Marine heatwaves have become longer and more frequent over the last century, a study on long-term changes in marine heatwaves has revealed.
The research shows that annual marine heatwave days have increased by 54 per cent from 1925 to 2016, with an accelerating trend since 1982. “This means a marine ecosystem that used to experience 30 days of extreme heat per year in the early 20th century is now experiencing 45 marine heatwave days per year. That extra exposure time to extreme heat can have detrimental effects on ecosystem health, with impacts on biodiversity as well as economic activities including fisheries and aquaculture.” says lead author of the research Dr Eric Oliver from Dalhousie University, Canada.
Marine heatwaves are defined as prolonged periods of unusually warm water at a particular location. Much like the worrying atmospheric heatwaves that make headlines each summer around the world, marine heatwaves are of concern too.
The authors suggest that given the likelihood of continued ocean surface warming throughout the 21st century, and the accelerating trend observed in recent decades, we can expect a continued global increase in marine heatwaves in future, with important implications for marine biodiversity.
Dr Dan Smale of the Marine Biological Association, who co-convened the working group and co-wrote the paper, has examined the ecosystem impacts of marine heatwaves. He says, “in In 2011, a marine heatwave off Western Australia caused an entire ecosystem to shift from being dominated by kelp forests to being dominated by turfy weeds, which in turn affected the fish and invertebrates that used the kelp forests as habitat. We also observed mass mortality events of corals, abalone and fish in response to extreme temperatures”.
“We needed ocean temperature measurements resolved daily. But on a global scale, that’s only been available since the early 1980s when satellites first began observing the ocean surface,” says Dr. Oliver. “But a few sites around the world have daily measurements going back to the early 20th century, and we also have monthly measurements of sea surface temperatures globally going back over a similar period.”
The research team was able to borrow information across these three sources of data to piece together a single global record of marine heatwaves, going back to the early twentieth century, for the first time ever.
“Our next step is to better understand how ecosystems will respond to an increased duration and frequency of marine heatwaves”, says Dr Smale. “We need to draw upon evidence from recent marine heatwaves, as well as from controlled experiments, to determine the likely impacts of extreme heat events on biodiversity and on the provision of ecosystem services such as fisheries production and nutrient cycling, which are hugely important to society”.
More information on this and related studies can be found on www.marineheatwaves.org
Figure 1 Marine heatwaves, discrete periods of extreme ocean heat, occur across the global ocean. For example, in summer 2014 multiple events were recorded in the northeast Atlantic, with temperatures soaring to 5°C above the long term average during the most extreme marine heatwave. Marine heatwaves can have major impacts on ecosystems, such as widespread loss of seagrass habitat and shifts in the ecology of commercially-important species like lobsters.