The ocean has buffered us from the worst of climate change by soaking up most of the heat and carbon dioxide we are generating. But new studies show this process is rebounding on us – with extreme consequences. by Veronika Meduna, New Zealand Listener.
Climate change may have taken a back seat this year as countries grapple with Covid-19, but it hasn't slowed down. Even long periods of lockdown barely made a dent in emissions, and the chain of wildfires, storms and floods is beginning to look like a global relay race in which one extreme weather event hands the baton on to the next.
But there is another less perceptible climate transformation under way, happening mostly out of sight below the waves of the ocean. Since the middle of last century, the world's oceans have soaked up 93 per cent of the extra heat energy we are generating by releasing greenhouse gases into the atmosphere – and they are now at their warmest in recorded history.
"Last year was by far the hottest in the ocean," says Kevin Trenberth, a New Zealander who spent much of his career studying climate change at the National Center for Atmospheric Research in Boulder, Colorado, but returned home just before New Zealand's lockdown.
"The past decade was also the warmest. If you look at the ocean heat content, 2018 is the second hottest, 2017 is third, 2015 is fourth, and 2016 is fifth."
Trenberth and several other climate scientists analysed global ocean temperatures going back to the International Geophysical Year in 1957-58, when systematic observations began in Antarctica and ships took measurements as they crossed the Southern Ocean. To work out how much extra energy the oceans have absorbed, the team combined ship-side measurements with data from a more modern fleet of autonomous Argo floats (see box, page 18) that have been tracking changes in ocean temperature and salinity since 2006.
This extra energy has been described as the equivalent of three billion Hiroshima atom bombs, but Trenberth prefers a more realistic comparison. "The overall warming of the ocean is about 100 times all of the electricity that is used around the world at any time."
Like other climate signals, ocean warming has been accelerating in recent years, and the heat has been seeping further into the deep. At first, only the top 200-300m of the ocean, known as the sunlight zone, were warming up. But since the late 1980s, heat has been diffusing further and further down through the twilight and midnight zones to depths of 2km and more.
More than sea-level rise
The most obvious consequence of a warming ocean is sea-level rise – warmer water expands and also helps to melt polar ice – but the repercussions reach much further, says Trenberth. "Marine heatwaves are now popping up increasingly, and we're about to see more. The ocean is rearing its head, but it's a much more ponderous and slow system, which also means it's going to continue to respond long after we stabilise emissions – if we can."
One of Trenberth's colleagues in this project, Michael Mann, is an equally high-profile and outspoken climate scientist in the US who was on sabbatical in Sydney during Australia's extreme wildfires early this year. He points to a clear link between warming oceans and extreme weather on land.
"Warmer oceans mean an intensified hydrological cycle, more rainfall but also more evaporation … and that leads to drying of the continents, a major factor behind wildfires, from the Amazon all the way to the Arctic and including California and Australia.
"It's also not a coincidence that we've seen the strongest hurricanes and worst floods on record during recent years as the oceans have been at their all-time warmest. Warmer oceans mean more energy to intensify storms, more moisture to turn into record floods. We've seen all of these things play out in recent years."
Locking in further change
It's hardly surprising that so much of the heat is going into the sea. Oceans cover more than two-thirds of the planet, in some places the ocean is deeper than the highest mountains, and water has a greater heat capacity than air.
This is an aspect of planetary physics we should be grateful for. By storing all this heat and drawing it into the deep, global seas have shielded the land from the worst effects of rising emissions.
"The ocean is buying us a lot of time," says New Zealand climate commissioner James Renwick. "[This] is fantastic for humanity, but the downside is that the oceans are so slow to respond that we're just locking in sea-level rise and further change for centuries."
Siphoning heat away is only half the story. The oceans also soak up about a third of the carbon dioxide we emit, preventing the gas from accumulating in the atmosphere and warming the climate. A special report published by the Intergovernmental Panel on Climate Change (IPCC) last year predicts that if the world's oceans continue their double climate duty at the same rate, they will enter unprecedented conditions this century.
In both these roles, the Southern Ocean, a churning beast that swirls around Antarctica, has been doing some heavy lifting. According to the IPCC, the Southern Ocean accounts for more than half the global heat uptake between 2005 and 2017 and is seen as the world's biggest ocean sink for carbon dioxide.
None of this comes without consequences, says University of Otago marine chemist Kim Currie. Scientists now often use the shorthand tag "hot, sour and breathless" to describe the main repercussions of ongoing warming, changes in chemistry that make the oceans more acidic and the loss of oxygen in some regions.
Currie's focus is on ocean acidification. Every two months, she goes on a 65km voyage off the Otago coast to collect water samples from the surface and 500m below. The Munida time series, named after the sample-collecting boat that first sailed out of Dunedin Harbour in 1998, is the longest record of ocean acidification in the Southern Hemisphere.
It corroborates what other groups are observing elsewhere: that the ocean's pH is changing towards more corrosive conditions. Currie's work is part of a nationwide project called Carim (Coastal Acidification: Rate, Impacts and Management), which shows that the twin effect of warming and acidification will probably change life in the sea for all food chains.
About half the oxygen we breathe is produced by marine phytoplankton, swarms of single-celled algae that use sunlight and carbon dioxide to grow just like land plants do. Niwa oceanographer Cliff Law's part of the Carim project was to figure out how phytoplankton would fare in warmer and more acidic seawater, as predicted for the middle and the end of this century. He filled nine massive tanks with seawater and set them up to mimic current and future conditions – and found that acidification alone made no significant difference, but when combined with rising temperatures, it fundamentally changed the ecosystem.
"The positive news is that we don't see a decrease in the overall biomass of phytoplankton, but the change is at the community level. Phytoplankton is as diverse as other ecosystems – there is a large range of sizes and families, each with a different role. What we see is a decrease in diversity. Smaller and some larger groups drop out and a middle-sized group of diatoms starts to dominate the phytoplankton in numbers and biomass."
The types of algae that almost disappear rely on carbonate to build their intricate armour. Carbonate is formed when carbon dioxide is dissolved in water, but as more CO2 is added, the increased acidity helps transform carbonate into bicarbonate.
Law's team investigated the composition of phytoplankton of the future further and found decreased levels in some essential fatty acids and nitrogen. The knock-on effect is that "despite the fact that there's more biomass, the food these primary producers represent … will be less nutritious".
For seabed grazers, crustaceans and fish larvae, this could mean they have to feed more to get the same level of nutrition, shifting their energy balance towards foraging at the cost of growth and breeding. "There'll be trade-offs," Law says. "We're not necessarily going to lose organisms completely, but in surviving that future ocean, they might have to trade off in one area, and that could mean smaller body size and less energy going into reproduction."
As more carbon dioxide continues to enter the ocean, it upsets a cascade of chemical reactions in ways that make life harder for many marine creatures that rely on carbonate to build their shells or skeletons – corals, shellfish, sea snails.
A spanner in the works
The ocean is the world's largest ecosystem, and most fish species are now moving poleward towards cooler waters. As the ocean continues to change, its own capacity to take up carbon dioxide could also shift.
The Southern Ocean is our largest marine sink for carbon dioxide, but using new high-tech floating probes, scientists recently discovered that it releases the greenhouse gas back into the atmosphere during winter in regions south of the polar front – around 60° latitude. This raises questions about how effective it will be as a climate buffer in decades to come – and it "throws a spanner in the works for our understanding of what's going on with the Southern Ocean", says Niwa climate modeller Sara Mikaloff-Fletcher.
As the Southern Ocean circles – connecting the Pacific, Atlantic and Indian basins – its main driver is a belt of strong and persistent winds. Its churning waters – notorious for shipwrecks – constantly haul up deeper layers that haven't seen the surface in years. They absorb carbon dioxide and pull it back down into the deep.
When this carbon sink slowed down during the 1990s, scientists figured it was because the westerly winds had ramped up and brought up water masses that were already rich in carbon, says Mikaloff-Fletcher. It's less clear what caused the carbon sink to strengthen again during the 2000s. Now, with the latest discovery of winter CO2 release, "we're back to the drawing board in trying to understand the ocean's mean state".
Last month, Trenberth published his latest findings, showing that the ocean is not only at its warmest but also becoming more stratified – its layers of colder or warmer, saltier or fresher waters are more rigid. This has profound and troubling implications, he says.
Surface waters will warm faster and take up less carbon dioxide (just as warm fizzy drinks go stale). With less mixing, the oceans will no longer draw as much heat and CO2 into deeper layers and more will stay in the atmosphere. "It's what we call 'positive feedback' – or a vicious cycle."
Climate change is only one stress factor for the ocean. Chemical pollution, overfishing and plastic waste all add to the challenge. Next year, the United Nations will launch a decade of ocean science to support research that tackles all these issues.
University of Otago marine scientist Abby Smith says it doesn't really matter what "is the straw that breaks the ocean's back – we need the ocean to be healthy and we're creating one that isn't".
The Intergovernmental Panel on Climate Change released a special report in September last year focusing on the world's oceans and icy landscapes. A synthesis of more than 6000 studies, it outlines the damage climate change has already done to oceans, polar ice sheets and mountain glaciers. This is what it says about the world's oceans:
• The global ocean has warmed unabated since 1970, taking up 90% of the excess heat in the climate system. Since 1993, the rate of ocean warming has more than doubled. Marine heatwaves have probably doubled in frequency since 1982.
• The global mean sea level is rising, accelerating in recent decades as a result of increasing rates of ice loss from the Greenland and Antarctic ice sheets as well as continued glacier melting and ocean expansion – as the ocean warms, water expands.
• Since about 1950, many marine species have shifted in geographical range and season in response to ocean warming and other changes, such as the loss of oxygen.
• Coastal ecosystems are all affected as marine heatwaves intensify, oceans acidify, oxygen levels drop, salt water intrudes and sea levels rise.
• The Greenland and Antarctic ice sheets are projected to lose mass at an increasing rate throughout the 21st century and beyond. Large reductions in greenhouse-gas emissions in the coming decades would limit this process after 2050.
• Over the 21st century, the ocean is expected to reach unprecedented conditions: increased temperatures, greater upper ocean stratification, further acidification and less oxygen. Changes will be smaller under scenarios with lower emissions.
• The sea level continues to rise at an increasing rate. At many locations, extreme sea-level events that are historically rare – once a century in the recent past – are expected to occur frequently – at least once a year – by 2050 in all emission scenarios and especially in the tropics.
• A decrease in the global biomass of marine animals and fisheries catches is expected over the 21st century from the ocean surface to the deep sea floor under all emission scenarios.
• Marine organisms and ecosystems will be better able to adjust if we achieve lower emissions. Sensitive ecosystems such as seagrass meadows and kelp forests could be under threat if global warming exceeds 2°C above pre-industrial levels.
Acidifying the Oceans
Niwa marine ecologist Vonda Cummings is particularly interested in pāua. She says the biggest challenge in the life cycle of pāua and other marine molluscs – clams and snails – is the brief window during which they transform from soft-bodied larvae to shelled juveniles.
"We know clearly from all the work that's been done around the world that the early life stages are the most susceptible, and it seems to be that stage when shellfish start to form their shells at about two days old. But if they can get through that stage, then what happens to the juveniles and adults?"
To find out, she kept adult pāua in tanks under end-of-century low-pH conditions, spawned them and tested if the adults' exposure to more acidic waters somehow conferred resilience on their young. It didn't.
The adults did okay – though Cummings is yet to complete analysis on whether the thickness and strength of their shells changed – but their offspring were no better off when put into future ocean conditions.
It's a different story for green-lipped mussels, New Zealand's major aquaculture species and a $200 million export earner. At Nelson's Cawthron Institute, Norman Ragg ran similar experiments on the mussels, using hatchery spat from a selective breeding programme. As with pāua, the weakest part of the life cycle of green-lipped mussels is their metamorphosis from a fertilised egg to a larval stage when they start making two closing shells to protect themselves. Among the 96 families Ragg tested, some had "genetic potential to develop resilience to ocean acidification".
"This is something they haven't experienced over evolutionary time, but we are seeing that different genetic lines have different levels of resilience to ocean acidification."
The Ocean's hotspots
Added heat is not distributed equally throughout the world's oceans. Thanks to a fleet of autonomous ocean probes, marine scientists have identified global hotspots, including the waters around Tasmania and a band at about 40° latitude near New Zealand. There, the rate of heat uptake is almost 10 times the global average.
Oceanographers began deploying Argo probes during the early 2000s, and 3800 floats now monitor the world's oceans, including the Southern Ocean. Each float sinks to a depth of 2km and surfaces regularly while measuring temperature and salinity along the way.
"Argo has collected 2.1 million of these depth profiles," says Niwa oceanographer Phil Sutton. "That's almost four times as much information as all the ship-based measurements collected in the entire history of oceanography."
As well as producing ocean heat maps, the Argo fleet records changes in salinity, showing fresh regions getting fresher and saltier regions getting saltier.
Water world: At risk NZ areas
Scientists have pinpointed the areas of New Zealand expected to be most affected by rising sea levels.
Only a few days into 2018, a monster storm drenched much of the North Island within 24 hours. The deluge swept over Auckland and massive waves surged inland from the Firth of Thames, ripping up coastal roads, flooding farms with seawater and cutting off communities in the Coromandel.
A marine heatwave in the Tasman Sea had aggravated a low-pressure system and the resulting tempest came barrelling inland, coinciding with a king tide and riding on top of higher seas. "Heat in the ocean was a big part of that event," says Tim Naish, a climate scientist at the Antarctic Research Centre.
Even a 40cm sea-level rise would mean such 100-year storms become annual – and that's a scenario we may well experience mid-century. Already, the global ocean has risen 25cm since 1880, in an expansion driven by warming and the melting of land glaciers and polar ice sheets. And the process is accelerating, with 9cm accrued in less than three decades since 1993.
Globally, the ocean will be between 0.46m and 1.1m higher by the end of the century, relative to 1986-2005, depending on if and how quickly we can curb emissions. "We could restrict global sea-level rise to less than 50cm if we can achieve the Paris target to be carbon neutral by 2050," says Naish. "But the thing we can say with a degree of certainty is that regardless of what we do, we're going to get 20-25cm of sea-level rise on top of what we've already got by about 2060 – that's already built in from the heat in the system."
The sea won't rise at the same rate everywhere, though, says Richard Levy, a climate scientist who leads the New Zealand SeaRise programme with Naish. The project combines the latest global sea-level rise projections with local data on land movement. Whereas some parts of the country have been uplifted by earthquakes, other areas are sinking, either because of tectonic effects or because the land is compacting. Sea-level rise will be relatively higher locally in areas that are sinking – a process most noticeable in places that have been drained, such as the Hauraki Plains, and urban areas on reclaimed land such as South Dunedin, where the ocean pushes the groundwater level up as it intrudes underground.
Dunedin, Napier, The Hutt
When it comes to the number of people affected by rising seas, South Dunedin is top of the list, says Belinda Storey, a climate economist and director of Climate Sigma. Other areas where local authorities already have to contemplate their options between defence and retreat include Napier, where rising seas increase the city's tsunami risk, and large river mouths such as the Hutt Valley at the northern end of Wellington Harbour, where an increased risk of flooding carries on up the river.
Across the country, $2.7 billion worth of roading and other public infrastructure is at risk if sea levels rise by 0.5m, according to a Local Government New Zealand report published last year.
One way of shielding communities is to build protective structures along the coast. Storey prefers the term defence to protection – the latter gives a false sense of permanence, she says. "People presume that if you build a seawall, then you are protecting the assets behind it indefinitely, and you're not. All you're doing is buying some time. Because sea-level rise is going to be continuous and because extreme events are becoming more frequent and severe, all you're doing is reducing your risk for a limited time."
Hard defence lines also come with equity challenges. "They may provide a level of defence for often private property that sits behind it, but when you put that hard structure in place, you shrink the size of the beach because you don't allow it to move inland. Members of the public who may otherwise enjoy that beach are no longer able to do so. It's only the people who sit behind the wall that continue to enjoy the beach view."
Rising seas not only wash further inland during storms but erode shorelines and cliffs and force saltwater into rivers and groundwater – and they don't just affect the coastline, says Judy Lawrence, a research fellow focusing on climate adaptation at the Climate Change Research Institute. The Hutt Valley is preparing for more frequent flooding upriver because "you only need about 30cm of sea-level rise before our gravity-fed drainage system is just not going to work properly and you have periodic, and then permanent, wet areas up the valley".
Lawrence has developed a new approach to planning for local authorities to be able to make decisions despite an ever-changing level of risk. Known as dynamic adaptive pathways planning, it's a staged approach that considers options with enough flexibility to adapt as new information becomes available.
She's been putting it into practice with three councils along the Hawke's Bay coastline, which have joined forces with communities and mana whenua to develop a coastal strategy for the century ahead. For some communities, a managed retreat from the coast will ultimately become unavoidable, but Lawrence says the local councils are signalling that as a problem now and putting in provisions not to build any more assets.
For Naish, there is another good reason to keep the focus on the goals of the Paris agreement. "One of the things we have learnt is that there's a threshold in the Antarctic ice sheet, and it's around 1.5-2 degrees [of warming]. If you go above that, not only are you going to go above half a metre of sea-level rise globally, but you're going to commit the planet to multi-metre, irreversible sea-level rise in the centuries to come."