Marine heat waves, periods of unusually warm ocean temperatures, are a significant and growing concern for the health of marine ecosystems globally. These events represent an intensifying threat to ocean life. Their increasing frequency and intensity worldwide highlight the urgency of understanding their nature and consequences.
Defining Marine Heat Waves
A marine heat wave is a prolonged period where ocean temperatures in a specific region are significantly warmer than the long-term average for that time of year. They typically last five days or more, distinguishing them from short-term temperature fluctuations. Scientists identify marine heat waves by comparing current sea surface temperatures to historical baselines, often looking for temperatures that exceed the 90th percentile of observations from previous years for that specific time and location.
While marine heat waves involve elevated ocean temperatures, they differ from natural climate patterns like El Niño. El Niño is a large-scale climate phenomenon with warmer-than-average sea surface temperatures in the central and eastern tropical Pacific. Marine heat waves can occur independently of El Niño, though these natural patterns can sometimes exacerbate them. Marine heat waves can occur at any time of year, even in winter, if the temperatures are much warmer than usual for that season.
Drivers of Marine Heat Waves
Multiple factors contribute to marine heat waves. A primary driver is anthropogenic climate change, which has led to the oceans absorbing approximately 90% of the excess heat generated by fossil fuel emissions. This absorption results in gradual warming, making naturally occurring heat waves more severe.
Atmospheric high-pressure systems reduce wind speeds and lessen evaporative cooling, allowing surface waters to warm more easily. Ocean currents can also transport warmer water into a region or suppress the upwelling of cooler, deeper water, further contributing to surface warming. Natural climate variability, such as the Pacific Decadal Oscillation or El Niño/La Niña events, can interact with these drivers. For instance, marine heat waves are more likely during El Niño years.
Ecological and Economic Impacts
Marine heat waves have severe consequences for both marine ecosystems and human activities. Ecologically, these events cause coral bleaching, where corals expel algae from their tissues, leading to whitening and potential death. Mass mortalities of marine species, including fish, seabirds, and invertebrates, also result from prolonged exposure to extreme temperatures.
Shifts in species distribution are common, as some marine organisms move to cooler waters, while non-mobile species like kelp and sponges cannot adapt and decline. The “Blob” marine heat wave in the Pacific from 2014-2016 caused significant shifts in species habitat and led to mass seabird die-offs due to food web disruptions. In the Mediterranean, marine heat waves between 2015 and 2019 resulted in recurrent mass mortality events affecting around 50 species, including corals and macroalgae.
Economically, marine heat waves significantly impact fisheries, leading to reduced catches, stock collapses, and closures. For example, the 2014-2016 marine heat wave on the U.S. West Coast and the 2011 event in Western Australia caused severe population declines, forcing fisheries to close for years to allow stocks to recover. Aquaculture also suffers from disease outbreaks and increased mortality. Tourism, particularly dive tourism, can decline due to the degradation of coral reefs and other marine habitats. A single event can result in hundreds of millions of dollars in direct economic losses.
Global Trends and Future Outlook
Marine heat waves have become more frequent, longer-lasting, and intense over the past few decades. Their global frequency has doubled since 1982, and the total number of days with marine heat waves has increased by 50% over the last century. These events are affecting an increasing area of the ocean, from surface to deep waters across all latitudes.
Scientific models project that with continued global warming, marine heat waves will become more common, severe, and prolonged. By the end of the 21st century, some regions, like the tropical Indian Ocean, may experience near-permanent heatwave conditions. This intensifying trend suggests that marine heat waves could become the “new normal” for many marine regions, posing ongoing challenges for ocean ecosystems and human societies.