By Laura Paddison, CNN

(CNN) — For around 2,000 years, global sea levels varied little. That changed in the 20th century. They started rising and have not stopped since — and the pace is accelerating.

Scientists are scrambling to understand what this means for the future just as President Trump strips back agencies tasked with monitoring the oceans.

Since 1993, satellites have kept careful watch over the world’s oceans, allowing scientists a clear view of how they are behaving. What they have revealed is alarming.

Sea level rise was unexpectedly high last year, according to a recent NASA analysis of satellite data.

More concerning, however, is the longer-term trend. The rate of annual sea level rise has more than doubled over the past 30 years, resulting in the global sea level increasing 4 inches since 1993.

“It’s like we’re putting our foot on the gas pedal,” said Benjamin Hamlington, a research scientist in the Sea Level and Ice Group at NASA’s Jet Propulsion Laboratory. While other climate signals fluctuate, global sea level has a “persistent rise,” he told CNN.

It spells trouble for the future. Scientists have a good idea how much average sea level will rise by 2050 — around 6 inches globally, and as much as 10 to 12 inches in the US. Past 2050, however, things get very fuzzy.

“We have such a huge range of uncertainty,” said Dirk Notz, head of sea ice at the University of Hamburg. “The numbers are just getting higher and higher and higher very quickly.”

The world could easily see an extra 3 feet of sea level rise by 2100, he told CNN; it could also take hundreds of years to reach thatlevel. Scientists simply don’t know enough yet to project what will happen.

What scientists are crystal clear about is the reason for the rise: human-caused global warming.

Oceans absorb roughly 90% of the excess heat primarily produced by burning fossil fuels, and as water heats up it expands. Heat in the oceans and atmosphere is also driving melting of the Greenland and Antarctic ice sheets, which together hold enough fresh water to raise global sea levels by around 213 feet.

Melting ice sheets have driven roughly two-thirds of longer-term sea level rise, although last year — the planet’s hottest on record — the two factors flipped, making ocean warming the main driver.

It’s likely that an increase of about 3 feet is already locked in, Notz said, because “we have pushed the system too hard.” The big question is, how quickly will it happen?

Ice sheets are the biggest uncertainty, as it’s not clear how fast they’ll react as the world heats up — whether they’ll melt steadily or reach a tipping point and rapidly collapse.

From studying things like ice cores and sediments, scientists know sudden and dramatic melting happened thousands of years ago, Notz said, but it’s still unclear how processes may unfold over the next decades and centuries.

Antarctica is “the elephant in the room,” he said. Alarming changes are unfolding on this vast icy continent, which holds enough water to raise levels by 190 feet.

Notzdescribes the ice sheet as an “awakening giant:” It takes a long time to wake up but once awake, “it’s very, very difficult to put it back to sleep.”

It will take time for scientists to unravel what the future holds for the ice sheets and what that means for sea level rise.

“There’ll be big uncertainties that persist for decades,” said Robert Nicholls, professor of climate adaptation at the University of East Anglia.

Where is most vulnerable?

The ocean is not flat like a bathtub and there are big variations in the way sea level rise is experienced.

Some parts of the world see higher levels because of a tangle of factors, including regional currents, erosion and land shifts — some due to natural processes such as tectonic plate movements, others to human activities including fossil fuel and groundwater extraction.

The US coastlines are tracking above global average and toward the upper end of climate model projections, NASA’sHamlington said.

The Gulf Coast, where land is sinking in large part due to the extraction of oil, gas and groundwater, is a hotspot.

Louisiana is particularly vulnerable as climate change-driven sea level rise meets fast-sinking land. The state has one of the highest rates of land loss in the world, with some areas experiencing relative sea level rise nearly four times the global rate.

Outside the United States, low-lying Pacific island nations bear the brunt of sea level rise, which already poses a threat to their existence.

Over the next three decades, islands such as Tuvalu, Kiribati and Fiji will experience at least 6 inches of sea level rise even if the world reduces planet-heating pollution, according to NASA.

What are the impacts?

The world’s coastal communities were built with an understanding of where high tides normally reach. “Now we’re shifting that normal” and even tiny shifts can cause big impacts, Hamlington said.

Already, coastal flooding is increasing, even in the absence of big storms or heavy rainfall. High tide flooding is now happening two to three times more often since 1990 along most US Atlantic and Gulf coastlines, said William Sweet, an oceanographer at the National Oceanic and Atmospheric Administration.

Sea level rise also contributes to coastal erosion, chokes sewage systems and causes salty water to seep into underground freshwater supplies.

Rising seas won’t be felt equally. In some places, 6 inches might mean more frequent flooding during high tides; in others, it could lead to constant inundation, with water covering streets and coastal land for months at a time. The difference depends on a range of factors, including land shifts.

The world can adapt to slow sea level rise, Notz said:“We might have to relocate cities; you might have to move people around,” but there are ways to build and prepare.

This is already happening. “Entire villages in Fiji have been formally relocated,” said Fijian activist George Nacewa, from climate group 350.org, “the incoming tides are flooding our roads and inundating our crops.”

However, if the pace accelerates rapidly, “it will be very, very difficult to adapt to, because things unfold too quickly,” he said. As ever, it will be the most vulnerable who feel the biggest impacts, he added.

Many parts of the world aren’t well prepared. Sea level rise is an “underappreciated” problem as “it’s hard to grasp the implications” of even a few inches of rise, the University of East Anglia’s Nicholls said.

Humans still have control over how fast sea level rises over the next decades and centuries by cutting emissions, Notz noted.

Global levels of planet-heating pollution, however, continue to tick up and the Trump administration is racing to undo climate policies, including slashing Earth science research at NASA, sweeping staff cuts at NOAA and withdrawing the US from global climate action.

“One of the big concerns with the current US administration is that they won’t renew satellites,” Nicholls said. This would take eyes off the oceans at a crucial time.

What the world chooses to do is an ethical issue, said Notz.

“The kind of sea level rise that we have to deal with is so much smaller compared to what future generations have to face,” he said; the real catastrophe will come further down the road when those who have caused the problem are no longer around.