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Summers are getting hotter and drier in the Eurasian landmass due to an atmospheric circulation pattern further aggravated by anthropogenic factors. The recent tandem heat-wave-drought events in the region stretching from Eastern Europe to East Asia are unprecedented, as confirmed by that analyzed tree-ring data going back 300 years and climate models.

The shift can have some serious consequences for the environment, affecting wildfires, agriculture, food security, water resources, and ecosystems, note the researchers of this study, published in Science Advances.

Research suggests that because of how heat and moisture interact in the atmosphere, and this thermodynamic relationship shows up at different periods. Since high temperatures often occur when there's little to no rainfall, it is reasonable to expect that heat waves and droughts can happen concurrently due to their shared dependence on atmospheric conditions.

It has also been observed that such heat-wave-drought compound events are fueled by persistent high-pressure systems that trap more heat and accelerate the evaporation. These processes also play a role in dictating larger atmospheric circulation patterns.

The geographical hotspots for heat-wave-drought compounds have primarily been areas like Australia and the western part of North America, and Europe. However, over the last couple of decades, the frequency of heat-wave-drought cycles has increased across Eurasia.

This raises the question of whether the shift in pattern is merely an outcome of local climate features or is global warming actively shaping the atmospheric conditions to drive this change?

To answer this question, the researchers collated daily temperature data from a dataset covering the years 1958 to 2022. They defined a heat wave as a period of unusually high temperatures lasting at least three consecutive days and analyzed heat wave data during the summer months (July and August) from 1979 to 2022. They also used data from a global drought index, covering the period from 1901 to 2022.

Trees are sensitive to local climatic conditions and factors like rainfall, CO₂ levels, and temperatures, which are reflected in the width and color of tree rings. The scientists, using tree-trunk proxies, got a sneak peek into the climatic conditions of the past three centuries.

A closer look at the information gathered alongside climate model simulations revealed a geographical shift in atmospheric patterns.

Over the past two decades (2001–2022), the global heat wave landscape and the pattern of heat wave occurrences has almost reversed. The Eurasian region is now experiencing nearly four times as many heat waves as it did in the late 20th century. A similar pattern and frequency shift was also observed for droughts, often alongside a heat wave.

The increase and shift in extreme weather events have been linked to the intensifying trans-Eurasian heat-wave-drought train (TEHD), a large-scale atmospheric circulation pattern.

The findings suggest that this pattern is driven by stronger Rossby wave activity—atmospheric waves that move along the jet streams—triggered by warming in the Northwestern Atlantic and increased rainfall in the Sahel, a semi-arid region of Africa. Both these atmospheric events are largely the result of global warming, and partly influenced by changes in the Atlantic Ocean.

The researchers note that projections show these changes will continue to intensify over time, making it more important than ever to develop mitigation strategies to manage the increasing risks.

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