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Climate 101 is a Mashable series that answers provoking and salient questions about Earth’s warming climate.

The oceans are the true keeper of climate change.

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Earth's seas soak up over 90 percent of the heat humanity is now trapping on the planet, a nearly an unfathomable amount of energy that expands the oceans. And as Earth's ice sheets and glaciers melt in a hotter world, this new water inevitably pours into the seas.

Since the late 19th century, sea levels have already risen by some eight to nine inches. But much more sea level rise is imminent, because the planet has warmed significantly over the last hundred years. In a new report authored by top researchers at a diversity of U.S. agencies — the National Oceanic and Atmospheric Administration (NOAA), NASA, the Department of Defense, and beyond — scientists project sea levels will rise by some 10 inches to a foot along the U.S. coast over just the next three decades.

In a hotter world, seas are rising faster. "Over the last 50 years, there's been an acceleration," William Sweet, a NOAA oceanographer and one of the report's lead authors, told Mashable.

And after 2050, the waters will continue rising. Crucially, how much more is contingent upon how much society heats the planet by burning ancient, decomposed deposits of carbon-rich organisms (stored underground as fossil fuels). Already, heat-trapping CO2 levels in the atmosphere are the highest they've been in some 3 million years. Sea levels could rise an additional foot and a half this century, or multiple feet.

Melting from Earth's massive ice sheets, which blanket Greenland and Antarctica, will figure prominently in the coming decades.

"The ice sheets are just getting warmed up," Josh Willis, a NASA oceanographer not involved with the report, told Mashable.

How we know sea levels are rising

Satellites have revolutionized our ability to measure changing sea levels.

That's because the sprawling oceans — influenced by the likes of varying temperatures, geography, ocean currents, and tides in disparate places — make it enormously difficult to measure how seas are changing globally. But satellites in space, like NASA's Jason-3, beam radio waves to the ocean surface that bounce back to the satellite. This gives oceanographers a precise recording of sea surface height over wide swathes of the ocean.

An extensive system of tide gauges confirms what satellites like Jason-3 record from space. There are other stark indicators, too. Along much of the U.S. coastline, high-tide flooding is now 300 to over 900 percent more frequentthan it was 50 years ago, notes NOAA. It's also why an octopus washed into a Miami parking garage amid a high tide event. The totality of the evidence is clear.

"We know the ocean is rising," said NOAA's Sweet.

Sea level rise each year more than doubled from 1.4 millimeters over most of the 20th century, to 3.6 millimeters by the early 21st century. From just the years 2013 to 2018, that number accelerated to 4.8 millimeters per year.

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Why scientists expect sea levels to rise significantly

In the coming decades, two factors will largely drive rising sea levels around the U.S., and the globe.

1. Thermal expansion:As the seas absorb more heat, they expand. The oceans have warmed each year for decades, and in 2021 the ocean "was the hottest ever recorded by humans," scientists concluded in a major study. Historically, thermal expansion has been responsible for one-third of sea level rise.

2. Melting ice sheets and glaciers: Globally, nearly all mountain glaciers are shrinking. You might consider glimpsing them while you can. Much of this glacial water ultimately enters the ocean. Separately, the colossal ice stores on Greenland and Antarctica are melting into the sea, too. The reasons for this are numerous: Warming air directly melts Greenland's ice. Warming oceans also melt the ends of Greenland's glaciers. In Antarctica, a warmer atmosphere is ultimately driving warmer seawater beneath the ends of gigantic Antarctic glaciers. This has destabilized them and amplified ice loss.

   Just how much ice have the ice sheets lost? A prodigious amount. Satellites beam lasers onto the great remote ice sheets to document their mass. The Greenland Ice Sheet, about three times the size of Texas, lost some 200 gigatons annually between 2003 and 2019. (A gigaton equals 1 billionmetric tonnes.) Meanwhile, Antarctica, a continent whose mountains are up to their necks in ice, lost some 118 gigatons each year.

Melting ice sheets and glaciers have accounted for two-thirds of sea level rise. But in the coming years, ice sheets will play a larger role. There's considerably more ice to melt.

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NOAA's Sweet, an author of the sea level rise report, has strong confidence in the sea level rise projections over the next few decades. That's because the graphed trajectory of how sea levels will rise, based on the reality that rates have observably increased, matches with careful computer simulations, or models, of how sea levels will rise.

"That's two lines of evidence pointing to similar numbers," Sweet explained.

But beyond 2050, the trajectory of the climate, and oceans, is more uncertain, and largely dependent on the most unpredictable part of the climate equation. That's us.

So the new sea level rise report created five different potential sea level futures between 2050 and 2150, based upon how much heat-trapping carbon global civilization adds to the atmosphere this century. (These five emissions scenarios were made, and deeply vetted, by the Intergovernmental Panel on Climate Change.)

It may sound awfully pessimistic to hear that sea levels will inevitably continue to rise past 2050. These ideas can fuel doomism and helplessness about our climatic future. But climate scientists emphasize that such doomist notions are misguided. Rather, it shows we still have enormous sway over how much the climate warms — and the seas rise — later this century. "We have a significant amount of influence over how much warmer it gets," Zeke Hausfather, a climate scientist and the director of climate and energy at the Breakthrough Institute, previously told Mashable.

The following five long-term (2050-2150) sea level rise scenarios cover a wide range of possibilities. The "low" scenario — involving an extremely ambitious climate target — requires global nations stabilizing Earth's warming at around 1.6 degrees Celsius (2.8 degrees Fahrenheit) above 19th-century temperature levels by mid-century.

Compared to sea levels in 2000, the "Intermediate" scenario for the U.S. below, which projects 1.3 feet of sea level rise by 2050 and several feet by 2100, is a world warmed by around 2 C (3.6 F) by mid-century.

Crucially, there are looming unknowns about how much ice sheets will respond to different amounts of warming later this century. (This is OK and expected: These scenarios are a "what if" guide.) That's because Earth's current, rapid warming is historically unprecedented. Humanity, and earth scientists, have never witnessed Greenland and Antarctica continually melt.

"We haven't watched these ice sheets melt before," said Willis. "The last time they did it was 20,000 years ago and we weren't paying attention."

The geologic record shows that our great ice sheets have experienced major ice loss in the past. During a warm period some 125,000 years ago, massive amounts of ice on Antarctica melted, raising sea levels by some six to nine meters. Fortunately, oceanographers don't think that type of melting will happen suddenly (think a process unfolding over hundreds of years). But that process could begin, perhaps this century.

Already, Antarctica's Florida-sized Thwaites Glacier has destabilized. "Thwaites is the one spot in Antarctica that has the potential to dump an enormous amount of water into the ocean over the next decades," Sridhar Anandakrishnan, a professor of glaciology at Penn State University, told Mashable in 2020. Anandakrishnan is a veteran researcher of this threatening, remote glacier.

Yet amid some future uncertainty, substantial sea level rise is still certain. Some 10 inches to a foot is due around the U.S. by 2050. That portends lots of American flooding.

"By 2050, moderate flooding ⁠— which is typically disruptive and damaging by today’s weather, sea level and infrastructure standards ⁠— is expected to occur more than 10 times as often as it does today," Nicole LeBoeuf, NOAA's National Ocean Service Director, said in a statement.

This is not ideal. But, importantly, Willis emphasized that we can limit significantly worse flooding if we slash our emissions of heat-trapping greenhouse gases. Nothing else is going to blunt sea level rise. The world isn't going to suddenly cool on its own.

"There are no big natural cycles that will save us," he said.

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