The ocean is coming for us. Global sea levels are now rising by 3.4 millimeters per year, up from an average rate of 1.4 mm per year last century. In just 80 years, the ocean could be a full 1.3 meters (4.3 feet) taller than it is today.

That kind of planetary sea change can be hard to fathom — unless you live in a low-lying place like Miami, the Maldives or the Marshall Islands, where the effects of sea-level rise are already apparent. But within just a few decades, the problem will become unavoidable in major coastal cities around the world, from New Orleans, New York and Amsterdam to Calcutta, Bangkok and Tokyo.

We all know why this is happening. Rising seas are one of the most salient effects of man-made climate change, triggered by thermal expansion of seawater as well as the influx of melting glaciers. Yet many people still see it as a distant risk, failing to grasp how (relatively) quickly the sea is swallowing shores worldwide. And since half of all humans now live within 60 kilometers (37 miles) of a coast, this isn't a niche issue.

To help put things in perspective, here's a deeper look at the problem:

1. Global sea levels have already risen by 8 inches (200 mm) since 1880.

sea-level rise, 1880-2014

The chart above was produced by NASA's Earth Observatory, based on data from the U.S. National Oceanic and Atmospheric Administration (NOAA) and Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO). Most of those historical data come from tide-gauge measurements, which are now complemented by satellite observations.

2. Not only are sea levels rising; the rate of their rise is rising.

sea-level rise, 1993-presentAverage global sea-level rise is now 3.41 mm per year, but the rate varies widely by location. (Image: NASA GSFC)

On average, sea levels rose by 1.4 mm from 1900 to 2000. The yearly pace had surpassed 3 mm by 2010, and now it's up to 3.4 mm per year.

3. That's the fastest sea-level rise Earth has experienced in 3,000 years.

If not for surging carbon dioxide in the atmosphere, sea levels should have only risen about an inch or two last century, and might have even fallen. Instead, thanks to the highest CO2 levels at any point in human history, global sea levels rose by 5.5 inches (14 cm) between 1900 and 2000. That's the fastest oceanic advance in 27 centuries, according to a study published Feb. 22, and it's still speeding up.

"The 20th century rise was extraordinary in the context of the last three millennia — and the rise over the last two decades has been even faster," says lead author Robert Kopp, a climate scientist at Rutgers University, in a statement.

"Scenarios of future rise depend upon our understanding of the response of sea level to climate changes," adds co-author Benjamin Horton. "Accurate estimates of sea-level variability during the past 3,000 years provide a context for such projections."

4. Every vertical inch of sea-level rise moves the ocean 50 to 100 inches inland.

Miami coastal floodingRising seas worsen regular flooding — like this 2015 high tide in Miami Beach — for many coastal cities. Miami is in the midst of a five-year, $400 million effort to upgrade its stormwater pump program. (Photo: Joe Raedle/Getty Images)

One inch may not sound like much, but it's an extra inch of ocean, not water in a rain gauge. Earth's oceans hold about 321 million cubic miles of water, and are generally more like a bowl than a beaker, with sloping sides. According to NASA, every vertical inch of sea-level rise covers 50 to 100 lateral inches (1.3 to 2.5 meters) of beach.

5. That's already causing flood problems in many big coastal cities.

As the ocean invades coastal cities, the first signs of trouble are often urban saltwater floods. These can also happen naturally, though, so to determine the influence of rising seas, a new report by Climate Central models "alternative histories simulating the absence of anthropogenic climate change" at 27 U.S. tide gauges.

Out of 8,726 days since 1950 when unaltered water levels exceeded the National Weather Service thresholds for local "nuisance" floods, 5,809 didn't exceed those thresholds in the alternative histories. "In other words," the report explains, "human-caused global sea level rise effectively tipped the balance, pushing high-water events over the threshold, for about two-thirds of the observed flood days."

Coastal flooding days have more than doubled in the U.S. since the 1980s, according to the report, in places ranging from Miami, Virginia Beach and New York to San Francisco, Seattle and Honolulu. According to a 2014 report, at least 180 floods will strike Annapolis, Maryland, during high tides every year by 2030 — sometimes twice a day. The same will be true for about a dozen other U.S. cities by 2045, not to mention many other low-lying urban areas around the world.

6. Sea levels could rise another 1.3 meters (4.3 feet) in the next 80 years.

sea-level rise mapThis map shows areas that would flood (marked in red) due to 1-meter sea-level rise. (Photo: NASA)

In another study published this month, scientists report that global sea levels will likely rise 0.5 to 1.3 meters (1.6 to 4.3 feet) by the end of this century if greenhouse gas emissions aren't rapidly reduced. Even if last year's Paris Agreement does spur ambitious climate policy, sea levels are still projected to rise 20 to 60 cm (7.8 to 23.6 inches) by 2100. Taken with the longer-term effects from melting ice sheets in Greenland and Antarctica, that means any strategy to endure sea-level rise must involve adaptation plans as well as efforts to slow the trend.

7. Up to 216 million people currently live on land that will be below sea level or regular flood levels by 2100.

coastal flooding in Typhoon FitowHigher sea levels can exacerbate storm surges, like this 2013 flood in Wenzhou, China. (Photo: STR/AFP/Getty Images)

Of the estimated 147 million to 216 million people in harm's way, between 41 million and 63 million live in China. Twelve nations have more than 10 million people living on land at risk from sea-level rise, including China as well as India, Bangladesh, Vietnam, Indonesia and Japan. Bangladesh is especially vulnerable, identified by the U.N. as the country most in danger from rising seas. Once the ocean rises by 1.5 meters (4.9 feet) next century, it will affect 16 percent of Bangladesh's land area and 15 percent of its population — that's 22,000 km2 (8,500 mi2) and 17 million people.

The situation is also urgent for low-lying island nations like Kiribati, the Maldives, the Marshall Islands and the Solomon Islands, where land is already so close to sea level that a few inches make a world of difference. Some are even mulling mass relocations — the government of Kiribati, for one, has a web page outlining its strategy for "migration with dignity." A town on Taro Island, the capital of Choiseul Province in the Solomon Islands, is also planning to move its entire population in response to rising seas. The small community of Newtok, Alaska, has already begun the difficult process of transplanting itself away from the encroaching coast.

8. Sea-level rise can contaminate water used for drinking and irrigation.

saltwater intrusionSea-level rise can aid saltwater intrusion of freshwater aquifers, as seen in this schematic illustration. (Image: NRC.gov)

In addition to surface flooding, sea-level rise can both push up the freshwater table and contaminate it with seawater, a phenomenon known as saltwater intrusion. Many coastal areas rely on aquifers for drinking water and irrigation, and once they're tainted by saltwater they may be unsafe for humans as well as crops.

It is possible to remove salt from water, but the process is complex and costly. San Diego County recently opened the Western Hemisphere's largest desalination plant, for example, and several other sites are proposed in the state. Yet that may not be practical for many coastal communities, especially in less wealthy nations.

9. It can also threaten coastal plant and animal life.

loggerhead sea turtle hatchlingFloods fueled by rising seas may harm baby sea turtles, like these South African loggerheads. (Photo: Jeroen Looyé/Flickr)

Humans aren't the only ones who'll suffer as sea levels rise. Any coastal plants or animals that can't quickly move to new, less flood-prone habitats could face dire consequences. As one 2015 study noted, sea turtles have a long-established habit of laying eggs on beaches, which need to stay relatively dry for their babies to hatch.

Inundation for one to three hours reduced egg viability by less than 10 percent, the study's authors found, but six hours underwater cut viability by about 30 percent. "All embryonic developmental stages were vulnerable to mortality from saltwater inundation," the researchers write. Even for hatchlings that do survive, being starved of oxygen in the egg could lead to developmental problems later in life, they add.

Other beach life may also be at risk, including plants. A recent study found that some salt marshes can adapt, both by growing vertically and by moving inland, but not all flora will be so fortunate. "Trees have to work harder to pull water out of salty soil; as a result, their growth can be stunted — and if the soil is salty enough, they will die, a common sign of sea-level rise," Climate Central explains. "Even trees that are especially suited to salty soil can't survive repeated flooding by seawater."

10. Global flood damage for large coastal cities could cost $1 trillion a year if cities don't take steps to adapt.

sea-level rise in TokyoThis Google Earth simulation shows a Tokyo neighborhood with 1.3-meter sea-level rise. (Image: Google Earth)

The average global losses from flooding in 2005 were about $6 billion, but the World Bank estimates they'll rise to $52 billion per year by 2050 based on socioeconomic changes alone. (That means things like increasing coastal populations and property value). If you add the effects of sea-level rise and sinking land — which is happening even faster in some places — the cost could surge to $1 trillion per year.

11. It's too late to stop sea-level rise — but not too late to save lives from it.

iceberg off GreenlandA full moon shines over an iceberg that broke off Greenland's Jakobshavn Glacier. If the entire Greenland ice sheet melted, sea levels would rise about 6 meters, or 20 feet. (Photo: Joe Raedle/Getty Images)

Unfortunately, CO2 emissions linger in the atmosphere for centuries, and today's CO2 levels have already committed Earth to dangerous sea-level rise. About 99 percent of all freshwater ice resides in two ice sheets: one in Antarctica and one in Greenland. Both are expected to melt if humanity's CO2 output isn't curbed quickly, but the question is when — and how much damage we still have time to prevent.

The Greenland ice sheet is smaller and melting more quickly. If it completely melted, sea levels would rise by about 6 meters (20 feet). The Antarctic ice sheet has been more buffered from warming so far, but it's hardly immune, and would raise the ocean by 60 meters (200 feet) if it melted. (Estimates vary widely on how long these ice sheets might survive — while most expect they'll take centuries or millennia to melt, a controversial 2015 paper suggested it could happen much more quickly.)

Sea levels have naturally risen and receded for billions of years, but they've never risen this quickly in modern history — and they've never had so much human help. It's unclear what effect they'll have on our species, but what is clear is that our descendants will still be dealing with this problem long after we're all gone. Giving them a head start on a solution is the least we can do.

"With all the greenhouse gases we already emitted, we cannot stop the seas from rising altogether, but we can substantially limit the rate of the rise by ending the use of fossil fuels," says Anders Levermann, a climate scientist at Columbia University and co-author of the new study on future sea-level rise. "We try to give coastal planners what they need for adaptation planning, be it building dikes, designing insurance schemes for flooding or mapping long-term settlement retreat."

As another recent study pointed out, any policy decisions made in the next few years and decades "will have profound impacts on global climate, ecosystems and human societies — not just for this century, but for the next ten millennia and beyond."