Intriguingly, all record-setting heatwaves based on this index occurred in the mid-latitudes, indicating that here heatwaves are becoming more intense at a pace that exceeds the global mean 20. A quantitative global analyses showed that the 2010 event was the most-severe heatwave ever recorded worldwide, based on a heatwave index that can be used across different regions 20. In 2010, Russia saw 33 consecutive hot-and-dry days (with temperatures above 30 ☌), resulting in an estimated 55,000 heat-related deaths, more than 500 wildfires near Moscow and grain-harvest losses of 30% 4. Many recent high-impact summer heatwaves occurred in the far-tail of the distribution and are difficult to explain by the direct radiative warming effect of greenhouse gas forcing alone 18, 19, 20, 21. In summer, the hot tail of the distribution is associated with persistent, blocking weather systems, and an increase in their persistence leads to more extreme temperatures. This is supported by recent studies that indicate that summer weather has become more persistent in several regions in the mid-latitudes 23, 24, 25. This increased variability indicates that more complex processes beyond simple radiative greenhouse gas forcing are important in driving heat extremes (Box 1). Clearly, over most mid-latitude regions, in particular over Eurasia but less so in the US, the hot tail has been warming faster than the cold tail and thus temperature variability in summer has increased 22. Figure 1 shows the warming trends in the 95th percentile (hot tail), 50th percentile (median), and 5th percentile (cold tail) of daily summer temperatures. Consistent with the increase in heatwaves, the hot tail of summer temperature distribution has been warming faster than the median and the cold tail. Such extreme heatwaves have been found to increase and intensify across most regions but more so in the mid-latitudes than over the rest of the globe 20. Many recent high-impact summer heatwaves indeed occurred in that far-tail of the distribution and cannot be explained by the direct thermodynamic effect of greenhouse gas forcing alone (Box 1) 18 – 21. For instance, any increased frequency in circulation regimes conducive to persistent heat extremes would act on top of the thermodynamically driven increase in heat, creating possibilities for very-extreme heatwaves. In summer, thermodynamic and dynamic drivers of extreme weather could act in the same direction, leading to tail risks 17. Links between AA and summer circulation have received far less scientific attention, despite the potential for synergistic effects that might favor high-impact extremes. Even though the exact pathways through which the Arctic influences the mid-latitude winter circulation are debated, a scientific consensus is emerging that AA has at least some influence on winter weather 7, 15, 16. The associated expansion of the near-surface air increases Arctic geopotential heights and can affect the circumglobal circulation directly as well as via feedbacks between the troposphere and stratosphere involving the stratospheric polar vortex 6, 10, 11, 12, 13, 14. The increased heat stored in the Arctic Ocean owing to sea-ice loss is released into the atmosphere in early winter. The stronger jet stream, the presence of the stratospheric polar vortex, and the post-1990s increase in abnormally cold winters over central Eurasia have drawn a lot of attention to the winter season 6, 8, 9. Most studies analyzing the role of AA on mid-latitude weather have focused on the winter season and the linkage with cold spells. The extent to which AA affects the mid-latitude circulation and possibly contribute to the observed increases in weather extremes has been a subject of active debate 7. Another pronounced signal of anthropogenic global warming is the rapidly increasing near-surface temperatures in the Arctic at a pace two to four times faster than the rest of the globe, known as Arctic amplification (AA) 6. Scientists are generally confident in the thermodynamic drivers of these changes but are less so in dynamic aspects 4, 5. The observed increases in the frequency and intensity of extreme heat and heavy rainfall events since the late 1980s, especially in mid-latitude regions, have been linked to anthropogenic global warming 1, 2, 3.
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