Plastic pollution and overfishing may currently be in the headlines. But rising CO2 emissions are also a key threat to the ocean that covers 71 percent of the planet, and a major challenge for future life on Earth.
Frank Bainimarama knows all about the need to protect the ocean. As the prime minister of Fiji, he runs a country made up of more than 300 islands in the South Pacific. This week, he's been co-hosting the first ever UN conference on the ocean in New York, along with Sweden's Climate Minister Isabella Lovin, focusing on sustainable development and conservation of the world's oceans, seas and marine resources.
Speaking to the assembled leaders from government, science, business and civil society, Bainimarama - also the incoming president of this year's UN Climate Conference in November - made it clear that ocean protection and climate change are inextricably linked.
"Climate change poses the biggest threat the world has ever known," he said. "And the quality of our oceans and seas is also deteriorating at an alarming rate. They are interlinked, because rising sea levels, as well as ocean acidity and warmer waters, have a direct effect on our reefs and fish stocks and the prosperity of our coastal communities."
In hot water
2016 was not only the warmest year on record, as far as our planet's atmospheric temperature is concerned. In its "Statement on the State of the Global Climate 2016," published earlier this year, the World Meteorological Organization (WMO) confirmed that globally averaged sea-surface temperatures had also hit record levels.
Scientists were surprised at the speed at which sea temperatures have risen. In the last century, the 15 years with the highest ocean heat anomalies have all been within the last two decades, according to data from the United States' National Centers for Environmental Information (NCEI). And the WMO report predicts that the trend will continue.
"The extra carbon dioxide that gets released into the atmosphere through our fossil fuels and deforestation is associated with extra heat," explained Susan Avery, an atmospheric physicist and president emerita of the renowned Woods Hole Oceanographic Institution, in an interview with DW last October. "And 93 percent of the extra warming is actually in the ocean."
Earlier this year, Avery became the first climate scientist to join the board of ExxonMobil.
Migrants and aliens
Warming seas play havoc with the organisms that live there, completely changing their living conditions to the extent where some are no longer able to survive in their traditional habitat. Mojib Latif, head of research on ocean circulation and climate dynamics at the Geomar Helmholtz Centre for Ocean Research in Kiel, told DW that the ocean isn't just warming on the surface, but down to a depth of 2,000 meters (around 6,500 feet), "a giant volume."
That heat will still be present - and will slowly escape from the ocean - for centuries to come. "This is basically why global warming will continue even if we don't really emit any greenhouse gases any longer," he said.
Fish species are moving toward the poles in search of cooler water, and alien species are moving in to replace the traditional residents in warming waters.
Coral reefs are perhaps the best-known victims of ocean warming. They are frequently in the headlines, most notably Australia's famous Great Barrier Reef, which has been devastated by bleaching.
"Corals don't have very great temperature tolerance. So if the oceans warm by more then 1.5 degrees [Celsius] or so, they would probably die," said Latif. "And it's well possible that without climate protection, all tropical corals will be gone by the middle of the century."
Warming seas are also influencing the world's weather patterns, and scientists are expecting increases in extreme weather events.
The El Nino weather event, a natural cycle, illustrates what happens when the ocean warms.
"El Ninos are events that are characterized by enormously high sea surface temperatures," said Latif. He attributed the extreme rainfall and flooding in Peru earlier this year to extremely warm water off the coast. "Sea temperatures off Peru are slowly warming."
The question is whether global change is having an influence on the climate of South America.
"What we can say is that El Ninos have become more frequent during the recent decades and they have somehow became stronger during the last 20 years," said Latif.
"Whether this is already a trend or not is hard to say, but some climate models actually project that this is exactly what's going to happen in response to climate change."
Warmer water, rising seas
Recent research shows sea levels are rising almost three times as fast as they were 25 years ago. Experts say ocean warming has been responsible for about half the sea level rise that we have observed during the last century, as warmer water expands.
The other key factor is melting ice, as the world warms. The Intergovernmental Panel on Climate Change (IPCC) is working on a special report on the ocean and climate change, to be finalized in September 2019. It will include information on ice melt in Greenland and in the Antarctic, including the East Antarctic, long thought to be immune to global warming.
Hans Pörtner, of Germany's polar and marine research center the Alfred Wegener Institute, is co-chair of an IPCC working group which focuses on impacts, vulnerabilities and adaptation possibilities for humans under climate change. He views this area as one of most challenging.
"We have increased awareness that previous estimates of sea level rise have been too conservative," he told DW. Looking back at the Earth's history, he said, "we know that during the last interglacial period, with 0.7 to 2 degrees global warming above pre-Industrial levels - these are the comparable numbers to where we are today - we had a sea level of around 7 meters [about 23 feet] higher. And if we compare and look back at the last period in Earth's history where we had 400 parts per million CO2 - and that's also where we are now - we had a similar degree of higher sea level."
So far, however, countries have based their adaptation and coastal protection measures on earlier, more conservative estimates.
"They will have to increase their ambition in coastal protection. And in several examples, we will have to give land back to the oceans. We may not be able to protect the land because sea level may actually reach 2 meters or even more by the end of the century," Pörtner said.
Upsetting the conveyor belt
The influx of melting ice doesn't just lead to rising sea levels, as atmospheric physicist Susan Avery explained.
"When the land-based ice comes into the ocean, you get a freshening of certain parts of the ocean, so particularly the sub-polar North Atlantic. So you have a potential for interfacing with our normal thermohaline circulation systems, which could dramatically change that," she said. "The changes in salinity are beginning to be noticed, and changes in salinity are a signal that the water cycle is becoming more vigorous."
Scientists describe these systems as the "global conveyor belt" that links the oceans of the world and in turn influences climate. One example is the Gulf Stream system, which is to a large extent responsible for the temperate climate of Europe and parts of North America.
A study released this week in the Proceedings of the National Academy of Sciences in the US suggests that if the ice influx increases far enough, the disruption of the ocean current system could also have devastating consequences in Africa, with a disruption in rainfall patterns playing havoc with agriculture.
An acid bath?
Alongside the extra heat, the CO2 we have indirectly been dumping into the ocean has another devastating impact.
"The extra carbon dioxide, when it gets dissolved into the ocean, through various simple chemical equations, will increase the PH or acidification in the ocean," Avery explained. Seawater is already at least 26 percent more acidic than it was before the onset of the Industrial Revolution in the 19th century, according to the "State of the Ocean" report published by the International Programme on the State of the Ocean back in 2013. The report found the ocean could be 170 percent more acidic by the end of the century.
Over the last 20 years, scientists around the world have been conducting laboratory experiments to find out what that would mean for the flora and fauna of the ocean. Ulf Riebesell, a professor of biological oceanography at Geomar, conducted the world's first experiments in nature off the coast of the Norwegian archipelago of Svalbard in 2010.
The on-site experiments showed that increasing acidification decreases the amount of calcium carbonate in seawater, making life very difficult for sea creatures that use it to form their skeletons or shells. This change will affect coral, mussels, snails, sea urchins and starfish, as well as fish and other organisms, Riebesell told DW.
"Some of these species will simply not be able to compete with others in the ocean of the future," he said.
That could have severe economic and social consequences, as acidification ultimately affects the food chain. Riebesell pointed out that coral reefs, for instance, are home to numerous species, serve as nurseries for fish, attract tourists and protect coastlines against waves and storms.
Riebesell has headed the German projectBIOACID (Biological Impacts of Ocean Acidification) for the last eight years. It's expected to publish a summary of its results ahead of the UN climate conference in Bonn in November.
Hot problem for cold water
Polar areas are most affected by ocean acidification, as cold water is able to more quickly absorb CO2. Experiments in the Arctic, which is already warming around twice as fast as the global average, indicate that the seawater there could become corrosive within a few decades, Riebesell said. "That means the shells and skeletons of some sea creatures would simply dissolve."
At a working meeting of the BIOACID group in Kiel last week, IPCC co-chair Hans Pörtner told DW that ocean acidification, a relatively young research field, had already made its way into the policy arena.
"It's being considered as one of the climate challenges for the ocean, and it's also considered in the context of sustainable food supply, especially for developing countries," he said.
But even countries like the United States are already feeling the effect, said Pörtner. On the country's west coast, ocean acidification is already threatening oyster cultures, and he said this economic impact has increased awareness of the problem.
Scientists are also concerned that the increasing amount of CO2 being stored in the ocean could, in turn, create a feedback effect that could further exacerbate global warming.
In the long run, the ocean will become the biggest sink for human-produced CO2, but it will absorb it at a slower rate.
"Its buffer capacity will decrease, the more acidic the ocean becomes," said Riebesell.
Science and policy
Pörtner told DW that the writing is on the wall. "We really have to go into ambitious mitigation, if we want to have a chance to keep this under control."
Carol Turley, senior scientist at the Plymouth Marine Laboratory in the UK, is highlighting the issue of ocean acidification at the UN conference in New York this week. She told DW that a combination of local management measures and global action were necessary to protect the ocean and implement the UN's sustainable development goals.
"Although acidification, warming, sea level rise and oxygen loss represent global challenges requiring international agreements, there are varied options for reducing other stressors while efforts continue to reduce emissions of carbon dioxide to the atmosphere," she said.
Turley said it was "inspiring" that UN Secretary-General Antonio Guterres and the co-chairs of the ocean conference, Fiji and Sweden, had recognized the key role of climate-related ocean stressors, including ocean acidification, and that the combined impact of these may threaten the implementation of the UN's Sustainable Development Goal 14 (SDG 14), which focuses on the ocean and life below water.
She said it was also heartening to hear their messages of hope. "It's not too late to fix the ocean. This shows how the evidence from science has been taken up by policy makers," she said.
Carol Turley of the Plymouth Marine Laboratory has been highlighting the issue of ocean acidification in New York this week
Martina Stiasny, a young climate scientist from the Geomar Helmholtz Centre for Ocean Research in Kiel, Germany, told conference participants in New York about her research on how climate change-induced ocean acidification is affecting fisheries.
"We are already starting to see the first effects in the oceans," she told DW. "It's imperative that we start to act now. Otherwise it poses a serious threat to the health of the marine ecosystems and food security. It's not only important for the SDG 14, but also links to most other SDGs in terms of human health, fighting hunger and poverty and most other goals."
The landmark ocean conference has been taking place against the background of US President Donald Trump's decision to take the US out of the 2015 Paris climate agreement, a move Stiasny describes as a "major disappointment."
But she takes heart from the widespread movement to press ahead with emissions reductions and a transition to renewable energy.
"The statement that America made by pulling out is of course the wrong message to send to the world," she said. "But fighting climate change is done every day by everyone on this planet and I believe we have no choice but to stay optimistic."