"NOAA Ocean Explorer: Pacific Deep Reefs 2011 Exploration: Mission Summary" by NOAA Ocean Exploration & Research is licensed under CC BY-SA 2.0.

How Clear Messaging Can Help Everyone Understand The Importance Of Marine Heatwaves

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The amount of heat in the ocean is accelerating and penetrating deeper, providing fuel for extreme weather. Recent record sea surface temperatures show the emergence of a warming signal that more clearly reveals the footprint of our increased interference with the climate system. The resulting marine heatwaves devastate ecosystems and the coastal communities that rely on them.

The ways in which marine life moves around the ocean differ in response to different types of warming. In slowly warming waters, displacement of many species might be steady and persistent rather than abrupt and temporary. Weeks, months, or years of unusually warm waters can bleach corals, spur harmful algal blooms, and wipe out seaweeds. They might kill or strand marine animals and disrupt food webs and fisheries. Billions of dollars are lost to such events around the world each year.

Resource managers, industries, and coastal communities have different options for dealing with slow, steady warming versus rapid, temporary temperature change. Clear messaging about the nature of ocean temperature change over short and long timescales is crucial so coastal communities can adapt time and resource allocation.

Chip in a few dollars a month to help support independent cleantech coverage that helps to accelerate the cleantech revolution! The world’s oceans have absorbed most of the excess heat caused by humanity’s carbon pollution. Climate change has increased surface temperatures across the planet, leading to atmospheric instability and amplifying extreme weather events such as storms. Michael Mann, presidential distinguished professor in the Department of Earth and Environmental Science at the University of Pennsylvania, advises, “Until we reach net zero emissions, that heating will continue, and we’ll continue to break ocean heat content records.” Mann suggests that better awareness and understanding of the oceans are necessary as a basis for the actions to combat climate change.

Marine heatwaves are like storm surges: relatively short-lived and intense changes in ocean height that lead to acute impacts. Increasing water temperatures and ocean salinity — also at an all-time high — directly contribute to a process of “stratification” where water separates into layers that no longer mix. This has wide-ranging implications because it affects the exchange of heat, oxygen, and carbon between the ocean and atmosphere, with effects including a loss of oxygen in the ocean.

The combination of long term temperature trends and marine heatwaves drive total heat exposure.

The global data for near-surface temperatures comes from ships, buoys, and satellite measurements of the oceans. While fluctuations are a normal phenomenon, an increase of the number of years that are warmer on average is expected due to climate change, which is the increase of global average and mean land and ocean temperatures. According to scientific findings, the continuing global warming will lead to changes in the strength, frequency, spatial extent, and duration of extreme weather events.

Words Matter: Defining Terms for Marine Heatwaves

Ocean scientists are striving to better understand whether climate change is making marine heatwaves more frequent and more intense. But right now the field has a problem: the definitions and communications describing what a marine heatwave is are confusing, according to a recent article in Nature. The members of the US National Oceanic and Atmospheric Administration (NOAA) Marine Ecosystem Task Force are proposing how to define basic terms to clarify what kinds of ocean warming the world has in store.

Scientists and the media use the term “marine heatwave” to refer to two different things:

  • extreme conditions compared with historical temperatures
  • extreme conditions compared with an evolving “new normal” of rising temperatures owing to climate change

Each definition results in different interpretations of frequency, intensity, duration, and spatial extent of marine heatwaves.

Why do marine heatwaves need communication messaging with clearer definitions?

  • for assessing ocean warming
  • analyzing extreme events
  • understanding how they will affect marine ecosystems and livelihoods in the future

A marine heatwave is a period of extreme ocean temperatures. Sometimes they appear over a few days or weeks, or they can seep in slowly over months or years. If temperatures exceed a particular threshold — such as the 90th percentile — above “normal,” they’re considered heatwaves.

Since pre-industrial times, ocean surface waters have warmed at an average rate of 0.06 °C, while the surface air temperature over land has been 0.1 °C per decade. However, a fixed baseline definition of marine heatwaves over the coming decades indicates that ever more frequent and more intense marine heatwaves will continue until the oceans warm so much that they reach a “perpetual heatwave” state. The fixed baseline approach might suit some studies of marine-organism physiology. Many species, such as corals, are severely affected when water temperature reaches a specific threshold. However, this fixed baseline approach doesn’t take into account that oceans are getting warmer over time because of climate change.

Then again, a shifting baseline definition of “normal” conditions accounts for specific patches of ocean and their long term ocean warming. This perspective looks to much warmer ocean temperature anomalies than those derived from a fixed historic baseline to qualify as a marine heatwave. This definition retains the idea that a heatwave should be an exceptional event in time and space. The usage would be consistent with other references to temporary surges in ocean temperature and other forms of climate variability, such as as the El Niño–Southern Oscillation, when long-term warming is removed from the equation. The public is less likely to become desensitized to the real threat of marine heatwaves if the term “extreme” has particular and significant meaning. A shifting baseline definition would likely arouse more public action and preparedness.

The US team that leads the annual SOE report issued by the NOAA Northeast Fisheries Science Center will detail marine heatwaves using the shifting baseline approach starting in 2023. Temperature changes relative to a fixed baseline will also be reported, but those excursions will not be described as marine heatwaves.

What language changes will we see about marine temperature variations?

  • The phrase “long-term temperature trends” will describe the relatively slow changes in ocean temperature that occur over decades or longer.
  • The term “marine heatwave” will be implemented only with the shifting baseline definition to describe ocean temperature changes that are transient and extremely warm relative to the expected conditions for a given place and time, as defined by an evolving, recent climatological reference period.
  • The term “total heat exposure” will describe the combination of long-term warming and marine heatwaves. This terminology is applicable to studies of impacts on ecosystems that are based on a fixed temperature threshold.

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Carolyn Fortuna

Carolyn Fortuna, PhD, is a writer, researcher, and educator with a lifelong dedication to ecojustice. Carolyn has won awards from the Anti-Defamation League, The International Literacy Association, and The Leavey Foundation. Carolyn is a small-time investor in Tesla and an owner of a 2022 Tesla Model Y as well as a 2017 Chevy Bolt. Please follow Carolyn on Substack: https://carolynfortuna.substack.com/.

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