Xiuwen Guo, Yang Gao, Shaoqing Zhang, Lixin Wu and Wenju Cai

Marine heatwaves (MHWs) are known to exert severe impacts on marine ecosystems, increasing mortality in seabirds, oysters and marine mammals, and causing coral bleaching, kelp forests shrinking, abnormal migration of marine life and decline in fish catches.

Understanding the impact of future marine heatwaves on large marine ecosystems is usually constrained by the resolution of climate models. Previous studies on MHWs are mostly based on models with a coarse spatial resolution at approximately 100 km or lower, which may lack the capability of resolving small-scale dynamics such as coastal processes.  

This study used a high-resolution eddy-resolving Community Earth System Model (CESM-HR). “CESM-HR, ported and optimized on the Sunway HPC, shows a substantial improvement in simulating global mean SST, mixed layer depth, and the frequency and persistence of MHWs, compared with a low-resolution (nominal 1°) version of the CESM, lending confidence in the analysis of MHWs and their future changes,” said Dr. Shaoqing Zhang, a professor at the Key Laboratory of Physical Oceanography, Ministry of Education, Ocean University of China.

“The high-resolution earth system model provides an unprecedented opportunity to examine MHWs over the Large Marine Ecosystems (LMEs) located in coastal regions, which account for 95% of global fisheries catches despite covering less than a quarter of the global ocean,” said Dr. Yang Gao, a professor at the Key Laboratory of Marine Environment and Ecology, Ministry of Education, Ocean University of China.

In terms of future changes in MHWs, most studies applied a fixed threshold based on the historical period – an approach which includes contribution from the long-term warming. This study defined future MHWs using both the “mean warming-inclusive threshold” and a “future threshold” to isolate the effect of mean background warming and effect such as variability that rides above the mean warming.  

The future threshold approach considers that species may have some capacity to adapt to future warming, and it is important to compare changes in MHWs under an assumption that marine ecosystems have no capacity to adapt, as opposed to another assumption that marine ecosystems can fully adapt to the mean warming.

Based on the “mean warming-inclusive threshold”, the majority of ocean areas is projected to experience almost permanent heatwaves with concomitantly stronger intensity by the end of the 21st century. “Can we still call it a wave? If we could, it would almost be a permanent wave,” said Dr. Wenju Cai, a Chief Research Scientist at CSIRO.

Therefore, we used the “future threshold” and found that even after eliminating the impact of the mean warming, there is still an increase in MHWs in almost all the LMEs and globally. “This is both surprising and disturbing”, said Miss Xiuwen Guo, a master student at Ocean University of China. An increase in SST variance and a stronger persistence combine to increase the MHW annual days and intensity. “Due to the geographical diversity of LMEs, dynamics are diverse for the increased variance, but enhanced SST variability of the north tropical Atlantic, and the Indo-Pacific associated with the Indian Ocean Dipole and El Niño-Southern Oscillation likely contribute”, said Dr. Wenju Cai.

“The result has far-reaching ecological, social, and economic implications and provides a scientific basis for developing a response strategy from the impacted communities and policymakers.” said Dr. Lixin Wu, president of Qingdao National Laboratory for Marine Science and Technology, China.