Marine heatwaves (MHWs), periods of extreme ocean warming lasting days to months, pose significant threats to marine ecosystems and coastal communities globally. The Arctic Ocean, with its unique and sensitive ecosystems, is particularly vulnerable. While global warming is increasing seawater temperatures, impacting marine species distribution and ecosystem structure, the future evolution of MHWs in the Arctic remains unclear. This uncertainty stems from the complex interplay between gradual, long-term warming and the occurrence of extreme events. To accurately assess the risks, it is crucial to differentiate between MHWs (extreme warming relative to a shifting baseline) and total heat exposures (THEs, relative to a fixed historical baseline). This study aims to address this gap by investigating historical and future (2071-2100) MHWs and THEs in the Arctic Ocean using the latest CMIP6 climate simulations, focusing on four key metrics: mean intensity, annual total days, mean frequency, and mean duration. Understanding these future changes will assist policymakers in developing effective conservation and adaptive strategies for Arctic marine ecosystems.

Previous research has established the global prevalence of MHWs, including in the Arctic marginal seas. Studies have shown that the mean intensity of MHWs in these seas has been comparable to other ocean regions over the past 40 years. The impacts of climate change on marine species have been widely documented, including poleward migrations to cooler regions, such as the Arctic. However, the literature lacks a comprehensive understanding of the future evolution of MHWs in the Arctic Ocean and the specific threats to Arctic marine species. Existing research highlights the importance of distinguishing between MHWs and THEs when assessing the impacts of climate change on marine ecosystems. Several studies have used CMIP models to project future changes in MHWs globally, but these often lack a focus on the unique characteristics of the Arctic and the specific implications for this region's vulnerable ecosystems. This study builds on this existing literature by providing detailed projections of both MHWs and THEs for the Arctic Ocean using the latest CMIP6 simulations and a sophisticated methodological approach.

This study utilized sea surface temperature (SST) and sea ice concentration (SIC) data from the Optimum Interaction Sea Surface Temperature v2.1 (OISST) dataset for the historical period (1985–2014) and from the Coupled Model Intercomparison Project phase 6 (CMIP6) for both historical simulations and future projections (2071–2100). Four Shared Socioeconomic Pathway (SSP) scenarios (SSP126, SSP245, SSP370, and SSP585) were used to represent different future climate forcing pathways. The methodology for defining and detecting MHWs and THEs followed Amaya et al., using a shifting baseline for MHWs and a fixed baseline for THEs. The 35% SIC threshold was used to distinguish between ice-free and ice-covered regions. MHWs were considered absent in ice-covered areas. Four key metrics were analyzed for both MHWs and THEs: mean intensity, annual total days, mean frequency, and mean duration. The CMIP6 multi-model mean (MMM) was used to synthesize results across multiple models, and statistical significance tests were performed to assess the robustness of findings. The spatial patterns and magnitudes of these metrics were compared between the historical and future periods, both globally and specifically within the Arctic Ocean. The relationship between changes in Arctic MHW intensity and sea ice area was further analyzed to understand the primary drivers of MHW amplification.

The study revealed a significantly more pronounced increase in MHW mean intensity in the Arctic Ocean compared to the global average in future climate scenarios. By the end of the 21st century under the SSP585 scenario, the increase in MHW mean intensity in the Arctic is projected to be 7.6 times the global average, while the increase in THE mean intensity is 1.5 times the global average. Historically, MHW intensity in the Arctic marginal seas was comparable to other regions; however, future projections show a substantial shift. The Arctic deep basin, previously largely free of MHWs due to sea ice cover, is projected to experience a substantial increase in MHW intensity and annual total days. This 'Arctic MHW Amplification' is primarily driven by sea ice decline, which increases open water and allows for greater heat absorption. In contrast, the increase in THE mean intensity in the Arctic is mainly attributed to the long-term warming trend, particularly evident in the Barents Sea. The projected changes in MHW frequency and duration in the Arctic Ocean were also significant, amplifying under all scenarios, unlike changes seen in lower latitudes. Analyses showed a significant anti-correlation between future changes in Arctic MHW mean intensity and sea ice area, confirming the importance of sea ice decline as a key driver. While the SSP585 scenario showed a weaker correlation at the end of the century due to near-complete sea ice melt, the correlation was much stronger when considering the months with some remaining sea ice. The seasonality of changes in Arctic MHW intensity was found to be linked to the seasonality of solar radiation and sea ice decline.

The findings highlight the disproportionate impact of climate change on the Arctic Ocean, with MHWs intensifying at a rate far exceeding the global average. This ‘Arctic MHW Amplification’ poses a severe threat to Arctic marine life, requiring targeted conservation and adaptive strategies. The significant role of sea ice decline in driving this amplification underscores the urgency of addressing climate change to protect Arctic ecosystems. While the long-term warming trend contributes to increased THEs, the dramatic increase in MHWs represents a unique and substantial threat. These projections emphasize the need for more detailed studies that consider the ecosystem-level impacts of these extreme warming events. The observed differences in the responses of MHWs and THEs to climate change further highlight the complexity of climate-driven changes in the Arctic and the importance of assessing both types of events.

This study provides robust projections of future MHWs and THEs in the Arctic Ocean, revealing a substantial 'Arctic MHW Amplification.' Sea ice decline is identified as the primary driver of increased MHW intensity, while long-term warming significantly contributes to increased THE intensity. The disproportionate increase in MHWs in the Arctic underscores the urgent need for targeted conservation and adaptation strategies. Future research should focus on refining model projections and investigating the ecosystem-level impacts of Arctic MHW amplification, informing effective policies for protecting this unique and vulnerable environment.

The study relies on CMIP6 climate models, which inherently contain uncertainties. Model spread in simulated MHWs and THEs was observed, highlighting the need for ongoing improvements in climate models. While the study considers a range of SSP scenarios, there is a limitation of the complexity of physical processes that are not accurately represented in the models. The study focuses on broader spatial scales and may not capture the full range of localized effects, especially in coastal areas. Additionally, understanding the biological and ecological impacts of these changes requires further research.