When heat waves strike on land, the ocean can provide a cool oasis. But the same climate forces that can make land less hospitable are having similar effects on marine environments, according to a recent study.

Researchers looked at the results of eight ocean heat waves and found they can have lasting effects on marine ecosystems — effects such as damaged coral, toxic algae and increasingly scattered populations of marine creatures. They published their findings in Nature Climate Change.

"Just as atmospheric heat waves can destroy crops, forests and animal populations, marine heatwaves can devastate ocean ecosystems," lead author Dan Smale, a researcher at the Marine Biological Association in Plymouth, England, tells AFP.

The oceans absorb more than 90 percent of the heat that results from greenhouse gases, and as a team of U.S. and Chinese researchers report in another recent study, marine warming may be our best metric for assessing the severity of climate change. The past five years have been the hottest ever recorded in the oceans, and 2018 now holds the title for highest ocean temperatures on record, the researchers report in Advances in Atmospheric Sciences, surpassing the previous record set in 2017.

"The numbers are huge," writes study co-author John Abraham, a professor of mechanical engineering at the University of St. Thomas in Minnesota, in an article for the Guardian. "[I]n 2018 the extra ocean heat compared to a 1981-2010 baseline amounted to 196,700,000,000,000,000,000,000 joules. The current rate of ocean warming is equivalent to five Hiroshima-size atomic bombs exploding every second."

In hot water

An ocean heat wave is relative and is based on ocean region having above-average temperatures for more than five consecutive days. Such heat waves now occur more frequently and with greater intensity, just like land heat waves. According to the Nature Climate Change study, there were 54 percent more heat wave days in the ocean per year between 1987 and 2016 than there were from 1925-1954.

"Globally, marine heatwaves are becoming more frequent and prolonged, and record-breaking events have been observed in most ocean basins in the past decade," Smale says.

To determine the effects of these aquatic heat waves, researchers looked at multiple events, including four El Niño events (1982-'83, 1986-'87, 1991-'92, 1997-'98), three events in the Mediterranean Sea (1999, 2003, 2006) and one in Western Australia in 2011. While the events all varied in their duration and intensity, what researchers found were negative impacts on marine ecosystems across the board.

For example, the 2011 heat wave in Australian waters killed off large swaths of seagrass and kelp and resulted in commercial fish species permanently moving to cooler waters. Seagrass death also occurred during two of the Mediterranean heat waves.

A bleached coral reef Ocean heat waves can threaten the health of coral reefs. (Photo: Sabangvideo/Shutterstock)

Or take "the blob." This mass of warm water stayed on the U.S. West Coast from 2014-'16 and increased temperatures by 10.6 degrees Fahrenheit (8 degrees Celsius). AFP reported that it resulted in toxic algae blooms, the closing of crab fisheries and the death of sea lions, whales and birds.

Damage to these environments creates ripple effects. The movement or loss of commercial fisheries can upend businesses and livelihoods that rely on the catching and selling of fish or ocean-based tourism. The destruction of foundational parts of the aquatic environment — kelp, seagrass and coral reefs — can drive away species that rely on those areas for shelter and food. Additionally, seagrass meadows serve as carbon stores in the ocean; their loss can lead to the release of carbon back into the atmosphere.

Similar to heat waves on land, ocean heat waves are expected to grow more severe and prevalent as climate change intensifies. And as Smale and his colleagues write in their study, the future of many species and ecosystems — along with human communities that rely on them — may depend on us confronting this crisis now.

"Given the confidence in projections of intensifying extreme warming events with anthropogenic climate change," they write, "marine conservation and management approaches must consider marine heat waves and other extreme climatic events if they are to maintain and conserve the integrity of highly valuable marine ecosystems over the coming decades."

Editor's note: This article has been updated with new information since it was originally published in March 2019.

Marine heat waves are transforming our oceans
Heat waves in the ocean are becoming more frequent and prolonged, scientists report, as ocean warming continues to break records.