The Case for Capping Sea-Level Rise

“Keep 1.5 alive.” For years, that phrase has been the rallying cry for climate advocates. Enshrined in the 2015 Paris climate accord, the 1.5-degree Celsius figure is the world’s aspirational limit for average global temperature rise. For those well versed in climate science, it serves as a shorthand for avoiding the worst consequences of climate change—the threats to nearly every aspect of human life that stem from rising global temperatures. Behind the 1.5-degree veil await more destructive storms, catastrophic coastal flooding, rampant hunger and disease, extreme and deadly heat waves, and the demise of the world’s remaining coral reefs.

But the 1.5-degree slogan does not adequately convey these threats. It communicates one figure about climate change—the average increase in global temperature since preindustrial times—and assumes knowledge of climate dynamics that most people lack. It fails to convey both the enormity of the stakes and the narrow window for corrective action. The Intergovernmental Panel on Climate Change (IPCC), the UN-sponsored body of scientists and economists that periodically assesses the science and risks of climate change, calculates that global greenhouse gas emissions must fall by 60 percent by 2035 (from 2019 levels) to limit global warming to 1.5 degrees.

Yet the world has made only modest progress toward curbing emissions. Despite the urgent need for action, greenhouse gas emissions are now higher than at any time in human history. In 2022, global carbon pollution from the burning of fossil fuels, deforestation, and other sources reached record levels, increasing the risk that the world will crack the 1.5-degree ceiling and invite irreversible damage to the planet’s ecosystems.

To galvanize a global effort to stave off catastrophe, climate advocates need a less abstract—and easier to visualize—goal around which to rally. An upper limit for average sea-level rise—say, of two feet or of half a meter, which is consistent with the most aggressive emission-reduction scenario—would offer exactly that. More so than rising temperatures, rising seas offer tangible evidence of the harms of climate change, including coastal erosion, abandoned communities, sewage backups and overflows, contaminated water supplies, and increased insurance costs. As countries prepare for the UN climate change conference in Dubai in November and December 2023, they should establish a ceiling for rising seas that would allow people to more easily grasp why it is so crucial to keep 1.5 alive.

RISING SURF, SINKING CITIES

In addition to warming the atmosphere, greenhouse gas emissions trap heat in the oceans, causing the volume of the world’s seawater to expand. Higher temperatures also cause glaciers to melt and ice sheets to shrink, further accelerating sea-level rise. Since 1880, global sea levels have risen by about eight inches on average. And the rate of sea-level rise is accelerating; since 1993, it has doubled.

Because some ocean heating is already “baked in” from past greenhouse gas emissions, sea levels will continue to rise at least somewhat even if the world cuts emissions. But the future rate of pollution will largely determine how much and how fast the oceans continue to rise. Curbing emissions to achieve the 1.5-degree goal could prevent runaway sea-level rise: the IPCC estimates that if temperatures remain at or below 1.5 degrees, the global mean rise in sea level will likely be somewhere between 1.2 feet and 2.5 feet by 2100. But if emissions go largely unchecked, the rise could be between 3.5 feet and 7 feet by 2100 in the United States.

Rising seas sow destruction. They increase the odds of flooding from storm surges, which can damage wetlands, spoil fields for agriculture, contaminate freshwater aquifers, and kill plants and trees. When sea levels are higher, storm surges can travel farther inland, affecting communities that have not previously experienced flooding. Roads become impassable, power and wastewater treatment plants flood, communication systems fail, and health care grinds to a halt. Higher sea levels also accelerate coastal erosion: land slips away and islands vanish, leaving people without ground under their feet.

More than ten percent of the world’s population lives along low-elevation coastlines that are vulnerable to sea-level rise. Many major global cities are highly vulnerable to coastal flooding, including Miami, Mumbai, New York, Shanghai, and Tokyo. In the United States, close to half of coastal communities surveyed have failed to plan adequately for rising seas. The result will be greater expenses for taxpayers down the road, since retroactively fitting a levee or a bridge, for instance, is typically far more expensive than building resilient infrastructure.

Already, rising seas are claiming land and forcibly displacing people. UN Secretary-General António Guterres has warned that sea-level rise could cause low-lying communities, including entire island countries, to disappear forever, driving a “biblical scale” exodus from coastal regions. This in turn would spur “ever fiercer competition for fresh water, land, and other resources,” according to Guterres. In recognition of these growing risks, the UN Security Council held its first session focused on sea-level rise in February 2023.

MAKE IT REAL

Scientists warn that the world has nearly run out of time to achieve the 1.5-degree goal and that countries must hasten their climate efforts. They should aim to eliminate emissions of carbon dioxide as well as other greenhouse gases, especially methane; develop technologies to remove carbon from the atmosphere; expand carbon sequestration efforts; accelerate research on climate interventions such as solar radiation management; and ramp up efforts to build resilience in the face of an already changing climate.

Establishing an upper limit for sea-level rise would help build support and provide momentum for all these efforts. A clear ceiling would enable people to visualize the impending harm from rising seas, make it harder for governments to continue to ignore climate risks, and intensify the pressure to deliver meaningful commitments in climate policy negotiations.

Rising seas offer visually arresting evidence of the threat of climate change: before-and-after photos of climbing seas reveal dramatic coastal erosion. Images of “sunny day” floods—in which high tides fueled by rising seas inundate coastal communities even in good weather—tell a story of hardship and economic loss. Photos of “ghost forests” resulting from saltwater floods show profound ecological damage. And charts of the growing reach of storm surges depict the march inland of rising seas as they swallow buildings and towns.

Threatened by rising seas and more intense storms, over 64,000 Florida homes could become “chronically inundated” by 2045.

An upper limit for rising seas could help coastal communities improve their planning. For instance, such a ceiling could help clarify how much sea-level rise is already unavoidable, even if local factors will have to be taken into account to make exact projections. A limit on sea-level rise could also illuminate the tradeoffs involved in climate adaptation and mitigation investments. Across the globe, countries have failed to adequately account for the long-term risks of climate change. That in turn has led to adaptation choices that fail to consider the dangers of increased sea-level rise, such as the continued development of flood zones. A limit on sea-level rise could incentivize better choices by, for example, highlighting the dangers of building infrastructure near shorelines.

State and local governments in the United States should be leading the charge for an upper boundary on sea-level rise. Consider the plight of Florida, which would be the 15th-largest economy in the world if it were a sovereign country—larger than Indonesia or Mexico. Threatened by rising seas and more intense storms, over 64,000 Florida homes worth a combined $26.3 billion could become “chronically inundated” by 2045. A limit on sea-level rise would help contain the threat that rising tides pose to Florida’s real estate and property insurance markets and thereby help ensure the state’s economic viability.

When world leaders gather in Dubai for this year’s UN Climate Change Conference, they should agree to set a limit for rising seas just as they have for rising temperatures. Doing so would underscore the existential threat faced by small island nations and thousands of coastal communities. It would also make discussions about climate “loss and damage,” which will be on the agenda in Dubai, more concrete. An upper limit on sea-level rise would reinforce the rallying cry to “keep 1.5 alive” and drive governments to act more swiftly on climate change.