Storm clouds form over the North Sea in Whitby, England

Sea levels could rise by up to 1.1 metres by 2100 if greenhouse-gas emissions continue to rise.Credit: Ian Forsyth/Getty

The world’s oceans have long helped to stave off climate change by absorbing heat and carbon dioxide from the atmosphere. But that is changing, with devastating consequences for humanity in the coming decades, leading researchers warn in a high-level report commissioned by the United Nations.

The rate at which oceans are warming has doubled since the early 1990s, and marine heat waves are becoming more frequent and intense ― trends that are reshaping ocean ecosystems and fuelling more powerful storms. And as the oceans absorb CO2, they are becoming more acidic, which threatens the survival of coral reefs and fisheries.

The special report on oceans and ice by the Intergovernmental Panel on Climate Change (IPCC) warns that without steep cuts to greenhouse-gas emissions, fisheries will falter, the average strength of hurricanes will increase and rising seas will increase the risk of flooding in low-lying areas around the globe.

The oceans “can’t keep up” with humanity’s greenhouse-gas output, says Ko Barrett, vice-chair of the IPCC and a deputy assistant administrator at the US National Oceanic and Atmospheric Administration in Washington DC. “The consequences for nature and humanity are sweeping and severe.”

More than 100 scientists from 30 countries contributed to the report. The IPCC released a 42-page summary of the analysis on 25 September at a meeting in Monaco.

High-water mark

The report projects that sea levels could rise by up to 1.1 metres by 2100 if greenhouse gas emissions continue to rise. That is about 10 centimetres more than the IPCC estimated in its last comprehensive report on the global climate, which it released in 2013.

Richard Alley, a geoscientist at Pennsylvania State University in University Park, says that the latest report’s sea-level rise projections are conservative. That’s because scientists still aren’t certain about when rising temperatures might trigger a rapid collapse of ice sheets, particularly in western Antarctica. If that happens, ocean levels will rise much faster than the IPCC’s latest estimate.

“Sea-level rise could be a little less, a little more, or a lot more” than the latest report predicts, Alley says. “But it’s not going to be a lot less.”

Those rising seas will increase the risk of flooding during storms, the report says, and high tides will become more frequent and severe. By 2050, flooding events that now occur once per century are likely to occur annually in many coastal cities and islands ― even with sharp emissions cuts.

But the report does make it clear that humanity can blunt the worst effects of climate change over the very long term. It projects that the sea level could be from 0.6 metres to 5.4 metres higher in 2300, depending in large part on whether and how quickly countries move to curb greenhouse-gas emissions.

“We’re going to get sea-level rise for centuries,” says Michael Oppenheimer, a climate scientist at Princeton University in New Jersey and coordinating lead author on the report’s chapter on sea-level rise. “The question is whether it’s going to be manageable or not.”

A draft version of the special report estimated that rising seas could displace 280 million people worldwide by 2100. The IPCC removed that figure from the final analysis, after scientists decided that they had misinterpreted the findings of an earlier study, Oppenheimer says.

Changing patterns

The IPCC report also examines the fate of the planet’s ice ― which it says will continue to shrink in the coming decades.

In the Arctic, where sea ice melts every summer and freezes every winter, the annual summer minimum extent has decreased by nearly 13% per decade since 1979. That rate of change is probably unprecedented in at least 1,000 years, the IPCC says. About 20% of the Arctic’s permafrost is vulnerable to abrupt thaw, followed by sinking of the soil left behind. By the end of the century, that could increase by half the area of the Arctic that is covered by small lakes.

And mountainous regions with small glaciers ― from the Andes to Indonesia ― could lose 80% of their ice by 2100.

The report’s overarching message, Barrett says, is that climate change is affecting water from the tops of Earth’s highest peaks to the depths of its oceans, and ecosystems are responding. Without steep emissions cuts, the total biomass of marine animals could shrink by 15% by 2100, and commercial fisheries could see their maximum catch decrease by up to 24%.

Such changes are already playing out in many locations, says Kathy Mills, a fisheries ecologist with the Gulf of Maine Research Institute in Portland. In the North Atlantic Ocean, for example, rising temperatures have sent right whales north in search of cooler waters. And that increases the animals’ chances of getting caught in lobster-trap lines.

“These ocean changes mean big problems for the future of people,” says Jane Lubchenco, a marine ecologist at Oregon State University in Corvallis and former head of the US National Oceanic and Atmospheric Administration.

Lubchenco is an adviser to the High Level Panel for a Sustainable Ocean Economy, which released its own report on climate change and the world’s oceans on 23 September. The analysis identifies a range of actions — including promoting renewable energy and sustainable fisheries, curbing marine shipping emissions and protecting coastal ecosystems — that could reduce global carbon emissions and limit the effects of climate change.

Lubchenco says those actions would also bolster coastal economies and help lift people out of poverty. “The reality is that the ocean is central to solving many problems,” she says. “The situation is quite dire and quite gloomy, but it is not hopeless.”