(CNN) Since the 1970s, the climate crisis has made our oceans warmer and more acidic, reducing the number of fish we rely on for our food and putting the future of fish in peril, according to a major UN report out Wednesday.
Rising temperatures mean oceans will have less oxygen, and this, along with more heatwaves and increased acidification, will make fish move further away from the coast and create larger deadzones, where life cannot survive.
Ultimately, the report said, this will lead to the extinction of some species of fish, which Americans have been eating an increasing amount of recent years.
The US dietary guidelines recommend 8-12 ounces of seafood a week to keep a healthy diet. Fish plays an even bigger role internationally, providing up to half of all animal protein eaten in developing countries and it remains a leading source of vitamins and minerals.
The "projected decreases in seafood availability from climate impacts on fisheries catch potential will elevate the risk of nutritional health impacts," the report concluded.
Ocean warming this century has contributed to an "overall decrease in maximum catch potential." Overfishing makes the issue worse, the Intergovernmental Panel on Climate Change (IPCC) report said.
Scientists are "virtually certain" that the ocean has warmed between 1970 and 2017.
Marine heatwaves, periods of extremely high ocean surface temperatures, have "very likely" doubled in frequency from 1982 to 2016, according to the report, lasting longer and becoming more intense due to the climate crisis.
Warmer waters hold less oxygen, and fish need that oxygen to survive. Warmer waters also lead to algae blooms, red tides that further decrease oxygen, and kill sea life. A ride tide triggered a state of emergency in Florida in 2018, killing thousands of animals and costing the state millions. The toxin created by the algal blooms can also make shellfish poisonous to humans.
Warmer waters endanger coral reefs that are homes to many fish. Foundation species, meaning fish food, also decline.
A February study noted that the increased ocean temperature has already led to more than a 4% global decline in sustainable catches. For a catch to be sustainable, it means the greatest number of fish, crustaceans like shrimp, and mollusks like sea scallops, that can be caught without depleting the stocks of these fish long-term.
It's "virtually certain" that the ocean has taken up between 20-30% of the total human produced carbon since the 1980s, changing the very chemistry of the ocean, making it more acidic, the IPCC report found.
Acidification eats away at the shells of some mollusks. The change in the ocean's pH makes carbonate ions less abundant. Oysters, mussels, clams, and other animals need these ions to build up their shells. The acidification also hurts creatures like calcareous plankton that many fish rely on for food
Ocean acidification has already caused massive die-offs of oysters in the Pacific Northwest.
In February, Margaret Pilaro Barrette, the executive director for the Pacific Coast Shellfish Growers Association testified in front of Congress that the corrosive water makes oyster shells dissolve faster than they can form. The oysters need that shell to grow into a baby oyster. There was a "severe oyster seed shortage" from 2007 to 2009 in the US, she testified, largely due to ocean acidification. The growers have adjusted, but there needs to be a better long-term plan to ensure oyster production can continue into the future, she said.
Without a plan of reducing carbon emissions, the ocean may be so acidic by 2080, that even creatures like some corals that had been able to withstand these conditions may erode quicker than they can rebuild.
The open ocean is also losing its oxygen, between 0.5-3.3% between 1970 and 2010, the IPCC report found, and it's expected to get worse. That means we can expect to see even larger "dead zones" where life cannot survive.
There are more than 400 coastal dead zones and they are growing, earlier studies found.
A reoccuring deadzone in the Gulf of Mexico is primarily linked to farming runoff, but it is exacerbated by the climate crisis that has led to harder rains and more flooding events on these farms, scientists have said in earlier research.
If the world continues as it is, the loss of oxygen, and the increased acidification and heatwaves, will make some species extinct and will move fish further away from the coasts.
The IPCC report predicts that by 2100 the ocean will take up two to four times as much heat since 1970, and globally marine heatwaves will very likely increase by a factor of 50 by 2081-2100 if the world doesn't curb its current emissions.
Globally, the average amount of oxygen in the ocean will decline by 3% to 4% and upper ocean nutrients will decline by 9% to 14% by 2100. By 2100, it's also very likely to have year-round corrosive conditions for shelled animals, the report predicts, for animals in the Arctic, Southern, and some parts of the North Pacific and North Atlantic oceans. All coastal ecosystems will be at high to very high risk as well.
"The key take away from this report is that fish in the ocean are the proverbial canary in the coal mine for climate impacts," said Malin Pinsky, an ecologist who studies marine communities, who was not involved in the IPCC report. He is an associate professor at Rutgers.
"This new report is a key step in helping everyone, including policymakers, understand exactly what could happen."
"We've seen progress with some of the stock coming back after we have managed it better, after overfishing, but we should push back against a growing optimism," Boettiger, the Assistant Professor in the Department of Environmental Science, Policy and Management at UC Berkeley said.
"Just because we've been able to rebuild some stocks that have been overfished, doesn't mean everything will be ok."
With the climate crisis, "there is no guarantee there will be recovery by mid-century. We don't exclude that or say it's impossible, but we need to be quite cautious."
Already many of the world's fishery stocks are overexploited or are in decline, according to the UN. Seafood many people love like Atlantic cod and certain salmon are challenged, and the continued climate crisis will not do these fish any favors.
"Fish are responding faster to climate change than many species on land, they are more vulnerable," Pinsky said. "They are outrunning our laws and regulations and are responding to climate change before we've had a chance to adapt."
Pinsky said he hopes that people will read this report and realize that we are at this "key tipping point in history." It is a time, he said, "where we have, as individuals and as a country, enormous control over what future we pick."
"If we are headlined for a future with clean energy and clean transportation and less carbon dioxide emissions, we don't need to be heading into a scary future," Pinsky said. "We can act and adapt."