Record temperatures are causing disruptions and destruction to the world’s seas from Antarctica to Florida. Clive Cookson reports.
Over the past few weeks, a large-scale rescue operation has been under way off the coast and keys of Florida. It began as water temperatures were rising towards a peak of 38.4C — similar to someone with a high fever — which was recorded on July 24 by a buoy off Manatee Bay.
Knowing that the heat was well above the level corals can tolerate, biologists at the Coral Rescue Foundation in Key West felt they had little choice but to evacuate specimens to onshore tanks.
Corals mount a two-stage response to heat stress, first bleaching and then dying. Some of the southernmost reefs, exposed to the hottest water, are already dead but those in slightly cooler locations have a better chance of survival and regeneration.
“We’ve taken representative colonies of all the species and all the generic strains that we’ve worked with and moved them into land-based facilities where they can be temperature controlled and stabilised,” says Alex Neufeld, a marine biologist with the foundation.
The coral crisis in Florida and the Caribbean is just one of the ways in which the oceans have shocked and alarmed scientists this year.
Perhaps the most striking evidence has been in the seas around Antarctica, where far less ice than usual is forming in the annual midwinter freeze. Marine biologists are worried too about the effect on fisheries of sea temperatures as much as 5C above normal in parts of the north Atlantic.
“This really is an extremely unusual year,” says Stefan Rahmstorf, professor of ocean physics at Potsdam University in Germany. “If you look at Atlantic temperatures and Antarctic sea ice, the changes are off the charts.”
Average sea surface temperatures across the globe have been rising by just over 0.1C per decade since 1970 — about half as fast as the atmosphere. The overall increase in the past year is closer to 0.2C, with much more intense marine heatwaves in some regions.
On August 1, average global sea surface temperatures reached an all-time high just short of 21C, Europe’s Copernicus Climate Change Service says. This record is particularly disconcerting because March is usually the hottest month for the world’s oceans.
The underlying cause of the warming is human activities pumping carbon dioxide and other greenhouse gases into the atmosphere, scientists say. But the reasons why marine heating is speeding up now are far from clear.
El Niño, the cyclical warming of the eastern equatorial Pacific Ocean which causes global changes in temperature and rainfall, “is still in its early stages and is not dominating what we are seeing”, says Chris Merchant, professor of ocean and earth observation at the University of Reading. He expects significant further warming over the next year as El Niño reinforces the long-term trend.
Many marine animals have a much narrower liveable temperature range than their terrestrial counterparts. Coral reefs are especially vulnerable to overheating because they are fixed in place.
Free-swimming ocean fish can and do move towards the poles and down to deeper waters in an effort to keep cool, as Carolin Dahms and Shaun Killen of Glasgow university confirmed recently in an analysis of 595 fish populations spanning all the world’s oceans.
“We observed a striking trend, with species living in areas that are warming faster also showing the most rapid shifts in their geographical distributions,” says Dahms.
“While relocation to cooler water may allow these species to persist in the short term, it remains to be seen how food-webs and ecosystems will be affected by these changes,” adds Killen. “If the prey of these species don’t also move, or if these species become an invasive disturbance in their new location, there could be serious consequences down the road.”
A Greenland-sized hole
In the depth of the southern winter, the ocean around Antarctica is freezing over. But this year sea ice is forming in slow motion, falling far short of any previous winter on record and leaving scientists shocked and mystified by the extent of still-open water.
“What we’re seeing is truly remarkable,” says Ella Gilbert, a polar climate researcher at the British Antarctic Survey (BAS). “The missing ice has left an area of open ocean bigger than Greenland.”
Statisticians often try to measure how unusual an event is by the Greek letter sigma, which denotes its deviation from the mean. The missing Antarctic ice has reached seven sigma, judged from the baseline of the past 40 years, says Edward Doddridge, an oceanographer at the Institute for Marine and Antarctic Studies in Tasmania.
That means it would not occur by chance in many billions of years. “What it really tells us is that the statistical model has failed,” he says. “We are so far outside the historical range that it no longer makes sense to talk about sigma and standard deviations. This must be a new state of the system, fundamentally unlike previous years.”
Over several decades up to 2015, the extent of Antarctic sea ice increased slightly, in contrast with declining Arctic ice cover — and contrary to the predictions of scientific modelling which shows less glaciation for both polar oceans in response to global warming. The expected decrease started in 2016 and gathered pace before this year’s precipitous fall.
“Many climate scientists will be viewing this period since 2016 as the real world catching up with the models, [showing] that climate change has finally ‘burnt through’ the natural barriers around the sea ice formed by the unique ocean and atmospheric circulation, which insulates the continent to a significant extent,” says Andrew Meijers, a BAS polar oceans researcher. “The question was always ‘when’ rather than ‘if’ under the present emissions trajectory.”
Scientists are struggling to understand what is driving the changes and the interplay between natural variability and human-driven warming in the far south. The components are extremely complex, as sea ice interacts with the vast continental ice cap and its ice shelves as well as ocean currents and the atmosphere.
“The ocean circulation around Antarctica has a hugely disproportionate impact on global heat and carbon distribution between the ocean and atmosphere,” says Meijers, “as well as driving sea level rise via ice sheet melt and supplying nutrients to much of the world’s oceans.”
More observations are desperately needed, he says, particularly in winter and under the sea ice and ice shelves, to understand how the system works and how it is changing.
The biggest gap in the evidence now is about the thickness of Antarctic sea ice, which cannot be measured directly by satellites. “Without knowing that, we cannot calculate the volume of ice with any confidence,” says Doddridge in Tasmania.
“It may be that the ice has been getting thinner and thinner over the years — and this year reached a critical point where it became so thin and fragile that it can’t expand out to the north as it has previously,” he adds.
The current lack of midwinter ice is likely to lead to another low point in cover as it melts during the coming Antarctic summer. “If there is more open water exposed, it can heat up much more quickly than if it was covered by ice reflecting back the sunshine,” says Kaitlin Naughten, a BAS ocean modeller.
Doddridge also predicts an adverse impact on glaciation of the Antarctic continent, which would accelerate global sea level rise. “We have preliminary results telling us that losing sea ice is going to accelerate the collapse of ice shelves and ice sheets,” he says. “The sea ice protects the coastline and coastal margins, by dissipating the energy from waves.”
The effects on wildlife and ecosystems will be many and varied. Mammals and birds that rely on sea ice to feed or breed, such as emperor penguins, will be hit hard. But ecologists are most worried about how krill will cope. These tiny crustaceans, which populate the polar oceans in vast numbers, “are basically the staple food source of everything around Antarctica”, says Doddridge.
“Krill come up as larvae under the ice during the winter and they feed on algae growing on the underside of the ice,” he says. “If that ice disappears and the algae aren’t present, we don’t know what the krill will do because we’ve never seen them do anything else.”
Looking further ahead, climate scientists are concerned about the impact of melting ice around both polar regions on the great “overturning” circulations, the network of currents that span the world’s oceans at various depths, carrying heat, chemicals and nutrients around the world.
The flow of fresh water from melting ice disrupts the sinking of cold and salty water to the ocean depths, from where it moves towards the tropics. There is evidence that both Antarctic and Atlantic overturning circulations are weakening as a result, with some experts suggesting that one or both might collapse at some point in the next few decades — with catastrophic effects on the world’s climate.
Courage, not hope
Heatwaves typically affect around 10 per cent of the oceans at any one time. The proportion experiencing one is now 44 per cent, according to the US National Oceanic and Atmospheric Administration. The agency forecasts that it will increase to 50 per cent in September and October.
A complex combination of factors on top of the overarching addition of greenhouse gases to atmosphere is accelerating ocean heating. One, ironically, may be recent global action to clean up shipping by switching to low-sulphur fuels. This has increased the amount of sunlight reaching the sea surface by cutting the production of pollutant particles that can stimulate cloud formation.
“There’s a debate about whether the impact of these aerosols is small or significant but we are certainly not going to turn back to the horribly dirty ships of the past,” Rahmstorf says.
To Shovonlal Roy, an ecosystem modeller at Reading university, the most alarming thing about the warming oceans is that they lose their ability to absorb oxygen. “This has a particular effect on larger fish, leading to fish kills. Within days, thousands of them can die,” she says.
Other ecological disruptions associated with marine heatwaves include harmful algal blooms and disrupted fish spawning.
While coral reefs take up less than one per cent of the ocean floor, their extraordinary biodiversity, rivalling tropical rainforests, makes them home to more than 25 per cent of all marine life.
A bleached coral has expelled the colourful symbiotic algae that live inside it, providing food and nutrients, but it is not dead. “It can recover and reincorporate algae back into its tissues if water temperatures return to more normal levels within a few weeks,” says Neufeld of Florida’s Coral Rescue Foundation.
Unfortunately, NOAA forecasts that Florida’s marine heatwave will persist in the region till October this year and Neufeld is sceptical that many of the region’s wild reefs will survive that long.
At the same time as transferring the corals to land-based aquaria, the Coral Rescue Foundation is moving its offshore “nursery structures” from their current depth of 3-4 metres down to 10m where the water is slightly cooler. The organisation expects to be able to preserve some wild Florida corals in the face of global warming. But in other places around the Caribbean with fewer conservation and rescue resources, this may not be possible.
The Coral Reef Alliance, an international conservation charity, estimates that 90 per cent of the world’s coral reefs will experience bleaching every year by 2050 and is developing a global warning and response network, including satellite observations, to help mobilise resources when necessary.
“Coral reefs are very important for coastal defences as well as incubators of biodiversity,” says Merchant, “but few will survive without human help.”
As researchers uncover yet more bad news about the oceans, many are aware of a growing political backlash against climate action and a sense that the public is growing weary of scientists’ doom and gloom stories.
“It’s something I struggle with all the time because you’re told you have to give people hope,” says Naughten at the BAS. “What do you do if your science tells you that it is just a bad news story?
“We need courage, not hope,” she adds, quoting the US climate scientist Kate Marvel. “Courage means doing well without the assurance of a happy ending. We need to be responsible and do the right thing.”
For Merchant, the Reading ocean scientist, the argument is easier to make than it used to be when there were few alternatives to fossil fuels. “Twenty years ago, when electric vehicles and green power were not available, it was difficult to know what could be done to drive people towards net zero,” he says. “Now we have enough answers to go a long way, though some people are still in denial about how bad it will get.”
In Florida, Neufeld says: “We are doing all we can to ensure that we continue to have coral populations, small though they may be relative to their heyday, so that we can buy the time needed to spur climate action on a much larger scale.
“If unfortunately it takes a statewide bleaching and mass coral mortality to raise awareness and make that change, then so be it.”
Written by: Clive Cookson
© Financial Times