Oceans and ice are absorbing the brunt of climate change

Climate change is here, heating the oceans and crumbling the planet’s ice sheets, a new report from the UN’s Intergovernmental Panel on Climate Change (IPCC) lays out.

On Wednesday, the IPCC released a major report on the state of the planet's oceans and ice. The 900-page report, which compiles the findings from thousands of scientific studies, outlines the damage climate change has already done to the planet’s vast oceans and fragile ice sheets and forecasts the future for these crucial parts of the climate system.

Climate change’s impacts, the report says, are already readily visible from the top of the highest mountain to the very bottom of the ocean—and tangible for every human on the planet.

The problems aren’t theoretical, the report stresses: Science shows that they are here, now. And the oceans, polar ice caps, and high mountain glaciers have already absorbed so much extra heat from human-caused global warming that the very systems human existence depends on are already at stake.

For example, Planpincieux glacier on the Italian side of Mount Blanc is expected to collapse at any time, prompting road closures and evacuations of structures in the area. And in the oceans, many fisheries have shifted and shrunken, impacting million-dollar businesses and subsistence fishers alike. The 27 percent of Earth’s human population that lives near coasts are bearing the brunt of higher seas and stronger storms. Marine “heat waves” sweep across the ocean twice as often as they did only three decades ago. And millions that rely on water from high-mountain glaciers and snowpack, the "water towers" of the world, are adjusting to both newly strengthened floods and devastating droughts.

These challenges are only going to get worse unless countries make lightning-fast moves to eliminate greenhouse gas emissions, the report says. But strong, decisive action could still forestall or evade some of the worst impacts.

"The oceans and cryosphere have been taking the heat of climate change for decades,” says Ko Barrett, the vice chair of the IPCC. “The report highlights the urgency of timely, ambitious, coordinated, and enduring actions. What’s at stake is the health of ecosystems, wildlife, and importantly, the world we leave our children."

Why we should listen to this report?

In 2015, world leaders gathered in Paris at a climate-focused meeting, where they agreed to try to limit planetary warming to an average of 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial temperatures—and to aim for a more ambitious goal of keeping warming under 1.5 degrees Celsius (2.7 degrees Fahrenheit).

At the time, 2 degrees Celsius was considered a “safe” target. Keeping the planet’s average temperature below that, world leaders said, would still result in great stresses on the economy, social systems, and natural environments, but would stave off the most devastating impacts.

Since then, two things have happened: First, science has made clear that the planet has already warmed about 1 degree Celsius, on average, while some regions, like the Arctic, have overshot that warming by at least four times. Second, thousands of scientists have diligently catalogued evidence that even 1.5 degrees of warming could push parts of the climate system in ways that would have devastating environmental, social, and economic impacts.

The IPCC gathers up evidence from scientists worldwide and summarizes the state of knowledge about the planet’s present and future, and sets about reassessing what the past few years of new science could tell us. Since 1990, it has prepared five comprehensive assessment reports, and it’s currently working on the sixth. It also prepares special reports on specific topics—including three important ones in just the past year.

The first, released last year, warned that even 1.5 degrees of warming would wreak havoc on the planet. The second, a few months ago, outlined both the already-observed impacts and likely future of lands and forests. This latest report, on the oceans and ice caps, rounds out the trio. (A related report, released earlier this year, summarized climate change’s impact on the planet’s biodiversity, warning of imminent collapses in many delicate ecosystems).

Taken together, the reports offer a bleak vision of the future, particularly because it is rapidly becoming apparent that both the 1.5- and the 2-degree Celsius goals will be difficult, if not impossible, to hit. The 1.5 degrees report said countries would have to aim for a “net-zero” greenhouse gas situation by 2050 in order to meet that target. But we’re currently on a very different track—one that leads us to 3.5 degrees or more of warming by the end of the century. (See how the planet’s carbon budget looks now).

Last week, an estimated four million people worldwide marched in a global climate strike, demanding that world leaders take action to address climate change. But earlier this week, when world leaders gathered at the UN Climate Action Summit in New York, they collectively failed to announce any major new commitments to solving the carbon problem. Meaningful action is still sparse—only a few countries are close to hitting the targets for reducing their emissions.

What’s at stake? Just about everything.

This report summarizes decades of research from scientists worldwide and focuses on two crucial parts of the climate system: oceans and ice. Climate change has already reshaped both.

The ocean has borne the brunt of the impacts, absorbing over 90 percent of the extra heat trapped in the atmosphere by excess greenhouse gases since the 1970s and somewhere between 20 to 30 percent of the carbon dioxide. That means water has buffered land-dwellers against the worst effects of climate change; without it, the atmosphere would have heated up much more than the average of 1 degree it already has.

“The rate of climate change has actually gone up since 1993, and that rate of warming of global oceans has actually doubled since then,” says Nathan Bindoff, a lead author on the report and an oceanographer at the University of Tasmania.

In tandem, marine heat waves—short bursts of hot marine weather—have also doubled, stressing out anything that they sweep over.

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Photograph by Brian Skerry, National Geographic

But the ocean’s buffering influence has come at a cost, with a fingerprint that is becoming ever clearer to scientists and anyone else paying attention to the natural world.

“The payback from oceans taking up all that heat is enormous,” says Matthew England, an oceanographer at the University of New South Wales. “We see it not just through warmer surface waters, but in melting ice caps, and also in things like the intensifying of tropical cyclones.

“A warmer ocean loads more moisture in the atmosphere and generates more intense rainfall. And marine life is obviously impacted. The list goes on: there’s all sorts of payback from the oceans’ absorption of the extra heat,” he says.

Hotter oceans fuel stronger hurricanes and rainier storms. But warming also has influences less obvious to humans. As the surface of the water warms, it gets lighter, making it harder to mix it with cold, nutrient-rich water below. So the top part of the ocean stagnates slightly, holding less oxygen and less of the critical nutrients that support marine life. And the carbon dioxide bleeding into the ocean is causing it to become more acidic, stressing out any organism that builds its shells out of acid-sensitive calcium carbonate, from tiny plankton to oysters to massive reef-building corals.

All together, the effects on marine life are far from subtle. Already, the report summarizes, about 30 percent of the world’s reefs have been stressed to near their breaking point, and 60 percent are heavily threatened. With 1.5 degrees of warming—the ambitious target, given that we’ve burned through a degree of that already—science tells us that 70 to 90 percent of reefs could collapse by 2100. At 2 degrees, that number jumps to more than 99 percent.

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That’s devastating for the reefs themselves, but also for the communities that live near and rely on them, the report says. Reefs operate as barriers to soften the blow of storms and waves on coastal communities; as nursery zones for many of the fish that feed humans worldwide; they sustain tourism and cultural practice for coastal communities; and more.

The warming of the top layer of the ocean also affects the fisheries humans rely on. Already, a range of marine species have been marching poleward in search of cooler climes. (Read about how “global fish wars” caused by climate change could be coming).

“Now, there’s tons of evidence, decades of change that we’ve observed, and now we can go in and say we are confident that climate change is influencing so many of these different species,” says Jennifer Sunday, a marine biologist at McGill University.

And if we continue tracking forward with the same high-carbon lifestyle the planet is currently on, science suggests that fish stocks could decrease by nearly 20 percent by the end of the century. Already, catches from open-ocean fisheries, like tuna, have stagnated, the report summarizes. That’s partly because of overfishing, but the pattern is exacerbated by climate change.

Ice isn’t immune

Ice everywhere—from the high mountains to the polar ice caps—is also changing, and fast.

Some impacts of melting ice are felt very directly by communities that live nearby. In the high mountains, like the Andes or the Himalaya, glaciers are retreating at unprecedented rates some 30 percent higher than a few decades ago, the report says.

Glacier melt currently provides fresh water to millions of high-mountain dwellers, as well as communities downstream. As glaciers retreat, the amount, timing, and quality of the meltwater change—and people have to respond.

In the high Himalaya, meltwater courses into lakes that sit at the edge of retreating glaciers. The lakes are often perched precariously above towns and villages, threatening to flood the settlements below.

And in Peru, the extra melt coming off the rapidly retreating glaciers of the Cordillera Blanca has an expanding high-elevation agricultural boom. But that glacier—like most of the high-mountain ice in the world—is likely to disappear or dwindle to a nub of its former self by the middle of the century, reshaping the economy of the region.

“These glaciers are critical for all kinds of water resources,” says Twila Moon, a glaciologist at the National Snow and Ice Data Center in Colorado. “Drinking water, agriculture, energy production, so much more. Even where they’re not perhaps problematic at this moment, they will be soon; we can very clearly see these problems coming down the line.”

Changes in Earth’s ice also affect people who live far from the high mountains or the poles. Since the early 1900s, average sea levels around the world have risen by about 16 centimeters (just over 6 inches). Now, the report says, melt from Greenland and Antarctica, the planet’s great storehouses of ice, is the primary driver of rising sea levels around the world. Melt from those ice sheets is now responsible for over half of the sea level rise happening today—about 1.8 millimeters each year.

That may not sound like a lot, but it adds up fast, and the number is projected to go way up. But the speed at which the ocean rises, and how much higher it will get, depends on how we collectively manage our carbon budgets now.

If all countries held themselves to their most ambitious goals, the polar ice sheets would add somewhere between 5 to 23 centimeters to the world’s oceans by 2100 (the mountain glaciers and the expansion of warmer ocean waters would add more). But if we continue on a path similar to today’s, by the end of the century the ice sheet could add much more melt to the oceans: between 11 to 55 centimeters.

Overall, that means sea level rise of just over 40 centimeters by 2100 if we’re careful, and over 80 centimeters—more than 2 feet—if we’re not.

And, the IPCC report points out, there is emerging evidence that those numbers could jump even higher if a major tipping point in Antarctica is crossed. Warm water, creeping closer and closer to a delicate part of the West Antarctic ice sheet, could initiate a runaway melting situation that would cause vast swaths of the ice sheet to collapse.

“It’s really this sort of doomsday scenario,” says Brooke Medley, a glaciologist at NASA’s Goddard Space Flight Center. “Once you trigger that retreat, it’s nearly impossible to stop.”

The report makes explicit mention of the possibility but doesn’t include those estimates in the 2100 projections.

No change occurs in a vacuum: It’s all connected

In West Antarctica, the ice responds to changes in the ocean, and vice versa. That’s a perfect illustration of the climate system, says Regine Hock, a glaciologist at the University of Alaska and a lead author on a chapter of the report: It's an interconnected set of phenomena. What happens in one part of the world is far from isolated.

“The changes we see are not only consistent across the systems, but they’re linked. From the highest mountains to the oceans, these systems are directly linked to each other,” says Hock.

And the choices humans make in one part of the world can affect every other. The future looks very different in a world where emissions drop quickly, the report shows.

“Our future depends on who we are and what we can do together,” says Heidi Steltzer, a lead author of the report and a mountain scientist at Fort Lewis College. “It’s a time when we must collaborate on solutions.”