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In an era when massive heat domes blanket large swaths of continents for days, wildfires burn through areas the size of small countries, and hurricanes regularly push the limits of what we once thought possible, sea level rise can seem like extreme weather’s low-key cousin. But with estimates suggesting that sea level rise will affect more than one billion people around the world in the next 25 years, this is one member of the dysfunctional climate change family that shouldn’t be ignored.
Why is this? Read on for the science you need to know about sea level rise, in seven parts.
1. Sea level rise is picking up speed
Globally, sea level is rising at more than double the 20th-century rate. Since 1901, global average sea level has risen at a rate of about 1.7 mm per year for a total rise of about 8 inches since that year. The pace of sea level rise since the start of the 20th century has been faster than at any point in the last 3,000 years.
As if that weren’t worrisome enough, studies show that it has been accelerating since the 1960s. Since 1993, sea level has risen by an average rate of 3.1 mm per year—just shy of double the 20th century average rate. And since 2006, it has been even faster: as high as 4.2 mm per year, according to recent estimates by NASA.
2. The primary cause of accelerating sea level rise is human activity
As people burn fossil fuels and emit heat-trapping gases like carbon dioxide, our atmosphere and our oceans warm up. As the ocean warms, it expands. Historically, this was the dominant cause of sea level rise. But a warmer atmosphere and ocean also contribute to a loss of land-based ice sheets (Greenland and Antarctica) and glaciers (like those in Glacier National Park). That adds water to the oceans, which raises their level. This loss of land ice has been the dominant contributor to sea level rise since 2006.
Since the 1970s, global sea level rise has accelerated significantly and ice loss from glaciers and ice sheets has become the dominant contributor to the global trend. Source: IPCC AR6, WGI, Chapter 9, Cross-Chapter 9.1, Figure 1Depending on how you slice it and what time period you’re looking at, at least 70% of global sea level rise since 1970 is attributable to human-caused climate change. A big chunk of the rise—roughly 30% since 1880 and roughly 10% since 1980—has been more directly attributed to heat-trapping emissions tied to the world’s largest fossil fuel producers and cement manufacturers.
3. Sea level rise along US coastlines is faster than the global average
Sea level rise rates vary significantly from region to region and even between locations in a given region. On average, sea level along the coastlines of the contiguous United States is rising more quickly than the global average.
Along much of the Eastern seaboard, for example, the pace and magnitude of sea level rise reflect both the global average and natural sinking—what scientists call subsidence—of the land. Much of that subsidence is happening for geological reasons. At the end of the last ice age, when the massive ice sheet that covered much of northern North America melted, much of the land sprung back up after having been pushed down by all that heavy ice for thousands of years. Like a spring bouncing up and down, the land is now sinking back down.
Along with the withdrawals of hydrocarbons, such as gas, human engineering has changed the amount of sediment flowing from rivers into the ocean. The settling of areas built on artificial fill has also contributed to land subsidence, land loss, and higher-than-average rates of sea level rise, particularly along the Gulf Coast. (Groundwater withdrawals globally have shifted so much water that it has affected the tilt of the Earth, but that’s another story!)
On average, over the 1993-2020 period, sea level has risen more quickly along the coastline of the contiguous United States than it has globally. Sea level rise rates are notably higher along the East and Gulf Coasts than they are on the West Coast. Source: Fifth National Climate Assessment, Figure 2.5.4. Sea level rise is making storm surge more extensive and damaging
Storm surge is often the most damaging aspect of hurricanes, nor’easters, and other coastal storms. “Surge” happens when the winds generated by a storm system push surface water toward the coast over a long expanse of ocean. Depending on the size of the storm, that water can build into a large surge where sea water many feet deep overtakes coastal areas. More than simply the presence of water, though, surge is made more damaging by the excessive wave energy that typically accompanies it. The combination can lead to catastrophic conditions.
Add higher sea levels and the threat is obvious: surge that can reach farther inland, bringing taller waves and causing greater destruction.
And when that destructive potential meets our increasingly- and often highly-developed coastline, disaster ensues.
Hurricane Sandy is a stark example of storm surge damage in a highly-developed region, made worse by rising seas. Scientists calculated the added economic damage inflicted on the region by the additional, climate-change driven water levels in 2012, and found that sea level rise caused an additional roughly $8.1 billion in damages (or approximately 13% of the $60 billion total) and affected approximately 71,000 additional people.
With 90 million people living along US coastlines and a coastal economy valued in the hundreds of billions of dollars, our nation is far too exposed to the “when,” not “if,” of storm surge damages.
Flooding from Hurricane Sandy’s storm surge in the New York City metropolitan area in 2012 affected 71,000 more people than it would have without human-caused sea level rise. Source: Metropolitan Transportation Authority.5. But storm or no storm, higher seas mean high tide flooding
Whether or not a storm ever strikes our coasts again, water is coming inland, delivered by one of nature’s gentlest cycles: the daily tides. Tidal flooding is now expanding in reach and frequency, along all the coasts in the contiguous U.S.
The tides are caused primarily by the gravitational force of the moon, which essentially pulls a bulge of water toward it (while inertia creates a bulge on the opposite side of the Earth).
(Cool fact: we say that the tides rise and fall, but a more accurate way to think about it is that, as the Earth rotates through that ^^ bulge of water, we move into and pass out of those higher water levels. Cool, right?) But sticking with the language we know, tides rise and fall once or twice daily along coastlines around the world, with a smaller change in water levels closer to the equator, and greater change farther from it.
Take Charleston, South Carolina, as an example: the average difference between high and low tide is 5-6 feet. Twice a month, when the moon is full and new, it exerts a stronger gravitational pull on the oceans and the tides are higher and lower than normal. There are more variations caused when the sun, moon and Earth align, but suffice to say, some tides are higher than others. And because the sea level in Charleston has already risen roughly 16 inches since 1901, normal high tides are now flooding Charleston in disruptive ways and with increasing frequency.
Charleston is far from alone; in little more than a decade, elevated sea levels along all the coasts have turned this into a disruptive national phenomenon. A 2019 UCS analysis, Underwater, found that Charleston can expect roughly 500 homes to flood on a chronic basis by 2035. The increasing reach and frequency of this flooding are inexorable; our agency lies in what we do to slow sea level rise and what we do to prepare the communities in its path.
When higher sea levels are added on top of the normal variations in tide height, the more extreme high tides can reach onto normally dry land. As sea level rises further, this occasional flooding can become chronic, as less extreme tides begin to cause flooding as well. Source: Union of Concerned Scientists.6. We know more sea level rise is unavoidable. How much more is uncertain.
With sea level rise accelerating, where are we headed? The many dynamics that contribute to rising seas mean there is a significant amount of uncertainty about future increases. That said, we have much more certainty than just a few years ago about the likely increases a few decades from now. Projections for the likely amount of global average sea level rise through 2050 now range between 7.5 and 11.4 inches while projections for the average rise on US coastlines is somewhat higher at 12.6 to 17.7 inches.
Longer term, two particular drivers loom large, with significant uncertainty around each—resulting in significant uncertainty around late century increases.
Currently, the projected range of global average sea level rise is between 2 and 6.5 feet by 2100. Though the higher end of that range is presently considered unlikely, it’s vital to future generations that we appreciate that sea level rise will continue long after the end of this century. We are unleashing a long-term process where ocean levels will rise over many centuries, depending heavily on what we do in the near term.
There is wide range in possible sea level rise amounts late this century for a couple reasons. First, we don’t know how much more carbon dioxide, methane, and other heat-trapping gases humanity will emit in the coming decades. Second, we don’t know exactly how Earth’s systems—especially land-based ice—will respond to those future emissions. Potentially large contributions from Antarctica, in particular, are not yet well enough understood. But we don’t need to pinpoint the time of, for example, the Thwaites Glacier collapse to say with certainty that we face an enormous threat that must be slowed.
Observations show that sea level rise is accelerating both globally and, as shown here, along the coastlines of the United States. The amount of sea level rise projected by 2050 is largely dictated by past human emissions of heat-trapping gases and is therefore more certain. The amount of sea level rise the US will experience by 2100 is much less certain, as it depends on current and future global emissions choices and the response of Earth’s systems—particularly the response of large ice sheets—to those emissions. Source: Fifth US National Climate Assessment, Figure 9.1.7. Now is the time to slow–and prepare for–sea level rise
Which brings us to the other primary source of uncertainty: our societal actions. Knowing what’s at stake, our leaders should be working with urgency to rein in emissions of the heat-trapping pollution that’s driving sea level rise. They should be working with urgency to enable communities, economic sectors, and whole nations to prepare for the increases that are inevitable and increasingly likely.
Collectively, coastal communities in the US and around the world face a massive deadline if we’re aiming to get ahead of the flooding that is coming our way. A deadline to prepare, protect, or move communities and their assets out of the ocean’s reach. A deadline that poorer communities and nations have done little to create and require resources of richer ones to meet.
Sea level rise is a tricky beast. At the rate of just a few millimeters per year, it’s easy to think that it’s a threat that won’t need to be dealt with for decades. But preparing, protecting, or moving the people, assets, communities, and economies located along the coast will be a massive, costly, sustained undertaking. As such, it needs to start now. But because sea level rise is just a few millimeters per year, if we start now and build flexibility into future planning and policies, it’s possible for coastal communities to meet this looming deadline.
