Sensitivity of Arctic marine heatwaves to half-a-degree increase in global warming: 10-fold frequency increase and 15-fold extreme intensity likelihood

Abstract We utilize the 50-member MPI-ESM-LR Earth System model to investigate the projected changes in Arctic marine heatwaves' (MHWs) characteristics caused by an additional 0.5°C increase in global warming, from 1.5°C to 2°C, with respect to pre-industrial levels. Our results indicate that this 0.5°C increase in global warming triggers an intensified response in both the Arctic's mean sea surface temperature (SST) and its variability. In a 2°C warmer world, one out of every four summer months would be warmer than the current climate. We detect a nonlinear increase in MHW intensity in a 2°C world, which is characterized by a break in slope occurring around the year 2042 ± 2 (across 50 ensemble members of the SSP5-8.5 scenario). At the estimated post-break dates, the intensity rate roughly doubles, leading to MHWs in a 2°C world with average cumulative heat intensity 100°C*days higher than in a 1.5°C world. Further results reveal that an extremely rare MHW with an intensity of 3.19°C, classified as a 1-in-100-year event in a 1.5°C world, is expected to transform into a 1-in-7-year event in a 2°C world. This transition signifies an approximately 15-fold increase in the likelihood of such events occurring due to a 0.5°C increase in global warming. Likewise, an uncommon event of years with 125 MHW days in a 1.5°C world is projected to become a 1-in-10-year event in a 2°C world, resulting in a 10-fold increase in occurrence probability. The main contributor to these changes is predominantly the rise in mean SST, with enhanced SST variability playing a minor role. These findings highlight that a 2°C world could lead to a substantial escalation of the frequency and intensity of MHWs in the Arctic compared to a 1.5°C world, transforming what are currently rare extreme events into more common events, with significant implications for global climate dynamics and the well-being of Arctic ecosystems and communities.