Chinese prof Lin uses climate models in CMIP6 to show how a warmer world will make heatwaves more frequent
![Composite of (a) 300-hPa geopotential height anomalies [shading, shading interval (SI) = 10 gpm] and (b) 2-m air temperature anomalies (shading, SI = 0.5°C) during the extreme heat summers over Western North America (WNA) in ERA5 data set. (c, d) are the same as (a, b), but for the MME from 15 CMIP6 models. The blue rectangles represent the region over WNA (40°–60°N, 128°–110°W). The anomalies are relative to 1981–2010.](/images/ezgif.com-webp-to-jpg_f5ba0.jpg "Composite of (a) 300-hPa geopotential height anomalies [shading, shading interval (SI) = 10 gpm] and (b) 2-m air temperature anomalies (shading, SI = 0.5°C) during the extreme heat summers over Western North America (WNA) in ERA5 data set. (c, d) are the same as (a, b), but for the MME from 15 CMIP6 models. The blue rectangles represent the region over WNA (40°–60°N, 128°–110°W). The anomalies are relative to 1981–2010.")
From late June to early July 2021, an unprecedented heatwave swept across Western North America (WNA), causing considerable regional societal and economic hazards. Many new records on maximum temperatures were broken, including 46.7°C in Portland, Oregon, and 49.6°C in Lytton, British Columbia, the latter representing the highest temperature ever observed in Canada. In addition, more than 1,000 deaths were believed to have been linked to the extreme heatwave. Such an extreme event raises questions about how the likelihood of a similar heatwave will change under global warming.
Recently, in a paper published in Earth's Future, Prof. WANG Lin from the Center for Monsoon System Research, Institute of Atmospheric Physics (IAP) at the Chinese Academy of Sciences, in collaboration with scientists from Yunnan University, revealed that heatwaves similar to the unprecedented WNA one in summer 2021 are projected to become more frequent in a warmer world based on the multi-model simulations from the Coupled Model Intercomparison Project, which began in 1995 under the auspices of the World Climate Research Programme (WCRP) and is now in its sixth phase(CMIP6).
They found that the likelihood of a similar heatwave to the 2021 WNA one will increase in the future if the global warming level continues to rise. Such a heatwave is projected to occur more frequently with increased extreme temperature and shortened return period, making a rare event in the current climate a common event in warmer weather, especially under a high-emission scenario like the Shared Socioeconomic Pathways 585 (SSP5-8.5). They also found a significant expansion of areas over WNA that will break the 2021 record in the future with an increasing emission scenario. However, some heat records west of the Rocky Mountains are still difficult to break even at the end of the 21st century, highlighting the specific extremity of the observed 2021 WNA heatwave. ![Spatial-temporal evolutions of the 2021 Western North America (WNA) heatwave in the observation. (a) Temporal variations of the spatial extent (shading, SI = 100 × 103 km2) of land areas over WNA that experience record-breaking temperature anomalies at different time scales. The purple dot indicates the date and time scale with the maximum record-breaking areas (i.e., June 29 at the 5-day time scale). (b) 2-m air temperature anomalies (shading, unit: °C; relative to 1981–2010) at the 5-day time scale centered on 29 June 2021. Contour lines [contour interval (CI) = 1 SD] indicate the normalized 2-m temperature, defined as the anomalies divided by the corresponding standard deviation among all summer days in 1981–2010. The black points highlight the record-breaking grids. The blue rectangle represents the region over WNA (40°–60°N, 128°–110°W), and the purple lines represent the Canadian and U.S. states' boundaries. (c) The daily evolution of the area-mean 2-m temperature anomalies (orange line) over WNA at the 5-day time scale in the June-August of 2021. The pink shading indicates the historical maximum temperature anomalies at the 5-day time scale on each summer day from 1950 to 2020. The black dot represents the temperature anomaly on 29 June 2021.](/images/ezgif.com-webp-to-jpg_1_e10d9.jpg "Spatial-temporal evolutions of the 2021 Western North America (WNA) heatwave in the observation. (a) Temporal variations of the spatial extent (shading, SI = 100 × 103 km2) of land areas over WNA that experience record-breaking temperature anomalies at different time scales. The purple dot indicates the date and time scale with the maximum record-breaking areas (i.e., June 29 at the 5-day time scale). (b) 2-m air temperature anomalies (shading, unit: °C; relative to 1981–2010) at the 5-day time scale centered on 29 June 2021. Contour lines [contour interval (CI) = 1 SD] indicate the normalized 2-m temperature, defined as the anomalies divided by the corresponding standard deviation among all summer days in 1981–2010. The black points highlight the record-breaking grids. The blue rectangle represents the region over WNA (40°–60°N, 128°–110°W), and the purple lines represent the Canadian and U.S. states' boundaries. (c) The daily evolution of the area-mean 2-m temperature anomalies (orange line) over WNA at the 5-day time scale in the June-August of 2021. The pink shading indicates the historical maximum temperature anomalies at the 5-day time scale on each summer day from 1950 to 2020. The black dot represents the temperature anomaly on 29 June 2021.")
"Our study indicates that the unprecedented heatwave will become more common in most areas of Western North America if we do not take adequate climate mitigation measures", said Dr. DONG Zizhen, the first author of the paper.
"We use multiple climate models that participate in CMIP6 and consider different emission scenarios and warming levels for the future heatwave projections over WNA, which may provide more information for decision-makers to plan their development routes and adaptation measures", said Prof. WANG, the corresponding author of the paper.
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