An extreme high temperature may not be caused by human activities, but many times?

  ”It is not magic, nor the activity of the enemy, that makes this damaged world impossible to revive. It is the people themselves who harm themselves.”
  Rachel Carson, American marine biologist, in Silent Spring, 1962 so wrote. In the middle of the 20th century, she discovered that the abuse of pesticides by humans was destroying the ecological environment, leaving the fields, woods and swamps that were full of life with only a silent cover.
  After the book was published, Carson was attacked by politicians and chemical business owners. The book presents a deafening critique of human activities, making the voices of resistance and questioning her in the real world equally deafening.
  How to prove the actual impact of human activities on the ecological environment and global warming? This key proposition always asks the governments, scientists and citizens who advocate environmental protection and energy conservation. Similar to the situation faced by Carson, the author of “Silent Spring”, even in the face of “global warming” that is deeply rooted in people’s hearts today, human beings are still unable to reach a consensus on the consequences of their actions.
  Are the warmer earth and the more extreme weather caused by ourselves? Meteorologists have been proving this issue for nearly 50 years.
  Wang Chunzai, director of the State Key Laboratory of Tropical Marine Environment, South China Sea Institute of Oceanography, Chinese Academy of Sciences, has long studied the impact of ocean and climate on extreme weather events. In recent years, he has studied the causes of super heatwave events in the western United States and Canada in 2021, and found the role of natural variability and human factors on extreme weather.
  Around the role of human activities in climate issues, a reporter shares his scientific approach to studying extreme weather and his findings that changes in Arctic sea ice and the Arctic Circle accelerate with global warming and affect extreme weather in the Northern Hemisphere.
Since the Industrial Revolution, humans have started to burn large amounts of fossil fuels. The rate of emission of greenhouse gases is unprecedented in human history and even in the history of the earth.

  Our conversation started with the high temperature and heat wave that swept the northern hemisphere in 2022, but the core of the discussion was “where will the earth go”. Wang Chunzai said that the rapidly warming earth is unprecedented, and human beings cannot get guidance from historical experience. But inexorably, intense climate warming has become the most severe and realistic environmental challenge facing all mankind.
Will global cooling come?

  South Wind Window: In 2022, China’s Yangtze River Basin will encounter extreme high temperatures, but many areas in my country will experience extreme weather such as heavy rain and drought. Scientific research points out that global warming not only increases the frequency of extreme heat, but also increases the likelihood of other extreme weather. why is that?
  Wang Chunzai: In the case of global warming, heat waves should be more likely to occur and last longer. At the same time, global warming can increase the saturated water vapor pressure of the atmosphere, that is, the higher the temperature, the higher the water vapor can be held in the atmosphere. On the other hand, global warming will change atmospheric circulation and water vapor evaporation in some regions of the world, providing a more abundant source of atmospheric water vapor for precipitation. The synergy of the two aspects increases the probability of extreme precipitation in humid regions.
  The global atmosphere is closely linked. An increase in precipitation in one area will cause a decrease in precipitation in neighboring areas, similar to a “seesaw”. As a result, dry regions may become drier.
  South Wind Window: Looking back at the history of the earth, there were also many moments when the climate suddenly increased. In the past, natural factors and solar activity were the main causes. In your opinion, when did human factors contribute to global warming? Why this time?
  Wang Chunzai: As you said, the Earth’s climate system is not static. Before the formation of human civilization and even human species, the earth’s temperature experienced many violent fluctuations, which promoted the evolution of the earth’s ecosystem. However, it is worth noting that the past climate change was the result of natural adjustment, and the organisms in the ecosystem have always played the role of adaptors, and basically cannot cause changes of magnitude to the climate system.
  However, since the Industrial Revolution, humans have started burning fossil fuels in large quantities. The rate of emission of greenhouse gases is unprecedented in human history and even in the history of the earth. Among them, the concentration of carbon dioxide has nearly doubled in the past one hundred years and reached a peak in the past few million years, which has obviously broken through the evolution of natural systems. The rapid rate of increase in global temperature during this period is obviously not caused by natural variability alone.
  In short, human activity has had an impact on global warming since the Industrial Revolution. The historic increase in greenhouse gases from the burning of fossil fuels is the root cause.
  South Wind Window: Human activities have led to global warming and become one of the factors affecting the formation of extreme weather, which has been accompanied by controversy and suspicion in the past. When did the countries of the world reach a consensus? Why are there so many different views on this in the scientific community?
  Wang Chunzai: As early as the first World Climate Conference held in Geneva in 1979, scientists proposed that the increase in carbon dioxide concentration would cause the earth to heat up. But there is no consensus in the scientific community due to a lack of research data. However, since 1990, with the release of the United Nations Intergovernmental Panel on Climate Change (IPCC) assessment report, the idea of ​​”global warming” has received widespread attention from governments and the scientific community. The view that “human activities cause global warming” has gradually become a consensus in the scientific community.
  The IPCC’s third assessment report, released in 2001, pointed out that global warming “may be” caused by the emission of greenhouse gases by human activities. The IPCC Fourth Assessment Report in 2007 considered it “probable”. The IPCC Fifth Assessment Report in 2013 considered it “very likely”. The sixth assessment report to be released in 2021 clearly states that global warming is “indeed” caused by human activities.
  However, some paleoclimatologists disagree with the idea of ​​global warming. From the perspective of paleoclimatology, since the past 800,000 years, the earth has been in a cycle of “glacial-interglacial”, with an average duration of each cycle of 100,000 years. In each cycle, the “glacial period” was longer and the “interglacial period” was shorter. During the “interglacial period”, atmospheric carbon dioxide concentrations were high, and global surface temperatures were also high. We are currently in an “interglacial period” that has lasted for more than 10,000 years.
  Therefore, some paleoclimatologists believe that this interglacial period may be coming to an end, and the earth is about to enter the next “glacial period”. The current greenhouse gas emissions have exceeded the amplitude of the four glacial-interglacial cycles in the past 800,000 years, which may cause the Earth’s climate system to approach the critical value of a rapid jump in the climate state and enter the next glacial period. Therefore, paleoclimatologists believe that there may be a “global cooling” in the future.
Human activities are the background of frequent extreme high temperature

  Can the extreme high temperature weather that occurs frequently today be expected by meteorologists?
  There have been many researchers in the direction of the atmosphere or ocean, who have studied the changing laws and impact mechanisms of extreme high temperature from different perspectives. In the long run, the frequency and intensity of summer extreme high temperature events in different regions around the world are increasing. Many studies have found that human activities dominated by greenhouse gas emissions have played an important role in the increase of extreme high temperature events in summer.

  According to the IPCC Sixth Assessment Report, human activities have caused a significant increase in the average summer temperature in most regions of the world, and further led to a significant increase in extreme high temperature events. The extreme heat events that occurred once every 50 years between 1850 and 1900 now average 4.8 times every 50 years. When the global average temperature is 4°C higher in the future, such events will occur on average 39.2 times every 50 years.
  South Wind Window: The climate is random and operates in a complex system. When scientists say that “human activities increase the probability of extreme high temperature”, how do they prove it? What are the basic principles and ideas?
When modeling extreme weather events, there is often greater uncertainty. This makes it difficult to accurately quantify the impact of greenhouse gases or human activity on an extreme heatwave event.

  Wang Chunzai: There are two ways to study the impact of climate change on the probability and intensity of extreme weather events. One is to analyze existing observations, extract statistical laws based on historical data, and give reasonable explanations and future predictions based on physical laws.
  However, there is a big difference between earth sciences and physical chemistry biology and other disciplines – we cannot conduct real experiments on the earth itself. Scientists can only give mathematical equations based on known physical laws, and gradually establish numerical models that simulate the climate and even the Earth system.
  Thanks to the rapid development of computers, it has become feasible to use supercomputers to simulate climate. Therefore, the second approach is to use numerical models to modify external forcing conditions such as greenhouse gases, and conduct simulated numerical model experiments to demonstrate the impact of climate change.
  At present, the average temperature of the earth has increased by 1.1 °C compared with the pre-industrial revolution (average from 1850 to 1900). The temperature change caused by natural external forcing (including solar activity and volcanic activity) simulated by the model is relatively stable and does not exceed 0.3 °C. Only when the model simulation includes human activity + natural variability can the simulation and the observed temperature change be achieved. This suggests that global warming is now largely attributable to greenhouse gas emissions from human activities.
  Why is it difficult for meteorologists to say, “Greenhouse gases or human activities have caused an extreme heat wave event”, but instead they say, “Human activities have increased the probability of extreme climate events”?
  The academic community generally uses numerical models to simulate the impact of greenhouse gases or human activities on climate change. However, in practical applications, numerical models are relatively accurate in simulating the mean state, while there are often greater uncertainties when simulating extreme weather events. This makes it difficult to accurately quantify the impact of greenhouse gases or human activity on an extreme heatwave event.
  It needs to be clear that atmospheric circulation is the direct cause of a specific extreme heat wave event, and greenhouse gases or human activities are only the climate background for frequent heat wave events.
The Arctic heats up, the oceans get warmer

  Has your team made any new discoveries in the study of extreme high temperature events around the world in recent years?
  The new results of our study of extreme high temperature events in various places are as follows: First, it is found that the “hot dome” effect caused by abnormal atmospheric circulation plays an extremely important role in the formation and development of the North American super heat wave event at the end of June 2021. .
  Second, seven main atmospheric circulation patterns in the northern hemisphere in summer were discovered and defined, revealing the influence of these seven atmospheric circulation patterns on the spatial distribution of surface air temperature anomalies in the northern hemisphere in summer.

Greenland, Denmark, June 30, 2022, an iceberg floats in Disko Bay in Ilulissat, western Greenland

  We also found that human-led changes in external forcing resulted in a general increase in the length of summer in the northern hemisphere. For every 1°C increase in temperature, summer can be extended by about 15 days. As future global warming continues, the length of summer will reach 1.5 times its 2014 level by the end of the century.
  South Wind Window: One of your findings in your study of the extreme heat in North America in 2021 is that atmospheric circulation anomalies are related to the Arctic vortex, which in turn is related to the loss of Arctic sea ice and changes in the Arctic Circle. In recent years, human observatories in the Arctic have repeatedly recorded high temperatures. For example, on June 20, 2020, the temperature in the Russian town of Verkhoyansk reached a record 38 degrees. What has happened to the Arctic Circle? How do these changes affect extreme weather?
In recent years, due to the Arctic amplification effect, the temperature difference between the Arctic and low latitudes has become smaller and smaller, and the Arctic polar vortex has become more unstable. This change could cause extreme climate events in the Northern Hemisphere by affecting the subtropical westerly jet stream.

  Wang Chunzai: Global warming has caused the rapid disappearance of sea ice in the Arctic Ocean, creating a large ice-free sea surface that absorbs solar energy. This additional energy input, and the corresponding influx of moisture and heat into the Arctic atmosphere, is helping to drive a strong local feedback on global warming known as Arctic amplification. As a result, surface temperatures in the Arctic are rising at twice the rate at lower latitudes.
  The Arctic polar vortex is affected by the temperature difference between the Earth’s high and low latitudes. The north-south temperature difference will lead to the formation of strong high-altitude westerly jets in the mid-latitude subtropics. There is a law in this: The greater the temperature difference between the North Pole and lower latitudes, the stronger the westerly jet stream, and the more stable the Arctic vortex, and vice versa. In recent years, due to the Arctic amplification effect, the temperature difference between the Arctic and low latitudes has become smaller and smaller, and the Arctic polar vortex has become more unstable. This change could cause extreme climate events in the Northern Hemisphere by affecting the subtropical westerly jet stream.
  In addition to extreme heat events, changes in the Arctic polar vortex can also cause extreme cold waves. For example, in the winter from the second half of 2020 to the first half of 2021, the westerly jet stream changed from a stable type to a fluctuating type. The arctic polar vortex moved southward and the cold air moved southwards sharply, triggering three cold wave events that swept across China.
  What do you think is the most difficult part of studying extreme climate events?
  The hardest part is to come up with and verify the hypothesis of the cause of the event. The correlation between extreme events and influencing factors does not represent cause and effect. Just because an event occurs at the same time or is related to an extreme event does not mean that it is the cause of the extreme event.
  In addition, there are few cases of studying extreme climate events, and the physical laws and mechanisms applicable to general anomalous events may not be applicable to extreme climate events. Moreover, there are many factors that affect extreme climate events, and the time scales are also different. It is necessary to study the impact of different factors on extreme climate events on different time scales.
  Research on extreme weather for many years, what are you curious about now?
  I have always been most interested in the impact of the ocean on climate and extreme weather events. In recent years, I have mainly promoted and engaged in the interaction mechanism of the three oceans (Pacific, Atlantic, Indian Ocean) and their impact on climate and extreme weather events.
  The ocean plays an extremely important role in climate change and global warming, but the three tropical oceans do not exist alone, but are closely linked through atmospheric and oceanic processes. Then, in the context of climate change, what will happen to the interaction between the three oceans, and how to regulate global climate change, this is what I am more curious about.

  In a biological mass extinction 250 million years ago (at the turn of the Permian and Triassic eras), a sudden rise in ocean temperatures led to the extinction of the vast majority of organisms. Do greenhouse gases cause warming of the oceans?
  The specific heat capacity of seawater is larger than that of the atmosphere, so seawater temperatures tend to fluctuate much less than air temperatures. Moreover, seawater plays a large role in storing energy during the warming process, slowing down the rise in temperature.
  However, despite the large specific heat capacity of seawater, its stored energy does not disappear into thin air, so that during the brief period of global warming stagnation from 1998-2012, sea temperatures have not stopped rising. In other words, SSTs are actually a better indicator of the global warming signal.
History is no guide, only a joint response

  Climatologists have modelled and predicted different climatic outcomes based on different atmospheric and oceanic models. Which scientists would you favour more? What are the more cutting-edge directions in the current study of human activity and extreme heat?
  The 2021 Nobel Prize in Physics was first awarded to climatologists Syukuro Manabe and Klaus Hasselmann for their contributions to the physical modelling of the Earth’s climate and the science of climate change. Manabe and his collaborators have played an irreplaceable role in climate change research by reliably predicting global warming due to a doubling of CO2 concentrations for the first time and by developing the world’s first climate model that takes into account the three-dimensional atmospheric circulation. Klaus Hasselmann laid the theoretical foundations for the development of attribution studies for climate change detection, providing an effective tool for quantitative estimation of the contribution of human activities to climate change in scientific research today.
Global temperatures have risen strongly in fluctuations during the last century or two of modern times and have been proven by scientists to be directly linked to greenhouse gas emissions from human activities.

  In addition to extreme heat, scientists are now focusing on the hazards of compound extreme events and the impact of human activities on them. the IPCC’s Sixth Assessment Report expands the definition of compound extreme events by providing the first comprehensive and systematic assessment of the attribution of, and future changes in, heat wave and drought compound events, wildfire-related compound weather events (combinations of heat, dryness, high winds), etc. . Current research extends from single extreme events to more influential composite extreme events.
  Are there any extreme heat events in Earth’s history that are more similar to the recent years? Can we use history or what the ancients did to give some inspiration for the future?
  From the perspective of the Earth’s geology and historical temperature changes (10,000 to several billion years), the Earth’s surface has always alternated between ice ages and interglacial periods, i.e. alternating cold and warm temperatures. In geological times, the Paleocene-Eocene extreme heat event 55.5 million years ago occurred against the background of massive carbon emissions from the oceans into the atmosphere. This process lasted between 20,000 and 50,000 years and resulted in a global average temperature increase of between 5°C and 8°C.
  Similar extreme heat events have occurred several times in geological time. The most recent one was the Last Interglacial, a historical period about 130,000 years ago. Scientists have found, based on a large number of ice cores, corals and other reliable proxies, that the temperatures at that time were similar to those of our current Holocene interglacial, and probably slightly higher than those of the modern period.
  The impact of this warming event on the Earth was far-reaching. The Paleocene-Eocene extreme heat event occurred during a period of mass extinction of life in the oceans. Terminal interglacial warming, later confirmed by scientists, led to the continued massive melting of the Antarctic ice sheet and the rise of the sea surface.
  For extreme heat events specifically, it is difficult to obtain proxy information from the Earth’s geological period, but only from historical records and recent modern observations. Overall, the negative impacts of extreme heat are widespread and include many aspects of global ecology, agriculture, the environment, and human health.
  Global temperatures have risen strongly in fluctuations during the last century or two of the modern era and have been proven by scientists to be directly related to anthropogenic emissions of greenhouse gases. As a result, we cannot derive any more valid guidance from human history and the practices of the ancients. At present, strong warming has become the most serious and real challenge facing all of humanity, and requires the cooperation and cooperation of all of humanity to ensure sustainable human development.

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