Of all the consequences of a warming world, the idea that our oceans would rise is, on its face, one of the easiest to understand – and maybe the most terrifying. We’re land animals, after all. It’s been a long, long time since we were all sea life.

Earth’s ice melts when it gets warmer, and the warming is greatest at the North and South Poles. With increasing melting of land ice – as has been the case in particular in Greenland in recent years – the oceans get higher, just as water in a bathtub rises from an open faucet.

But that’s not the whole story, which is more complicated. For one thing, the top 100 meters of the world’s oceans are also expanding as they get warmer – a phenomenon called thermal expansion. That too is causing seas to rise.

It’s important to recognize that melting ice doesn’t always cause sea levels to rise. Only ice that melts on land masses such as Greenland and the North American continent and flows into the sea causes it to rise. Consider that to be “land ice.” Ice floating in the ocean – and here we switch to “sea ice” off Greenland and at the edge of Antarctica – does not cause sea levels to rise, as least not directly, although it can contribute to further warming as the dark watery surface replaces white ice, as explained below.

There are a few things well worth knowing about melting sea ice:

• Ice shelves hold back glaciers that otherwise would slide toward the open ocean. When ice shelves shrink as a result of melting, there can be an acceleration of glacial ice melt – something already seen after the Larsen B Ice Shelf on the Antarctic Peninsula splintered and collapsed in just over a month back in 2002. After that collapse, the Larsen A and B glaciers abruptly accelerated off the Antarctic continent. In 1996, those glaciers were losing two to four billion tons of ice per year to the sea. By 2006, they were losing 22 to 40 billion tons per year.
• That sea ice in the Arctic? By itself, it doesn’t cause sea levels to rise when it melts, as explained earlier. But sea ice at the roof of the world is white, and it radiates heat back into space. The Arctic, in fact, acts as a great global air conditioner. So, when sea ice melts, it leaves a dark blue ocean behind, which absorbs heat, causing more ocean warming. That leads to more thermal expansion of the ocean, an acceleration of glacial melt where glaciers meet ocean water, and even changes to the jet stream over the Northern Hemisphere, altering storm tracks across North America, Europe, and elsewhere. Climate scientists refer to this as a feedback loop, and there are many of them in the planetary system, where one consequence leads to another consequence, or intensifies an existing one.

So how much have the world’s oceans risen? On average, sea levels have gone up more than eight inches since 1880 – and three of those inches have been over just the past 25 years, which suggests that the globe’s seas are rising faster in this century than they did in the last.

Indeed, the world’s oceans are now rising at an accelerated rate, primarily as a result of increased melting in Greenland and Antarctica. If this accelerated rate keeps up through the end of this century, sea levels around the globe could rise more than two feet by 2100 – twice the amount expected if sea levels were rising at a steady rate.

At least one recent study suggests that if global greenhouse gas emissions continue unabated, it’s possible that by 2100 the rise in average global sea levels could exceed six and a half feet. Under this scenario, millions of people would be displaced, and major food growing regions, such as the delta region of the Nile River, could be lost.

As seas rise, high tides are becoming higher, and storm surges are growing more destructive. Island nations such as Tuvalu in the South Pacific face life-threatening challenges as they confront the relentless rise in sea level. Given such concerns, coastal communities around the globe are planning for the future; either discussing how to fortify their coastlines against rising seas – arguably a losing battle – or systematically moving inland, a process known as managed retreat. There’s also talk, particularly along the Gulf Coast, about restoring coastal wetlands, thereby creating buffer zones that could better absorb the blows of storm surges and increasingly higher tides.

While seas are rising, they are not rising by the same amount everywhere. One reason has to do with thermal expansion: ocean water is warmer in some places, like the tropics, than other places, like the Arctic, so sea levels are generally higher where sea water is warmer. Another reason involves wind, which shapes the contours of the surface of the ocean and can have a direct impact on local sea levels. Ocean currents, meanwhile, also shape the oceans, having large local effects on some coastlines and smaller ones elsewhere. Finally, the shape of a coastline and tectonic forces can shape how a local coastline responds to rising sea levels. In many areas, land is naturally sinking over time and becoming more vulnerable to rising seas. The Chesapeake Bay region, for example, is sinking today as a result of a shift of underlying rock since the last ice age. Overall, U.S. sea levels are rising the fastest along the East Coast and the Gulf of Mexico.

Researchers also have pointed to a slowing Gulf Stream, shifts in North Atlantic weather, and El Niňo climate cycles for making Eastern Seaboard cities such as Norfolk, Virginia, Baltimore Maryland, Charleston, South Carolina, and Miami, Florida, particularly vulnerable.

Coastal areas along the West Coast are subject to the same combination of phenomena, and flooding in places like the San Francisco Bay Area is a big concern. Some scientists say natural cycles in the Pacific have masked the effects of sea-level rise on the West Coast for the past two decades, but that’s now changing.

There has been plenty of troubling news in 2019 about melting ice and rising seas. This summer, the Arctic has sweltered under a severe heat wave, and melting of Greenland’s ice sheet has gone into overdrive. Over the month of July alone, 197 billion tons of ice from the Greenland ice sheet melted into the ocean – enough to raise sea levels around the entire planet by about 0.02 inches.

For additional information on sea-level rise and climate change, here are some worthwhile online sites:

Intergovernmental Panel on Climate Change (IPCC) AR5 – Sea Level Change.

Federal government

U.S. Global Change Research Program: Sea level rise

NOAA Climate.gov: Climate change: Global sea level

NASA Global Climate Change: Sea level

NOAA Ocean Service: Is sea level rising?

Independent groups

Union of Concerned Scientists: Sea level rise and global warming

Climate Central: Sea-level rise

Open Knowledge Repository: Climate change and sea-level rise : A review of the scientific evidence

Media

National Geographic: Sea-level rise, explained

Recent news articles

“Extreme ice melt in Greenland threatens coastal communities across the world, scientists warn,” Aug. 1, 2019.

“Sea-level rise could flood Bay Area homes, according to new map,” Aug. 1, 2019.

“How sea-level rise will change Orange County,” Aug. 1, 2019.

“High-tide flooding on the rise, especially along the East Coast, forecasters warn,” July 10, 2019.

“Glaciers are melting underwater. It’s worse than previously thought,” July 25, 2019.

“Climate change SHOCK: Sea-level rise forecasts were wrong by 80 percent, scientists warn,” June 21, 2019.

“The California coast is disappearing under the rising sea. Our choices are grim.” July 7, 2019.

“The leaders of these sinking countries are fighting to stop climate change. Here’s what the rest of the world can learn.” June 13, 2019.

“Climate change: Global sea-level rise could be bigger than expected,” May 20, 2019.

“Climate change: Sea-level rise could displace millions of people within two generations,” May 20, 2019.

Editor’s note: This story is part of Covering Climate Now, a global collaboration of more than 250 news outlets to strengthen coverage of the climate story.