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Marine cold spells, a potential buffer against warming seas, are fading away

  • A new study has found that marine cold spells have decreased in number and intensity since the 1980s due to climate change.
  • Marine cold spells can have both negative and positive impacts on the environment; they can wreak havoc on ecosystems like coral reefs, but they can also buffer the impacts of heat stress during marine heat waves.
  • While marine cold spells are decreasing, marine heat waves are increasing — but the relationship between these two kinds of events still isn’t clear, the study says.

In 2010, an influx of cold air blew over the lower Florida Keys, chilling the subtropical waters to temperatures as low as 11° Celsius, or 52° Fahrenheit. The coral reefs in the area, which had already been struggling against warm-water bleaching events, disease outbreaks and other stressors, grew weak in the cold. By the time the event was over, about 11% of the reefs had died, one study suggested, with inshore reefs sustaining the worst losses.

While this event, known as a marine cold spell, was detrimental to the marine environment, others can be helpful, especially as climate change heats up the oceans and causes more marine heat waves that disrupt the ocean’s ecological balance. For instance, a series of marine cold spells that happened off the west coast of Australia helped buffer the impacts of heat stress and allowed local abalone, scallop and crab fisheries to recover, according to a study.

A new study published in Geophysical Research Letters looks at the global trend of marine cold spells — defined as events in which water temperatures dip below a certain threshold and hold steady for at least five days — as our planet shifts and changes in response to human-induced climate change. The researchers found that there are 25% fewer marine cold spells today than there were in the 1980s, and that the intensity of these events is 15% lower than what they used to be. The researchers drew these conclusions by analyzing sea surface temperature data between 1982 and 2020.

“In our paper, we found that cold spells are decreasing and becoming less intense because of global warming,” lead author Yuxin Wang, a Ph.D. student at the University of Tasmania, told Mongabay in a video interview. “So that means you might lose some cold refugia under the warming ocean and the warming world. But on the other hand, because the marine cold spells are getting less extreme, they won’t cause huge impacts on the ecosystem directly.”

An endangered Florida manatee sleeping in shallow water in order to benefit from the sun’s heat on his back, while submerged in the cold-spell waters, 2012. Image by David Hinkel/U.S. Fish and Wildlife Service via Flickr (CC BY 2.0).

While marine cold spells are decreasing and becoming less intense, marine heat waves are intensifying. Similar to marine cold spells, marine heat waves are events that last for at least five days, but with unusually high temperatures instead of unusually low ones. A study found that marine heat waves increased by 50% between 1925 and 2016, and that these events have intensified in two-thirds of the global ocean since 1982.

However, the relationship between marine cold spells and marine heat waves remains unclear since these events happen at different rates, Wang said.

“The increasing rates of marine heat waves are not aligned with the decreasing rates of marine cold spells, and that could be explained by the internal temperature variability changes,” Wang said, adding that further research is needed to understand the relationship between these two kinds of events.

What is clear is that the decrease in marine cold spells has to do with a hastily heating world driven by human activity. According to the theory of planetary boundaries, there are nine Earth subsystems and processes that help stabilize the Earth, but that have limits to which they can deal with anthropogenic pressures. Once a critical threshold, or boundary, has been crossed, humanity moves away from a “safe operating space” and into a danger zone.

One of the core boundaries, climate change, was surpassed in 1988 when carbon dioxide levels surpassed 350 parts per million (ppm) in the atmosphere. This means that the planet will keep warming and changing as humanity continues to expel carbon dioxide into the atmosphere. According to the latest report from the Intergovernmental Panel on Climate Change, the changes brought on by global warming are now threatening the future of humanity.

Marine cold spells are naturally occurring phenomena driven by atmospheric fluxes that bring cold air to a particular region, or cold-water currents such as upwelling. Yet climate change is quickly warming both the air and water, changing the dynamics of the ocean.

A study found that ocean temperatures are the hottest they’ve ever been in recent history, breaking records for the sixth year in a row. Another study co-authored by conservation ecologist Kyle Van Houtan found that marine heat extremes are in fact the “new normal.”

From 2014-2016, a marine heat wave spread across the northeastern Pacific Ocean. In the summer of 2019, an expanse of warm surface water returned to the region. Image via NASA Earth Observatory.

“What was once by definition rare, is now ‘common,’ occurring more than half the time,” Van Houtan told Mongabay in an email. “In [2019], this reached 57% of the entire ocean.” In certain regions, such as the waters off the coast of the northeast United States and the boundary waters of Somalia, about 90% of the ocean is experiencing extreme heat, he added. “It’s hard for the ecosystems that we’re used to enduring those persistent extreme values and maintaining their integrity.”

While it is important to understand the rise in marine heat waves as the ocean warms and changes, experts say it is equally important to understand the changing nature of marine cold spells. And there is much still to learn about these phenomena. For instance, Wang noted that in some regions, including western boundary currents such as the Gulf Stream, marine cold spells are persisting and even becoming stronger.

“​​[Those would be] very interesting locations to study marine cold spells because they can provide cold refugia [amid] global warming in those particular regions,” she said.

However, it’s unclear what will happen to these remaining cold spots as the world’s oceans continue to warm, causing more extreme events such as marine heat waves and fewer marine cold spells.

“Extreme events affect coastal communities and economies, but members of the public might not be aware of how they’re going to intensify in the future,” said Sofia Darmaraki, a Ph.D. candidate at Dalhousie University who studies both marine heat waves and marine cold spells, but was not involved in this current study. “We need to get the word out. Information about the underlying, physical causes of these extreme events can help improve forecasting, which can lead to the development of early warning systems. That information can be provided to fisheries and other stakeholders, and they can collaborate on the best adaptations, the best path forward.”

Banner image: Bleached coral, Acoropora species. Image by Vardhan Patankar via Wikimedia Commons (CC BY-SA 4.0).

Citations:

Cheng, L., Abraham, J., Trenberth, K. E., Fasullo, J., Boyer, T., Mann, M. E., … Reagan, J. (2022). Another record: Ocean warming continues through 2021 despite La Niña conditions. Advances in Atmospheric Sciences, 39(3), 373-385. doi:10.1007/s00376-022-1461-3

Feng, M., Caputi, N., Chandrapavan, A., Chen, M., Hart, A., & Kangas, M. (2021). Multi-year marine cold-spells off the west coast of Australia and effects on fisheries. Journal of Marine Systems, 214. doi:10.1016/j.jmarsys.2020.103473

Lirman, D., Schopmeyer, S., Manzello, D., Gramer, L. J., Precht, W. F., Muller-Karger, F., … Thanner, S. (2011). Severe 2010 cold-water event caused unprecedented mortality to corals of the Florida Reef Tract and reversed previous survivorship patterns. PLOS ONE, 6(8). doi:10.1371/journal.pone.0023047

Steffen, W., Richardson, K., Rockström, J., Cornell, S. E., Fetzer, I., Bennett, E. M., … Sörlin, S. (2015). Planetary boundaries: Guiding human development on a changing planet. Science, 347(6223), 1259855. doi:10.1126/science.1259855

Tanaka, K. R., & Van Houtan, K. S. (2022). The recent normalization of historical marine heat extremes. PLOS Climate, 1(2). doi:10.1371/journal.pclm.0000007

Wang, Y., Kajtar, J. B., Alexander, L. V., Pilo, G. S., & Holbrook, N. J. (2021). Understanding the changing nature of marine cold-spells. Geophysical Research Letters, 49(6). doi:10.1002/essoar.10508759.1

Elizabeth Claire Alberts is a staff writer for Mongabay. Follow her on Twitter @ECAlberts.

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