세종대학교

The Heatwave-Drought Increase in Eurasia Due to Rapid Climate Change and Its Impact on the Climate of South Korea

Professor Jeong Ji-hoon, Senior Researcher Kim Min-seok, Department of Environment & Energy/SAIST Climate Change-Hydrogen Center

1. Introduction

The increase in anthropogenic greenhouse gas emissions has caused the average global temperature to rise by only about 1oC since the Industrial Revolution. However, this has led to fundamental changes across the entire climate system, which resulted in a frequent occurrence of devastating extreme weather events, such as extreme heatwaves, torrential rains, massive floods, arctic cold waves, super typhoons, and large-scale wildfires. What climate scientists are recently particularly concerned about is that these extreme weather events are not merely individually occurring. They are manifesting as compounding climate extremes that occur simultaneously or in sequence, which cause unprecedented degrees of immense damage (Ridder et al., 2020).

Compounding disasters among these where record-breaking heatwaves and droughts simultaneously occur, which subsequently escalate into large-scale wildfires, have recently caused severe damage worldwide. Representative examples include the record-breaking droughts and wildfires in Australia from 2019-2020 (Udy et al., 2024), the large-scale wildfires across the Western United States, such as in California in 2020 (Ayars et al., 2023), and the widespread heatwaves in Western Europe and their resulting increases in deaths and instances of drought in 2003 and 2022 (Stott et al., 2004; Tripathy & Mishra, 2023). The frequency and intensity of simultaneous heatwaves and droughts in East Asia are also on the rapid rise (Choi et al., 2020; Ha et al., 2022). These phenomena profoundly impact agriculture, water resources, ecosystems, and the socio-economy, which unequivocally demonstrate that mitigating global warming and adapting to abrupt climate change are issues that are directly linked to human survival.

It is ultimately necessary to reduce greenhouse gas emissions in regards to mitigating the speed and intensity of climate change in order to minimize the damage from these types pf destructive heatwave-drought compounding disasters. However, the scientific reality is that it could take decades or centuries in order to reverse or alleviate the already ongoing changes in the climate system (Eby et al., 2009). A more realistic short-term countermeasure is to therefore elucidate the mechanisms of these extreme climate phenomena and develop adaptation strategies for early prediction and preparedness of these occurrences. The recent studies show that the increased spatial concentration and simultaneity of these compounding disasters are not just due to regional characteristics. They are closely linked to changes in large-scale atmospheric circulation patterns and land surface conditions, which are altered by climate change.

This study (Jeong et al., 2025) confirmed, which is via observational data, the increasing trends of heatwave-drought occurrences across Eurasia over the recent decades and presents how these types of phenomena are connected to anthropogenic climate change. Furthermore, the changes in heatwaves and droughts are forecasted in this study based on future climate scenarios, which ultimately examined the potential impacts of these changes on South Korea.

2. The increasing trends of heatwaves and droughts in Eurasia over the recent decades

Figure 1. The warming-drying trends and the prominent changes in heatwave-drought occurrence patterns in Eurasia over the recent decades 

(A) The linear trends for the July to August (JA) mean surface air temperature (SAT) and the 6-month Standardized Precipitation Evapotranspiration Index (SPEI6) from 1979 to 2022. The gray dots indicate grids where both the SAT and SPEI6 trends are statistically significant at the 90% confidence level. The SAT trend per decade and the SPEI6 trend per decade are set as the x and y axes of the color matrix in the bottom right, respectively, which represent the corresponding colors. Total number of JA heatwave (days, shaded) and drought (months, contoured) occurrences during 1979–2000The total number of JA heatwave days, which are shown as shaded areas, and the drought months, which are shown as contours, during the period from 1979 to 2000 (B) and from 2001–2022 (C). The gray and black contours indicate regions with over 8 and 14 drought occurrences, respectively, which are more than the Eurasian average frequency during 1979–2000 and 2001–2022 as per Science Advances 2025 (Jeong et al.).

A comprehensive look at changes in the Eurasian summer temperatures and the drought index (SPEI: Standardized Precipitation Evapotranspiration Index), which are shown in Figure 1A. reveals a clear warming and drying trend in Eastern Europe, the Black Sea coast, the Iranian Plateau, the Mongolia-Northern China region, and Ukraine based on observational data since the 1980s. This outcome might be somewhat expected, which is due to global warming that is a result of increased greenhouse gases. This trend is particularly pronounced in semi-arid regions, whereas it appears relatively weaker in Central Asia and high-latitude areas.

The analysis on the heatwave and drought occurrences before and after 2000, which is revealed in Figure 1B-C, shows that the distribution patterns of heatwave-drought occurrences have fundamentally changed across Eurasia over the past few decades. The frequency of heatwave occurrences has sharply increased in areas where they were historically rare, such as European Russia, Southern Europe, inland East Asia, and Southern China, which reached approximately four times their past levels. Droughts have shown a similar trend. The most severe droughts over the past 20 years occurred in Eastern Europe, Southern Europe, the Middle East, North Africa, the Caucasus, Mongolia, and Northern China, which  almost perfectly spatially and temporally align with the increased heatwave occurrences. There was no clear spatial correlation in the past between the main occurrence areas of heatwaves and droughts. These two extreme phenomena are now recently on the simultaneous increase in similar patterns, which show a tendency to become more synchronized.

Many of the previous studies suggest that this simultaneous occurrence of heatwaves and droughts is likely due to enhanced land-atmosphere interactions that stem from dry soil (Miralles et al., 2019; Seo & Ha, 2022). Heatwaves dry out the land surface, which lead to droughts. This increases the release of sensible heat from the ground to the atmosphere as a result, which strengthen the high pressure in the atmosphere and further intensify heatwaves and droughts. The distinct hot and dry trend, which is in particular observed in inland East Asia around Mongolia, also indicates the possibility that the region's climate in undergoing irreversible changes (Zhang et al., 2020). This phenomenon is not limited to just the vicinity of Mongolia, and it is commonly observed across many different regions of Eurasia, which are shown in Figure 1. However, the specific mechanisms of this type of rapid and systematic climate change are not yet sufficiently elucidated. 

3. Atmospheric flow inducing compounding heatwave-drought occurrences: The Trans-Eurasian Heatwave-Drought Train (TEHD)

Figure 2. The Trans-Eurasian Heatwave-Drought Train (TEHD) pattern

 A representative variability pattern of the summer upper tropospheric pressure anomaly during the 1979-2022 period. It is presented as a result of the empirical orthogonal function analysis of the average 250hPa geopotential height data for July-August, and this pattern has rapidly been intensified during this period. The black boxes indicate two regions in inland Eurasia where high pressure is particularly strengthened in this pattern. The square and triangle markers indicate the locations of the tree-ring width data that is used in order to reconstruct the changes in this pattern's intensity from 1741 to the present. Science Advances 2025 (Jeong et al.).

The heatwaves and droughts occur simultaneously across Eurasia, which are in particular in specific regions, so they aren't just a coincidence. They're the result of complex physical interactions that involve large-scale atmospheric circulation and land surface changes. Figure 2 shows the trend of the intensified pressure systems in the upper troposphere over the Eurasian continent in the recent decades. The red areas in the figure indicate the strengthening of anticyclonic circulation, and the blue areas show the strengthening of the cyclonic circulation. The most distinct pattern is a clear wave train that extends from Eastern Europe through Central Asia and to the Kamchatka Peninsula. The most powerful centers within this wave train are the positive pressure anomalies in the Western European Russia region and the Northern China-Mongolia region, which are almost consistent with the areas where heatwaves and droughts have increased.

We define this distinct atmospheric circulation pattern as the Trans-Eurasian Heatwave-Drought Train (TEHD), and we set the magnitude of the pressure anomalies at the two main anticyclonic centers as the TEHD intensity index. We analyzed changes in the TEHD intensity during the period since 1958 based on them, and the observational data is available, which is shown inside the box in figure 3. The TEHD showed a somewhat weakening trend from the 1960s to the early 1990s, but it has almost been continuously strengthening since the late 1990s, which reached historically high levels in the 2000s. The TEHD index notably exceeded ±2 standard deviations (σ) of past variability in 2010, 2016, 2018, and 2022, and record-breaking heatwaves and droughts occurred across Eurasia. For example, the 2010 Russian heatwave caused approximately 550,000 deaths, which was accompanied by drought and wildfires. Acute droughts occurred in only 1-2 weeks due to heatwaves in East Asia in 2022.

Several important scientific questions arose in East Asia in 2022. Is the recent rapid intensification of TEHD over the past two decades, which is in addition to the increase in record heatwave-drought occurrences, truly a result of anthropogenic climate change beyond natural variability? Why does it appear in a specific type of pattern, such as the TEHD assuming that this is the case? Will these compounding heatwave-drought events continue to persist and become the new normal in the future?

4. Changes in the TEHD over the past several centuries via a tree-ring data reconstruction

Figure 3. Long-term changes in the intensity of the past Eurasian Heatwave-Drought pattern (TEHD index), which is represented by a tree-ring and observational data 

The black line represents the TEHD index, which was reconstructed from tree rings, and the red line illustrated the TEHD index, which is represented by observational data. The upper left box shows the TEHD index from 1958-2023, which was derived from observational data. Science Advances 2025 (Jeong et al.). 

It is necessary to analyze long-term observational data that extends back to before the Industrial Revolution in order to identify whether the recent intensification of the TEHD pattern is a result of anthropogenic climate change due to greenhouse gas emissions as well as examine that the change surpasses the natural climate variability. However, climate variables, such as temperature, pressure, and soil moisture were fully recorded only after the mid-20th century, which was when global observation networks were established. A very limited amount of data existed prior to that. Climatology utilizes various types of climate proxy data that record past climates in order to overcome these limitations. Marine sediments, ice cores, fossils, pollen, historical documents, corals, speleothems, and others are used. Tree rings are the most widely employed among them for reconstructing climate variability for over hundreds to thousands of years, because they annually record climate conditions, such as temperature and precipitation with relatively high resolution.

We reconstructed centuries-long TEHD variability in this study by utilizing the data about the growth rings of trees across Eurasia. Trees that specifically grow near the alpine tree line in high-altitude areas are adapted to low temperature and moisture conditions. Growth becomes vigorous and the tree-ring width increases when the temperature is high or precipitation is sufficient in a given year, whereas it narrows in the opposite scenario. Tree rings have traditionally been primarily used for reconstructing temperature or hydroclimatic variables due to these characteristics. Furthermore, atmospheric circulation complexly influences plant growth through temperature, solar radiation, precipitation, and soil moisture, so tree rings are also highly suitable in order to capture the traces of large-scale atmospheric patterns, such as TEHD. For example, subsidence caused by high pressure in its central regions suppresses convection and precipitation and increases solar radiation and temperature when the TEHD pattern is strong, which thereby promotes tree growth. Growth is conversely inhibited when the TEHD is weak. It is accordingly possible to estimate past the TEHD variability.

We extracted tree-ring chronologies from 33 regions across the Eurasian continent from the International Tree-Ring Data Bank for this purpose, which in particular focused on areas deemed sensitive to the TEHD variability. These data sources were evenly distributed across the core and adjacent regions of the TEHD, so they were suitable for reconstructing large-scale and widespread TEHD patterns as opposed to localized signals. We decomposed both the target variable (TEHD index) and the predictor variable (tree-ring indicators) into long-term and short-term variabilities, reconstructed them separately, and then combined them in order to estimate the variations in the TEHD index over the past 282 years by referring to the methodology that is proposed in a previous study (Zhang et al., 2020).

The reconstructed TEHD index, which is similar to the global average temperature increase, began to show a clear upward trend from the early 20th century, which followed the Industrial Revolution. It has notably rapidly increased over the past two decades since the late 20th century, and it reached its highest level in observational history. Four out of the top five years with the strongest observed TEHD in fact occurred from 2010 onward, which included 2010, 2016, 2018, and 2022, and these periods exactly coincide with periods of record-breaking heatwaves and droughts across Eurasia. The TEHD intensity has consistently remained at a level since the 2000s by exceeding approximately 3 standard deviations (3σ) based on the average and variability of the past 300 years. This strongly indicates that the intensification of the TEHD is a result of anthropogenic climate change that extends beyond simple natural variability.

5. Mechanism of the Trans-Eurasian Heatwave–Drought pattern intensification

The recent intensification of TEHD is fundamentally driven by a rise in the baseline temperature within the climate system due to global warming. However, the reasons why anticyclonic circulations, which cause heatwaves and droughts, are intensively enhanced only in specific regions need to be understood through a separate physical mechanism.

Large-scale atmospheric circulation takes the form of numerous superimposed waves, much like ripples on the ocean. Especially as the Coriolis force, resulting from Earth's rotation, acts differently at various latitudes, large-scale atmospheric flows generally meander. At this time, a relatively high-pressure system appears at the center of a clockwise atmospheric flow, while a low-pressure system appears at the center of a counter-clockwise flow. These massive waves, formed by the continuous connection of such atmospheric pressure variations, are called Rossby waves. Typically, Rossby waves, which occur on scales of hundreds to thousands of kilometers, are formed and propagate long distances when air crosses large mountain ranges, when extreme temperature differences arise at the boundary between oceans and continents, or when large-scale convection occurs in the tropics. Under certain conditions, these flows can also stagnate in one region for a long period of time, forming a stationary pressure pattern. The essence of the TEHD pattern is precisely such a stationary Rossby wave, and the high-low-high pressure pattern illustrated in Figure 2 is a typical example of large-scale wave train.

Analysis of observational data reveals that there are primarily two reasons for the recent intensification of the Rossby waves that trigger TEHD. First, as global warming has led to a significant increase in the sea surface temperatures (SST) in the Northwest Atlantic particularly, energy is released from the ocean—which is over 800 times denser than the atmosphere and stores immense thermal energy—into the atmosphere. This is presumed to create perturbations in the upper atmosphere, serving as the seeds for Rossby waves, which then propagate eastward and form a wave similar to TEHD that spans Europe to East Asia. Furthermore, as a natural rising cycle of sea surface temperatures in this region around the 2000s coincided with this, global warming and natural variability seemed to make large-scale waves like TEHD even more prominent. Second, during the same period, increased precipitation in the African Sahel region activated tropical convection. This also released vast amounts of energy into the atmosphere, acting as another source for forming TEHD-like waves. Consequently, the high-pressure anomaly pattern formed in the Eastern Europe-Western Russia region passed over the Black Sea-Caspian Sea and then intensified again into a localized high-pressure anomaly in the high-altitude regions of Mongolia-Northern China.

Notably, the core high-pressure affected regions of TEHD, including the areas around the Black Sea-Caspian Sea, the Caucasus, and Mongolia, are representative semi-arid regions of Eurasia. Here, rapid desiccation occurring alongside the high-pressure system led to strong surface heating, further warming the upper air and promoting an interaction that intensified the high-pressure again. Ultimately, it appears that the combination of global warming and natural variability amplified the Rossby waves originating from the northwest Atlantic as the waves propagated across Eurasia, and thereby led to prolonged heatwaves and droughts lasting for over several weeks.

Figure 4 schematically illustrates the TEHD pattern and its impact.

Figure 4. Occurrence and Impact of the Trans-Eurasian Heatwave-Drought Train (TEHD). 

H and L represent the high-pressure and low-pressure variation observed in the TEHD pattern, respectively. The red dashed lines indicate regions where simultaneous heatwave-drought occurrences have recently surged. The left side shows the sources of the Rossby waves that create the TEHD, which are Northwestern Atlantic warming and Enhanced Sahel Precipitation. 

6. The TEHD in future climate scenarios based on climate models

Will the occurrences of heatwaves and droughts in the Eurasian region continue as the TEHD intensifies in the future? This study analyzed the changes in the TEHD based on the results from the future climate change scenarios that were projected by the Intergovernmental Panel on Climate Change (IPCC)  in order to investigate it.

The analysis results, which are available in Figure 5, revealed that the climate models used for future climate prediction successfully reproduced the spatial pattern of the TEHD and its associated heatwave-drought occurrence trends, which are similar to the observational data. They in particular simulated the distinct strengthening of the TEHD observed in the late 20th century to some extent. This provides stronger evidence that the recent amplification of the TEHD and the increase in Eurasian heatwave-droughts result from anthropogenic climate change.

The future simulation results, which extended to 2100, predict that the TEHD will continue to intensify in all greenhouse gas concentration and socioeconomic scenarios. However, there was a possibility that the intensification trend might gradually ease after the mid-21st century according to the scenarios that assumed significant greenhouse gas reduction efforts (SSP126, SSP245). The TEHD intensity is on the contrary projected to increase 2 to 3 times compared to current levels in scenarios where greenhouse gas reduction fails and high emissions persist. This indicates that if substantial efforts to reduce greenhouse gases lack, the occurrence of compounding heatwave-droughts disasters, such as the TEHD pattern will intensify in Eurasia during the latter half of the 21st century.

Figure 5. The TEHD changes observed in the IPCC CMIP6 future climate simulation 

This shows the temporal changes in the average pressure anomaly in the high-pressure regions at the center of the TEHD from the past-to-future climate model simulation results. Each color represents a different emission scenario experiment, and the larger the number, the higher the greenhouse gas emission scenario it indicates. Science Advances 2025 (Jeong et al.).

7. Impact on South Korea’s climate

The primary regions experiencing a surge in heatwaves and droughts due to the TEHD pattern in East Asia are the inland areas, which span from Mongolia and Northern China. South Korea is also directly and indirectly within the sphere of influence of these changes. Summer heatwaves typically occur in South Korea when hot and humid air flows in from the southwest as the North Pacific High expands after the monsoon season ends. 1994 was a representative case of an extreme heatwave. However, the extreme heatwaves that occurred in 2016 and 2018 in South Korea exhibited different characteristics from the characteristics that occurred in the past. A high-pressure system with relatively hot and dry air extended eastward from the Western Mongolian Plateau or the Tibetan Plateau and combined with the lower-level North Pacific High. The high pressure remained over South Korea for 2 to 3 weeks during those two years, which caused a so-called heat dome phenomenon. This rapidly evaporated soil moisture lead to an acute drought in a short period. Droughts rarely occur during summers with a typical monsoon season. However, as evapotranspiration largely increased due to record-breaking heatwaves, soil moisture rapidly disappeared especially in mountainous areas, which large-scale field crops damage occurred. This even led to wildfires, which rarely occur in the summer, which turned the situation into compounding disasters. A similar severe heatwave occurred in July 2025, which was when this article was written. This demonstrates that this type of rapid climate change is no longer just a scientific research topic. It is a direct threat to our daily lives.

8. Summary and Conclusion

According to scientific evidence based on a tree-ring and observational data, the geographical distribution of summer heatwaves and droughts across the Eurasian continent in recent decades has fundamentally changed. This is because the large-scale atmospheric circulation pattern, which is the Trans-Eurasian Heatwave-Drought Train (TEHD), has strengthened to an unprecedented level. The emergence of the TEHD results from a complex interaction of global warming and natural variability, which is specifically from the Rossby wave activity that is generated by rising sea temperatures in the Northwest Atlantic and increased precipitation in the Sahel region. Furthermore, the interaction between the land surface and the atmosphere served as the mechanism of intensifying heatwaves and droughts mutually, so therefore this trend has therefore been accelerated.

The study findings reveal that human-induced warming creates the TEHD atmospheric circulation pattern, which thereby escalates the extreme risk of compounding heatwave and drought events across Eurasia. These changes are expected to have significant impacts on wildfires, agriculture and food production, water resources, and ecosystems as a whole. Future climate simulations also project that this trend will continue to strengthen. It is therefore urgently necessary to develop systematic mitigation and adaptation strategies for these climate disasters. 

References

Ayars, J., Kramer, H. A., & Jones, G. M. (2023). The 2020 to 2021 California megafires and their impacts on wildlife habitat. Proceedings of the National Academy of Sciences, 120(48), e2312909120.  https://doi.org/10.1073/pnas.2312909120 

Choi, N., Lee, M.-I., Cha, D.-H., Lim, Y.-K., & Kim, K.-M. (2020). Decadal Changes in the Interannual Variability of Heat Waves in East Asia Caused by Atmospheric Teleconnection Changes. Journal of Climate, 33(4), 1505–1522. https://doi.org/10.1175/JCLI-D-19-0222.1 

Eby, M., Zickfeld, K., Montenegro, A., Archer, D., Meissner, K. J., & Weaver, A. J. (2009). Lifetime of Anthropogenic Climate Change: Millennial Time Scales of Potential CO2 and Surface Temperature Perturbations. Journal of Climate, 22(10), 2501–2511.  https://doi.org/10.1175/2008JCLI2554.1 

Ha, K.-J., Seo, Y.-W., Yeo, J.-H., Timmermann, A., Chung, E.-S., Franzke, C. L. E., Chan, J. C. L., Yeh, S.-W., & Ting, M. (2022). Dynamics and characteristics of dry and moist heatwaves over East Asia. npj Climate and Atmospheric Science, 5(1), 49. https://doi.org/10.1038/s41612-022-00272-4 

Jeong, J.-H., Kim, M.-S., Yoon, J.-H., Kim, H., Wang, S.-Y. S., Woo, S.-H., & Linderholm, H. W. (2025). Emerging trans-Eurasian heatwave-drought train in a warming climate. Science Advances, 11(18), eadr7320.  https://doi.org/10.1126/sciadv.adr7320 

Miralles, D. G., Gentine, P., Seneviratne, S. I., & Teuling, A. J. (2019). Land–atmospheric feedbacks during droughts and heatwaves: state of the science and current challenges. Annals of the New York Academy of Sciences, 1436(1), 19–35. https://doi.org/10.1111/nyas.13912 

Ridder, N. N., Pitman, A. J., Westra, S., Ukkola, A., Do, H. X., Bador, M., Hirsch, A. L., Evans, J. P., Di Luca, A., & Zscheischler, J. (2020). Global hotspots for the occurrence of compound events. Nature Communications, 11(1), 5956. https://doi.org/10.1038/s41467-020-19639-3 

Seo, Y.-W., & Ha, K.-J. (2022). Changes in land-atmosphere coupling increase compound drought and heatwaves over northern East Asia. npj Climate and Atmospheric Science, 5(1), 100. https://doi.org/10.1038/s41612-022-00325-8 

Stott, P. A., Stone, D. A., & Allen, M. R. (2004). Human contribution to the European heatwave of 2003. Nature, 432(7017), 610–614. https://doi.org/10.1038/nature03089 

Tripathy, K. P., & Mishra, A. K. (2023). How Unusual Is the 2022 European Compound Drought and Heatwave Event? Geophysical Research Letters, 50(15), e2023GL105453.  https://doi.org/10.1029/2023GL105453 

Udy, D. G., Vance, T. R., Kiem, A. S., Holbrook, N. J., & Abram, N. (2024). Australia’s 2019/20 Black Summer fire weather exceptionally rare over the last 2000 years. Communications Earth & Environment, 5(1), 317. https://doi.org/10.1038/s43247-024-01470-z 

Zhang, P., Jeong, J.-H., Yoon, J.-H., Kim, H., Wang, S. Y. S., Linderholm, H. W., Fang, K., Wu, X., & Chen, D. (2020). Abrupt shift to hotter and drier climate over inner East Asia beyond the tipping point. Science, 370(6520), 1095–1099. https://doi.org/10.1126/science.abb3368