If the ground is what you stand on, can you tell when it moves?
Sometimes it's obvious, in road-cracking earthquakes. Or imperceptible, in the gradual drift of tectonic plates.
It contracts and rebounds, settling from the weight of a house and rebounding from glaciers that long ago retreated.
And its subtle movements are key to the understanding of sea level rise because, really, the ocean isn't only going up — the ground is going down.
That's the case in the Charleston area, at the edge of South Carolina's sloping coastal plain. A sinking ground accounts for about 40 percent of the relative sea level rise — the combined motion of land and sea — in the past century. That sinking land is falling in a process known as subsidence.
But this variable, one among many in the discussion on sea level rise, is often misunderstood.
Some point to subsiding land as the main, or only, driver of Charleston's tidal flooding threat. They claim Charleston is a city built on loose soil and trash, leaving it unstable under the weight of its buildings. Experts in sea level rise and the area's geology say that's cherry-picking the facts to fit their own skepticism. While subsidence is a factor, scientists broadly agree that the greater threat to the low-lying coast comes from oceans swelling in the midst of climate change.
"The rate of sea level rise is faster," said Norm Levine, who runs the Lowcountry Hazards Center at the College of Charleston.
Levine has heard members of the public blame sinking land, rather than a heating globe. He said the signs people point to, like cracks in their walls after their homes settle, are misleading — signs like these often show localized problems, not regional ones.
That doesn't mean subsidence isn't worth exploring. It remains one of the most challenging factors to measure in the sea rise puzzle.
Levine and several other researchers are hoping to do just that.
Land and sea
In Charleston, the past rate of sea level rise is clear because of a long-running tidal record.
The highs and lows were all recorded by a gauge affixed to a pier at the mouth of the Cooper River. Housed inside a large plastic pipe that blocks sudden wakes from passing boats, this monitor pings the water with acoustic waves every six seconds. Those readings are transmitted in near real-time to a satellite.
The National Oceanic and Atmospheric Administration, which owns the sensor, is testing an eventual replacement that should be even more accurate. The new probe will pinpoint the water's surface with microwave rays.
But those who track tidal flooding know the Charleston station, like many NOAA operates, is anchored to ground that's sinking.
NOAA accounts for these changes in its measurements. About 5 inches of the more than a foot of sea level rise over the past century has come from subsidence at the gauge station, the agency estimates.
In future sea level rise scenarios, rising waters will move faster and play an even bigger role. The middle range of scenarios in sea rise projections show between 2 and 6 feet of ocean rise by the end of the century.
Meanwhile, NOAA predicts that rate of ground subsidence will hold steady — another 5 inches in the next 100 years. But that's a guess, said Doug Marcy, a NOAA coastal hazards specialist based in Charleston.
"Vertical land motion’s a big deal, and we don't really have a great handle on the differences regionally," Marcy said.
NOAA used surveying methods to measure the dip at its own station, but that number says little about what's happening across the region. Matt Wellslager of the South Carolina Geodetic Survey called local estimates a "WAG" — in other words, a "wild ass guess."
Wellslager's group establishes stations around the state that help verify locations on the ground by continuously pinging satellites. These stations could, eventually, provide an answer for how much the region is moving.
Knowns and unknowns
So if the ground is sinking, what might be causing it?
Suspect No. 1: Groundwater withdrawal.
Communities that suck water from underground aquifers essentially deflate the structure under their feet. Soil that was supported by the liquid collapses together, said Adem Ali, an associate professor at the College of Charleston.
If too much water is removed, the aquifer cannot replenish itself. In California's San Joaquin Valley, for example, the U.S. Geological Survey recorded subsidence as deep as 28 feet.
Charleston doesn't yet face this problem, said Ali, who works at the college's hazards lab with Levine.
"We haven't been populated enough to do major extraction of groundwater, although we are being tested, that’s for sure," Ali said.
Suspect No. 2: Glaciers.
Ice sheets melted away from the modern-day United States about 12,000 years ago, but the East Coast is still feeling the effects. Like lying on a water bed, the mass of mile-thick ice pushed down Earth's crust, said Erin Beutel, who studies tectonic plates at the College of Charleston.
The pressure squeezed out the slow-moving mantle below. On the edge of the glacier, displaced mantle made the ground bounce up.
But unlike the fluid in a waterbed, mantle is thicker than Play-Doh, Beutel said. It takes a long time to return to its original place. So the Northeast is rising from where it was pressed down, and the bubble at the end of the glacier is sinking back down, particularly around the Chesapeake Bay.
But that mantle displacement doesn't extend far enough south to reach Charleston. Suspect two eliminated.
That leaves one suspect left: Compaction, or the compression of soil. Unlike the first two options, this isn't an effect that would show up across a broad region. But it might be visible on your house.
Charleston's soils — thousands of feet of dirt and muck — are what a geologist would call "unconsolidated," meaning that, for the most part, heavy equipment isn't needed to break them apart.
They're also weighed down simply by sitting on top of each other, with soils on the surface compacting soils below, said Simon Ghanat, an assistant professor of civil engineering at The Citadel.
The effect is stronger in urban environments. In some cases, the signs are obvious: a buckled road; a spiderweb of cracks on a home's walls; a tilting backyard swimming pool.
Residents of the area might see these signs and think the whole area is sinking, but that's not the case, Levine said. It's only the ground immediately under the road or house or swimming pool.
Still, all together, this weight and pressure has some effect. Levine and Ghanat want to one day calculate exactly how much the land is being weighed down. They would do it by collecting data on what's sitting on the Charleston peninsula, lot by lot.
"We’ve had buildings on the peninsula. Now we’re getting much bigger buildings on the peninsula," Levine said. "If we get out there now and kind of measure things, we can start understanding the susceptibility."
But the gold standard of measurement for regional effects would be continuous GPS data. Reference stations on tall structures could continuously ping satellites in the atmosphere. If the distance between the reference station and satellite grows over time, that means the ground is sinking.
There's only one station in the Lowcountry that's actually equipped to do that right now. It's at NOAA's Fort Johnson facility on James Island, and it's only been there for a little over a year, said Wellslager, of the state Geodetic Survey. There used to be other sites, on a College of Charleston building and on S.C. Ports Authority land near the tidal gauge. Both of those stations have been taken down, in one case because of a renovation, and in another because the property changed hands.
That means that there's no long-running station still in operation. Ideally, researchers would want a grid of these sites across the region to see if some areas are sinking faster than others. But each one costs tens of thousands of dollars to install.
Scott Howard of the S.C. Geological Survey said the sparse reference stations present "an interesting data point" but don't show the whole picture. He's working on a different effort, using radar that would similarly measure the distance from a satellite to the ground.
But until Howard's group collects more information, he said existing understanding of subsidence in South Carolina is limited.
"We have some preliminary (radar) data that looks promising, and we're trying to move forward with additional work," Howard said. "But current circumstances are slowing us down. Hopefully, we will get back to this project soon."
Sea rise certainty
While researchers work out the nature of Charleston's subsidence, there's no question the rising ocean will outstrip it.
Global data backs this up, collected in tidal gauges like the one in the Cooper River and through satellites that measure the heights of the oceans.
The vast majority of the scientific community agrees on the process that lifts up the tides. The planet is being warmed by heat-trapping gases like carbon dioxide. That extra heat makes polar ice melt, adding water to the oceans. That also makes the water increase in volume: when it's warmer, the molecules sit farther apart.
"What I don't want to do is give people the feeling that this is so uncertain we don't know that it's happening, because that's not true, we know that it's happening," Marcy said.
And as Beutel points out, sea rise is still happening in places without roads and bridges to weigh the land down, including the uninhabited Santee Delta.
"Water — sea level and climate change — is a more important factor for what we’re experiencing," Ali said.