High-impact weather extremes such as heatwaves, floods, and tropical cyclones exert a profound impact on human society and ecosystems all around the world. East Asia is one of the regions with frequent occurrences of weather extremes. For example, Korea had a record hot and long summer heat wave last year, which affected our society and economy considerably including 48 heat-related deaths. This year, we are experiencing more typhoons than usual, which bring extreme winds and rainfalls, creating devastating impacts on our lives and infrastructures. Thus, understanding past and future changes in these high-impact weather events is critically important to minimize associated damages and prepare for appropriate adaptation strategies.
Observational studies show that the intensity and frequency of weather and climate extremes have been increasing over many parts of the world during recent decades. Extreme temperatures have warmed the mean temperature and occurrences of extreme precipitation have also increased. Have human activities resulted in increases to greenhouse gases and contributed to the observed intensification of climate extremes? To answer this important question, climate scientists devised a method, the so-called “fingerprinting”, which compares observations with climate model simulations performed under different forcing’s. They first estimate space-time patterns of climate responses to a concerned forcing like anthropogenic increases in greenhouse gas emissions, and then investigate whether such “fingerprint” patterns are present in the observations. Using this method, climate scientists identified human influences on the observed increases in temperature and precipitation extremes over the global land and many continental regions. Based on these pioneering studies, in their 2013 assessment report, the Intergovernmental Panel on Climate Change (IPCC) concluded that there is high confidence for human induced increases in warm extremes and decreases in cold extremes.
Searching for human contributions to “weather extreme events” is more challenging than examining the long-term trends in temperature and precipitation extremes averaged over a large area. First, long-term daily observations are scarce. Daily measurements are required to analyze the extreme events that occur in short time scales from days to weeks. Precipitation extremes also have smaller spatial scales, making it harder to get observations. Secondly, our numerical models, which are computer codes developed for simulating Earth’s weather and climate, generally are less skillful at simulating the observed extreme events, providing less confidence in the simulation results. Thirdly, extreme weather events have larger natural fluctuations, more easily affected by internally-driven atmospheric circulations, which hinders us from finding responses in extremes to external forcing. For example, heat waves in Korea are usually accompanied by an anomalous high pressure system with less clouds and increased insolation. This heat wave pattern is expected to become more intense due to background global warming. However, it can also be strengthened or weakened by atmospheric circulations, which occur randomly through complicated air-sea interactions.
In spite of these limitations, climate scientists now can attribute some of the observed extreme events to human activities. Utilizing a novel approach of comparing the “odds” of extreme event occurrences between two conditions, with and without human influences, they have found strong evidences for human influences on the intensified heat waves in many places around the world. The extreme heat waves observed in Korea during 2013 summer and 2014 spring have been attributed to human-induced global warming. We can expect the overall shift of daily temperature distribution to the warmer direction under global warming, and this shift of temperature in turn increases the chances of extreme heat waves. European countries as well as India and other Asian countries are suffering from unusually high temperatures during recent years, and scientists are highly confident that many of those are largely contributed by human-induced global warming. In addition, as oceans are heating up, marine heat wave events are occurring over some ocean basins, which have been found affected by human influences as well. These expanding hot areas in land and ocean are in accord with the poleward expansion of tropical climate zones. In this respect, human influence has caused the early summer heat observed in Korea during 2017, in so far that man-made greenhouse gas increases have more than doubled the probability of a 2017-like ‘spring heat wave’ that started in late May.
As mentioned, the biggest cause of global warming, which bring the increased probability of extreme weather events, is anthropogenic emission of greenhouse gases. Global annual mean CO2 concentration has increased by more than 45% since the Industrial Revolution, from 280 ppm to 415 ppm as of 2019. And fossil fuel burning and land use change are largely responsible for this increase. A recent report by IPCC warns that an additional half degree warming will further increase the chance of extreme events in a disproportionate way. To decrease the adverse impacts of weather extreme events in the coming future, we need to take immediate actions to reduce CO2 emissions. Local community, society, and government all need to work together to avoid dangerous climate changes. Each of us can participate in warming mitigation in everyday life by saving energy, reducing waste, using public transportation, planting trees, and so on. Lastly, it should be noted that extreme heat waves are unavoidable due to the already emitted greenhouse gases. Together with mitigation efforts, our society needs to find the best adaptation options to cope with stronger and more frequent weather extreme events.