two types: warm and cold. The “subtropical high” we often hear in weather forecasts refers to warm anticyclones; and they are called subtropics because they are usually active in the subtropical regions of the southern and northern hemispheres. (20°~35° north-south latitude).
  The high-pressure center of an anticyclone is like a thermal dome, a pressure cooker lid, where the sinking air is continuously compressed, capturing latent heat previously absorbed by the environment, and often drying out. Those cooler, fast-moving airstreams are pushed outward by the center of high pressure, and even the clouds are pushed out—which results in dryness and little rain, while sunlight hits the ground unobstructed.
  The soil, gravel, concrete and asphalt on the ground are subjected to high-intensity baking by sunlight, and rapidly accumulate heat during the long days and short nights in summer.
  Heat waves are especially common in arid regions, and at high altitudes where high-pressure systems are prone to form. Evaporation is one way to cool the environment, but in arid regions, the water content of the ground, waterways, and vegetation is scarce, and nothing but air can be used to absorb heat.
  ”These heatwaves consolidate themselves,” said Jonathan Martin, a professor of atmospheric sciences at the University of Wisconsin-Madison. “Dry conditions lead to higher temperatures, higher temperatures lead to stronger anticyclones, and stronger anticyclones lead to fewer clouds, There is less precipitation, drier conditions and better heating of the ground by incoming sunlight.”

Wet Bulb Temperature and Heat Island Effect

  Heat waves can complement each other with drought, and they can also work with humidity.
  According to climate experts, the air absorbs 7% more moisture for every 1 degree Celsius warming, and the increased water vapor can indirectly intensify the greenhouse effect and bring about even greater warming.
  In low latitudes with humid environments, such as the Persian Gulf and countries in South Asia, high temperatures tend to increase air humidity, giving local residents a doubling steamer experience and greater health risks.
  If the air is too hot and humid, it will be more difficult for sweat to evaporate and the body to cool down, and the risk of heat stroke is higher.
  A key thermodynamic concept is introduced here, wet-bulb temperature.
  To measure wet bulb temperature, place the thermometer in an environment with a relative humidity of 100% (wrap the thermometer with a damp cloth). Since 100% relative humidity means that water vapor can no longer evaporate and absorb heat, it is measured in this environment. The temperature represents a cooling limit – you want to achieve cooling through evaporation, the lowest value is the wet bulb temperature, and it can’t be lower.
  The highest wet bulb temperature that humans can withstand is 35°C. If this threshold is exceeded, the human body will lose the ability to regulate its own temperature and can only survive for a few hours.
  Heatwaves usually last about 5 days, but can last longer if blocked by high-pressure systems. The high-pressure system eventually weakens, letting in cooler air and precipitation, ending the heatwave. However, as the warm season continues, more high-pressure systems will turn on heat waves.

  In addition, urban areas are covered with natural landscapes such as roads, parking lots and buildings, which can absorb more heat and higher temperatures than the surrounding environment, resulting in an urban heat island effect and aggravating the oppression of heat waves on urban residents.
How climate change is exacerbating heatwaves

  In recent years, scientists have been trying to uncover the link between greenhouse gases-warming-extreme weather through models and experiments. Efforts in this area belong to a branch of climatology—climate attribution science.
  A series of past studies seem to tell us that climate warming is accelerating the pace of extreme heat:

  Generally speaking, if a place has 3 or more high temperature days, we can judge that the place has experienced a heat wave.

  A 2019 study found that a warming climate makes Europe five times more likely to experience a mega heatwave than it would be without warming.
  In 2020, scientists found that the probability of heat waves in the oceans increased by more than 20 times compared to pre-industrial times, and the probability of sustained high temperatures in Siberia increased by at least 600 times.
  Recently, the World Weather Attribution Organization concluded that climate change has increased the likelihood of deadly heat waves in India and Pakistan by 30 times from March to May…
  According to climatologists, the burning of large amounts of fossil fuels produces large amounts of greenhouse gases , the greenhouse effect pushes up the global average temperature, the extreme high temperature also goes up, and the heat wave becomes longer, stronger and more frequent.
  Furthermore, warming effects vary by latitude. Colder regions are warming faster than regions closer to the equator, and the polar regions are warming three times faster than the global average, resulting in more intense heatwaves in the Arctic.
34 Years of Humanity and Extreme Heat

  A study of 13,115 cities around the world found that since the 1980s, residents of these cities have tripled the frequency of exposure to extreme humidity and heat.
  Combining infrared satellite imagery and readings from thousands of ground-based instruments, the authors’ team determined the daily maximum temperature and maximum humidity for more than 13,000 cities between 1983 and 2016, and converted them to daily wet-bulb temperatures.
  They define extreme heat as a wet-bulb temperature of 30°C—the equivalent of a dry-bulb temperature of 41°C—a temperature at which even the healthiest people can’t get enough of their outdoor activities, and the unhealthy can become seriously ill and even die.
  The analysis results show that the frequency of extreme heat exposure of urban residents has increased from 40 billion times per year in 1983 (1 time = 1 person × 1 day) to 119 billion times per year in 2016; 1.7 billion people have experienced extreme heat for many consecutive days. high temperature.
  On a global average, two-thirds of this increase is attributable to rising urban populations, with the remaining one-third attributable to climate warming, but this varies widely across regions and cities.
  The cities with the largest growth are concentrated in low latitudes, especially in South Asia, where India accounts for half of the total growth, followed by Bangladesh; Dhaka, the capital of Bangladesh, Delhi and Kolkata in India are the three cities with the largest growth.
  Dhaka has seen 575 million more extreme heat exposures over the 34 years, 80% of which came from population growth. This “city of mosques” has grown rapidly and is now one of the major cities in South Asia, and its population has ballooned from 4 million in 1983 to 22 million today.
  Cities with population-led growth similar to Dhaka also include Bangkok, Thailand, Yangon, Myanmar, Dubai, UAE, and Shanghai and Guangzhou in China. Of course, population dominance does not mean that these regions have not experienced significant warming.
  In contrast, European cities have stable populations, so the increase in heat exposure is almost entirely driven by warming.
  The study isn’t the academic’s first documentation of the dangers of overheating in cities around the world. In 2020, a scientific team discovered that a hot-humid environment beyond the limit of outdoor human survival, that is, a wet bulb temperature exceeding 35°C, has actually appeared briefly around the world.