Arctic Amplification of Marine Heatwaves under global warming

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Marine heatwaves (MHWs) and total heat exposures (THEs), extreme warming events occurring across the global oceans, seriously threaten marine ecosystems and coastal communities as the climate warms. Understanding the changes in MHWs  and THEs is critical to the assessment of their impacts on marine ecosystems and socioeconomic systems, enabling better prediction, adaptation, and mitigation strategies.

MHWs refer specifically to episodes of anomalously high sea water temperatures that persist for several days to weeks or longer. These extreme events can be caused by a variety of factors, including changes in ocean currents, wind patterns, and heat fluxes from the atmosphere. THEs are the combination of the MHWs and the ocean long-term warming. MHWs and THEs are widespread globally, including the Arctic Ocean. Previous studies point that the intensity of MHWs in the Arctic marginal seas has been comparable with that observed in other regions of the world’s oceans. However, future changes in MHWs and THEs in the Arctic Ocean, where unique marine ecosystems are present, are still unclear.

To address this gap, we investigated the future changes of MHWs and THEs in the Arctic Ocean, using the latest the Coupled Model Intercomparison Project phase 6 (CMIP6) climate simulations. We found that both MHWs and THEs in the Arctic Ocean are anticipated to intensify in a warming climate. Particularly striking is the projected rise in MHW intensity during the 21st century in the Arctic Ocean, surpassing the global average by more than sevenfold under high CO2 emission scenario. We call this phenomenon the 'Arctic MHW Amplification', which underscores an impending and disproportionately elevated threat to the Arctic marine life, necessitating targeted conservation and adaptive strategies. The above finding has been published on Nature Communications (https://www.nature.com/articles/s41467-024-52760-1).

CMIP6 climate models project that under the high CO2 emission scenario, MHWs with mean intensity surpassing 1°C and THEs with mean intensity exceeding 2°C are expected to become widespread across the majority of the global oceans over 2071–2100. The Arctic Ocean is expected to experience stronger MHWs and THEs in the future, with mean intensities of about 2°C and 3°C, respectively. The most substantial increase in MHW mean intensity is projected to take place in the Arctic deep basin. The changes in the MHW mean intensity in the Arctic Ocean considerably surpass the global ocean average. To investigate the main drivers of changes in MHWs and THEs in the Arctic Ocean, we studied the relationship between the changes in MHW intensity and sea ice area in the Arctic Ocean and the relationship between the changes in THE intensity and long-term ocean warming in the Arctic. We found that future changes in the mean intensity of Arctic MHWs are significantly anticorrelated with future changes in Arctic sea ice area across the models, which suggests that the reduction in sea ice cover is a key driver for the projected increase in Arctic MHW intensity. A remarkable decline of Arctic sea ice in a warming climate enhances air-sea interaction, and the resulting strengthened atmospheric effects could induce temperature anomalies in open waters, consequently fostering increased MHW intensity in the Arctic. The strong correlation between future changes in THE mean intensity and future changes in SST exists in the CMIP6 models, which indicates that the long-term warming trend is the primary driver of increases in THE intensity.

Figure 1. Mean intensity and its changes of marine heatwaves (MHWs) and total heat exposures (THEs) during 2071–2100 based on CMIP6 multi-model mean under the high CO2 emission scenario. The changes in MHW mean intensity are MHW mean intensity in 2071–2100 relative to that in 1985–2014.

We also studied the changes in annual total days and mean frequency of Arctic MHWs and THEs in the future. Annual total days are the total MHW/THE days per year. Mean frequency is the annual number of occurrences of MHW/THE events averaged over the considered period. CMIP6 climate models project that  MHW annual total days in the Arctic Ocean are about 20 days, which are shorter than those in the mid and low latitudes (about 30 days) during 2071–2100 under high CO2 emission scenario. However, the most noteworthy increase in the annual total days of MHWs is expected within the Arctic deep basin. Projections indicate that in the period 2071–2100, THE annual total days are expected to exceed 300 days across about 80% of the global ocean expanse, whereas in the central Arctic, they are projected to hover around 200 days. The mean frequency of MHWs and THEs in the Arctic Ocean is projected to markedly increase in the future warming climate. In contrast, a decrease or no remarkable change in frequency is anticipated for the majority of other ocean regions.

Confronted with thermal stress under global warming, some marine species have started migrating poleward to cooler regions, including the Arctic Ocean. Despite this migration, the threats that stronger Arctic MHWs and THEs will pose to Arctic marine species in the future remain unclear.

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