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MAPP-led Marine Ecosystems Task Force emphasizes the need for clearer definitions of marine heatwaves as warming increases

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                        Image credit: Marine Heatwaves : NOAA Physical Sciences Laboratory   The Modeling Analysis Prediction and Projections program (MAPP) of NOAA’s Climate Program Office (CPO) coordinates the Marine Ecosystems Task Force (METF) which brings together scientists to collaborate on MAPP-funded marine ecosystems-related projects.  Recently, one of the undertakings of the METF was to properly define marine heatwaves by taking into account the nonstationarity of climate. This effort culminated in a nature commentary that addresses the need for clearer definitions that properly capture the changing nature of our climate and the shift to warmer conditions.  Due to the long-term trends in Sea Surface Temperatures (SSTs), the frequency and intensity of marine heatwaves have increased, and marine heatwaves definitions based on thresholds that do not account for this “shifting” baseline may no longer accurately reflect the nature of extreme warm events driven by internal processes. The distinctiveness of individual Marine Heat Wave (MHW) events has declined as SST warming in many regions is becoming comparable to the magnitude of MHWs.  Studies have shown that there is a significant shift in the distribution of sea surface temperatures as a result of climate change which is expected to become even more pronounced as climate continues to warm, with serious implications for marine ecosystems. In the Alexander et al., 2018 study, a figure (shown below) depicts a shift in the temperature regime of  SSTs such that in the future, even the coldest days would be considered hot extremes by prior definitions, and thereby losing the meaning of the term “extreme”.  Billions of dollars per year is lost globally as a result of the impact of marine heatwaves on the ocean’s ecosystems. These losses include the mortality of species such as lobsters, scallops and snow crabs which significantly impact the economy of coastal communities, as well as damage to coral reefs and other marine habitats which impacts tourism. MHWs can also lead to other extreme events such as hurricanes and severe storms which result in loss of lives and damage to property over land.                             Projected sea surface temperatures over the 21st century: Changes in the mean, variability and extremes for large marine ecosystem regions of Northern Oceans (Alexander et al., 2018 DOI: https://doi.org/10.1525/elementa.191.f10)   Hence, the METF embarked on this project to distinguish between extreme marine heatwave conditions derived from long-term historical data (fixed baseline) and those extremes that account for the new average temperatures (shifting baseline). This distinction is essential to decision-makers and stakeholders for adaptation and accurate assessment of risks. Both definitions have their uses; for instance, extremes based on a fixed baseline are helpful to understand coral bleaching while a definition based on a shifting baseline provides information for the future variability of warming oceans. Understanding the difference in the definition of marine heatwaves allows for better decision making in the short and long term. Planning strategies for rapid warming differ from those for slowly varying changes over time highlighting the importance of clearer definitions for these extremes. It is also necessary to consider these definitions when approaching the dissemination of information to the general public as the urgency in response to marine heatwave events could be impacted when too many events are considered extremes as is more likely to be the case when a fixed baseline is adopted. This issue may also be relevant to other studies of extremes as the climate shifts, particularly drought. The researchers involved argued for terms that best distinguish the baseline considered in the definition of the event. For more on this article, read here

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