The escalating frequency, intensity, and duration of heatwaves globally, including the Arctic, are widely recognized consequences of climate change. However, the expanding land area impacted by these events has received comparatively less attention. This study addresses this gap by quantifying the observed and projected changes in the spatial extent of Arctic heatwaves. The Arctic, experiencing amplified climate change effects, is particularly vulnerable to heatwaves. These extreme events can trigger significant ecosystem disruptions, such as increased wildfire activity fueled by warmer, drier conditions and the release of greenhouse gases from thawing permafrost. Heatwaves also pose a substantial public health threat, particularly to Arctic communities. Understanding the spatial expansion of heatwaves is crucial for assessing and mitigating their impact on vulnerable Arctic ecosystems and populations. This research utilizes advanced atmospheric reanalysis data (ERA5-Land) and a suite of global climate models (CMIP6) to analyze the spatial extent of heatwaves over the terrestrial Arctic, providing crucial insights into the long-term variability and future projections of this phenomenon.

Previous research has extensively documented the increasing frequency, intensity, and duration of heatwaves worldwide and in the Arctic. Studies using various metrics, including frequency, intensity, and duration, have highlighted the accelerating trend in heatwave occurrences. However, a recent shift towards cumulative indices, such as the heatwave magnitude index daily (HWMId), is gaining traction. HWMId, a percentile-based index, effectively integrates duration and temperature anomalies, providing a more robust comparison of heatwaves across different regions with varying climate characteristics. Dobricic et al. (2020) demonstrated an increasing trend in HWMId in the Arctic, particularly in northeastern Canada and Greenland, using ERA-Interim reanalysis data. This study builds on these previous findings by leveraging the higher-resolution ERA5-Land reanalysis and CMIP6 model simulations, providing a more comprehensive and detailed assessment of heatwave spatial extent.

This study analyzed boreal summer (June-August) data from 1950 to 2022 for observational analysis and 1950 to 2099 for model projections. The terrestrial Arctic (land areas poleward of 60°N) was the focus area. Observational data were obtained from the ARCLIM dataset, derived from the high-resolution ERA5-Land reanalysis (0.1° resolution). The ERA5-Land reanalysis, developed by the European Centre for Medium-Range Weather Forecasts, offers comprehensive hourly meteorological data, allowing for a temporally and spatially consistent analysis of climate patterns. Model data were obtained from 21 CMIP6 models (43 simulations in total), utilizing historical simulations (1850-2014) and SSP2-4.5 scenario simulations (2015-2099) for future projections. Data from these models were remapped onto a common 1° latitude-longitude grid, focusing solely on land areas. The heatwave magnitude index daily (HWMId) was the primary metric. HWMId aggregates excess temperatures above a normalized 90th percentile threshold (calculated using 1981-2010 data), combining duration and temperature anomaly into a single value. Three HWMId thresholds (3, 6, and 9) were used to categorize heatwaves as severe, extreme, and very extreme, respectively. Trend analysis was conducted using the Mann-Kendall trend test. Sensitivity analysis regressed HWMId, heatwave length (HWL), and heatwave intensity (HWI) against summer mean temperature using CMIP6 data (2000-2099). The spatial extent of heatwaves at various global warming levels (0.5°C, 1°C, 1.5°C, and 2°C above the 1850-1900 average) was also investigated.

The analysis of ERA5-Land reanalysis data (1950-2022) revealed a dramatic increase in the spatial extent of heatwaves in the terrestrial Arctic. Compared to the 1950-1979 period, the area affected by severe heatwaves doubled, extreme heatwaves tripled, and very extreme heatwaves quadrupled by 1993-2022. The most significant increases occurred in Greenland, the high Canadian archipelago, and Siberia. The time series of annual heatwave coverage further illustrates this escalating trend, with severe heatwaves exceeding 30% of the terrestrial Arctic area multiple times since 2010. CMIP6 model projections (under the SSP2-4.5 emission scenario) suggest this expansion will continue throughout the 21st century. By mid-century, very extreme heatwaves are projected to occur across virtually the entire Arctic region. By the end of the century, the majority of the terrestrial Arctic is projected to experience severe heatwaves annually, with a substantial increase in the occurrence of extreme and very extreme heatwaves. Regional differences in the projected increase of HWMId were observed, with the strongest increases projected for the High Arctic (Canadian archipelago, Greenland coasts, Svalbard, and some Russian islands). The analysis shows that this difference is mostly due to longer heatwaves rather than more intense heatwaves. The Canadian archipelago shows a three times faster increase in the length of heatwaves compared to Siberia for every degree of warming. The sensitivity of heatwave intensity to the summer warming is similar in both locations.

The study's findings highlight the significant and accelerating spatial expansion of heatwaves in the terrestrial Arctic, exceeding historical climate norms and underscoring the region's increasing vulnerability to extreme heat. The observed and projected increases in heatwave extent have substantial implications for Arctic ecosystems and societies. The observed manifold increase in heatwave area underscores the urgent need for climate change mitigation and adaptation strategies tailored to the specific vulnerabilities of the Arctic. The disproportionate increase in heatwave magnitude in the Canadian archipelago and northern Greenland, primarily attributed to prolonged heatwave durations rather than intensified heatwave temperatures, highlights the critical role of sea ice loss in this phenomenon. Reduced snow and ice cover could lead to a feedback effect where less heat is used for ice melt, instead increasing air temperatures. This emphasizes the importance of considering region-specific dynamics when assessing and addressing the impacts of climate change.

This study provides compelling evidence of a dramatic increase in the spatial extent of heatwaves across the terrestrial Arctic, both in recent decades and projected into the future. The manifold increase in the area affected by heatwaves of various intensities highlights the escalating severity of this climate change impact. The region-specific dynamics, particularly the disproportionate increase in heatwave duration in the High Arctic linked to sea ice loss, warrant further investigation. Future research should focus on more detailed energy budget analysis to understand the complex interactions between snow, ice, and atmospheric conditions in shaping local heatwave characteristics. This enhanced understanding is vital for developing effective adaptation and mitigation strategies for the vulnerable Arctic ecosystems and communities.

The study's reliance on reanalysis data and climate models introduces uncertainties. While ERA5-Land offers high-resolution data, it is still a model-based reconstruction of the climate, and there might be some biases and uncertainties. Similarly, CMIP6 model projections are subject to inherent uncertainties related to climate model structure, parameterizations, and emission scenarios. The use of a single emission scenario (SSP2-4.5) limits the exploration of the full range of potential future scenarios. Additionally, the focus on the heatwave magnitude index (HWMId) as a single metric could potentially overlook other relevant aspects of heatwave impacts.