In the chilly waters off Iceland and Norway, the world's largest populations of Atlantic cod have kept fishing fleets thriving for centuries. They feature in Viking legends and grace today's Icelandic coins.

They're also facing a rising climate threat that could crash these critical fisheries.

As greenhouse gas pollution increases, it will create a double whammy for Atlantic and polar cod populations, with warming and more acidic water projected to undercut their reproduction and alter their habitat dramatically, new research shows.

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The study projects that Atlantic cod hatchling numbers around Norway and Iceland will decline 60 percent if temperatures warm more than 3 degrees Celsius above pre-industrial times—considered likely in the second half of this century if greenhouse gas emissions aren't cut back significantly—according to scientists with the Alfred Wegener Institute for Polar and Ocean Research.

A dramatic decline of cod populations would pose grave risks for commercial fishing, and because cod are a key food source for other species, a population crash could have a "disastrous" effect on regional ocean ecosystems, said Flemming Dahlke, lead author of the study, published Wednesday in the journal Science Advances.

Some changes are already happening.

"In the Barents Sea, there have been shifts in populations. The Arctic fish communities are retreating," he said. During a temporary regional ocean warming phase in the 1940s, Atlantic cod populations shifted northward by almost 1,000 kilometers, showing how fast and dramatic climate impacts can be, he said.

A marine biologist from Fisheries and Oceans Canada tags cod in 2016. The Atlantic cod populations off Newfoundland and New England have been slow to recover after overfishing contributed to a population crash in the 1990s. Credit: Craig F. Walker/Boston Globe via Getty Images

The rapidly warming climate is also affecting other species, and it can have international implications. Before 2000, the Iceland fishing fleet caught almost no mackerel. Since then, the fish have moved northward as waters warmed, increasing tensions between Iceland and other countries over quotas. By 2016, mackerel made up 8 percent of Iceland's total catch.

If global warming can be kept to the goals of the Paris climate agreement, cod populations will have a better outcome. "Limiting global warming to 1.5 degrees Celsius means that both species will be able to find optimum temperatures at their current spawning sites," Dahlke said.

Warming Waters + Acidification

Dahlke said the findings can help make better predictions of where fish will go as the climate changes, helping fisheries managers and communities adapt to climate change impacts.

Another recent study provided similar insights in North America, projecting big shifts in habitat for hundreds of fish species on the Atlantic and Pacific coasts. The recently released U.S. National Climate Assessment also warns that by mid-century, if greenhouse gas emissions continue at a high rate, 86 percent of U.S. marine ecosystems will experience combinations of temperature and acidity that have never before been experienced by modern species.

Other Atlantic cod populations—including along the New England and Newfoundland coasts—crashed by as much as 95 percent in the 1990s with overfishing and warming waters and have yet to recover.

Cod are especially sensitive to both temperature and increased acidity because their larvae float near the surface, where even small changes in temperature and acidity can combine to kill the eggs or cause serious deformation, said the new cod study's co-author Daniela Storch.

Polar cod thrive under the sea ice and play a key role in the Arctic food web. This fish was exposed as a research vessel broke through the ice off Norway. Credit: Jasmine Nahrgang/UiT The Arctic University of Norway

The study found that increasing acidification makes cod embryos more sensitive to higher and lower temperatures, effectively narrowing the optimal temperature range for reproduction. That means both species' potential spawning grounds will shrink as the planet warms.

"If we have the combined effects of ocean warming and acidification—production of embryos will further decrease," Storch said.

If emissions continue at a highrate, conditions for young Atlantic cod will deteriorate in the North Atlantic near the end of this century. Atlantic cod populations in the Northeast Atlantic will likely shift into the Arctic, she said.

That would put additional pressure on young polar cod—the most ice-adapted of fish—which drift with the ice, living in crevices and feeding on ice algae. Polar cod will decline if larger, more aggressive Atlantic cod start occupying more of their habitat as sea ice melts, and that would ripple through the ecosystem, all the way to seals, birds and whale species like orcas and humpbacks that feed in the far north.

Dahlke and Storch said the results of their lab research on temperature and acidity impacts to eggs, larvae and spawning habitat are clear, but there are also natural variables that will affect future development of cod populations, including ocean currents and the availability of food.

Climate Change Impact Goes Beyond Cod

The new findings on cod fit with a 2016 study showing that climate change could cut global fishing revenues by $10 billion dollars by 2050. The biggest impacts would be to developing countries in the global south where local fishing is a primary source of daily food, according to Vicky Lam, a University of British Columbia fisheries scientist who led the economic study.

Iceland and Norway's annual cod fishery catches 800,000 tons worth about 2 billion Euros. Hundreds of years ago, transportation of preserved cod deep into inland areas of Europe may have helped fuel the growth of cities by providing supplemental nutrition.

The new research also dovetails with information in the recent National Climate Assessment. It describes how "warming water temperatures in the Gulf of Maine exacerbated overfishing of Gulf of Maine cod, and the subsequent low quotas have resulted in socioeconomic stress in New England."

A recent heat wave in the Gulf of Alaska sent a shock through the Pacific cod population as well, with cod numbers dropping so much in 2016 and 2017 that fisheries managers cut the quota by 80 percent in 2018.

Extreme events like ocean heat waves are still a wild card in projecting long-term climate change impacts to fisheries, according to Lisa Kerr, with the Gulf of Maine Research Institute, which closely tracks cod in the northwestern Atlantic.

"It's a topic for us in the Gulf of Maine. We can't pinpoint all the effects, but we think they (extreme events) have lasting impacts," Kerr said.

Dire Impacts, Possible Adaptation for Fishers

The National Climate Assessment lists a series of impacts on fisheries attributed to ocean heat waves, including a collapse of lobster prices in New England in 2012. An outbreak of toxin-producing algae in 2015 caused a shutdown of the Dungeness crab fishery on the West Coast.

Kerr said researchers are also studying whether a warming ocean will also cause more problems with infectious disease and parasites. The science in that field for marine species is emerging, but freshwater biologists say warmer water has spurred more such problems in lakes and rivers.

More research on global warming impacts will improve projections for disease outbreaks in pelagic and shellfish fisheries, protecting human health and fishing economies.

And the National Climate Assessment found that human-caused climate change likely contributed to the ocean heat waves, which increased the "intensity and frequency of toxic algal blooms."

"In the absence of significant reductions in carbon emissions, transformative impacts on ocean ecosystems cannot be avoided," it concluded.

The new cod research also makes it clear that limiting global warming to 1.5 degrees Celsius is the best way to avoid the worst long-term impacts to the important cod populations. That can only be achieved by steep, rapid reductions in emissions of carbon dioxide from burning fossil fuels, eventually bringing this pollution to zero.

But the detailed mapping by Dahlke's team can also help fishing industries and communities adapt to ongoing changes that are already locked in because of past emissions.

"We can identify critical habitats or areas that could become future habitat, and they could be protected from human impacts like oil and gas drilling," he said. The mapping can also improve seasonal management of fisheries to protect by populations by preventing overfishing.