What Are Marine Heatwaves? Are They Getting Stronger? Impact and Mitigation

Most of us know what a heatwave is or have experienced one if not many. Similar to a land-based heatwave, a marine heatwave marks a sustained period of time when the temperatures in a marine area are well above average. 

How much above average? Typically 90%, although the exact percentage depends on the season. An official marine heatwave also must last a minimum of five consecutive days. Even if the temperature dips during a particular marine heatwave, it is considered part of the same heatwave when the thermostat hits above the 90% threshold within two days’ time.

Marine heatwaves sometimes last much longer than one week and can cause changes in ocean ecosystems, impacting marine biodiversity, human health, and economies. Here, we explore these impacts as well as what can be done to lessen the effects caused by marine heatwaves.

How Marine Heatwaves Form

One of the most common causes of marine heatwaves has to do with ocean currents. These currents contribute to marine heatwaves by allowing very warm water to accumulate in concentrated areas. 

Another big driver of marine heatwaves is something called air-sea heat flux. This is when the heat in the atmosphere penetrates the surface of the ocean and becomes absorbed by it. High-pressure systems combined with a scarcity of cloud cover can stagnate the air in the area. In other words, there isn’t much wind. As atmospheric temperatures above the surface of the ocean rise with this lack of circulation of air, the temperatures of the ocean’s surface rise as well. Meanwhile, without cloud cover, the sun’s rays further warm the water. 

El Niño can also play a role in marine heatwaves, as by definition it is atypical warming of the surface water of the ocean. In fact, one study found that the years with the most marine heatwave days spread across an area encompassing the coast of Queensland, Australia each happened directly after El Niño events.

Yet, while El Niño can influence marine heatwaves and the two intersect, they are not necessarily the same and can occur independently of each other.   

Impact on the Environment

Since oceans absorb the vast majority of heat associated with greenhouse gas emissions, marine heatwaves can serve as an important gauge of how severe the impacts of climate change are and might become. Studying marine heatwaves offers an opportunity to not only understand how they impact environments but also analyze their ripple effects throughout the wider ocean systems, along with systems outside of the ocean. 

Disruptions Caused by "The Blob"

One of the most notorious marine heatwave events in recent history is “the Blob,” which hit the Pacific Coast near Alaska in 2014 and lasted until 2016. 

As a result, the zooplankton in the area decreased in size. This meant that the species relying on zooplankton—such as fish, marine mammals like whales, and even seabirds (which eat the fish that eat the zooplankton)—became undernourished, which makes them more vulnerable to disease, pollution, and inclement weather. 

In addition, the Blob triggered extreme algae blooms that caused parts of the fishing industry to close down entirely and led to the deaths of thousands of animals, including fin whales, sea otters, sea lions, and Chinook salmon. Algae blooms caused by marine heatwaves often last longer than naturally occurring ones. They can kill wildlife directly by depriving species of light and oxygen, whereas some species suffer from the loss of their food source.

Habitat Displacement

Marine heatwaves can also force many species that are reliant on cold-water ecosystems to move from their familiar habitat or veer from their historical migration routes in order to survive. Since marine heatwaves can potentially impact hundreds of thousands of miles of ocean, some species may be entirely displaced from their traditional habitat during these events. This can make it more difficult for predator species to find their prey, or for some species to find mates and breed. 

Unfortunately, the Blob and similar events are harbingers of the things that will likely become more commonplace due to climate change. 

What Can Be Done About Marine Heatwaves?

One of the most effective courses of action to prevent marine heatwaves from becoming even more prevalent is to pass legislation that would help curb carbon emissions.

In the interim, being able to anticipate and better plan for these events can also help avoid some of the worst impacts. This means advancing the tools that predict marine heatwaves and adopting methods that help us adapt to our changing climate and its effects on our oceans. 

The Marine Heatwave International Working Group was formed in order to develop a better understanding of heatwaves, by tracking them and identifying patterns that can help to predict future events. Similarly, after the Blob, the Southwest Fisheries Science Center of the National Oceanic and Atmospheric Administration (NOAA) created a tool called the California Current Marine Heatwave Tracker.

Researchers hope we will soon advance our technology to get dynamic modeling of marine heatwaves, which will do a better job of forecasting events than standard modeling because it won't just rely on historical patterns but newer trends as well.

In addition, many scientists think better modeling can help identify what seeds and plants should be stored for future cultivation. Improving marine heatwave predictions could also shed light on what species are most at risk and allow governments to enact restrictions on harvesting those species during certain times of the year or altogether.

By better planning for future marine heatwaves, fishing professionals, wildlife managers, oceanographers, and others with the common interest of preserving our oceans can work together to prevent the worst impacts.