For the hundreds of millions of people who live alongside the world's coasts, the scariest sea-level rise scenario is the idea that ice sheets could collapse.
Seas are already rising rapidly, threatening to swamp cities like Miami within the lifetimes of people alive today.
If the sheets of ice that sit on top of Antarctica and Greenland were to collapse, the rate of sea level rise could skyrocket, making coastal cities uninhabitable and destroying trillions of dollars of property and infrastructure.
To prevent or slow these floods from washing over cities, humanity may need to embark on the biggest civil engineering project in human history, according to a study published Thursday in the European Geosciences Union journal The Cryosphere.
The project: building massive walls under the ice sheets to stop them from falling apart.
It would be a geoengineering effort - a way of reworking our planet - that might buy time for coastal areas to adapt and for humanity to reverse some of the warming we've caused by burning fossil fuels and changing the climate.
"Doing geoengineering means often considering the unthinkable," said John Moore, one of two authors of the new study and a climate scientist at Beijin Normal University and Finland's University of Lapland Arctic Centre, in a statement.
The project is still theoretical. This sort of "ice sheet intervention today would be at the edge of human capabilities," the authors wrote in the study.
But it's possible that catastrophic ice sheet collapse could happen in the foreseeable future, and the processes that could trigger it at the Thwaites Glacier in West Antarctica - one of the most vulnerable glaciers - could already be happening.
"Thwaites could easily trigger a runaway [West Antarctic] ice sheet collapse that would ultimately raise global sea level by about 3 metres (10 feet)," Michael Wolovick, a geosciences researcher at Princeton and the other author of the study, said in a statement.
Predicting collapse
There's enough ice stacked on top of Antarctica to raise seas around the globe by almost 200 feet (60 metres). While it takes time for major changes to occur with that much ice, Antarctica is melting faster than we thought, according to a study published in June in the journal Nature.
While it would take thousands of years for seas to change by hundreds of feet, people have already caused seas to start rising.
In the 20th century, sea levels around the globe rose about six inches on average, Michael Oppenheimer, a professor of geosciences at Princeton, said during a media briefing earlier this year on sea level rise. That was enough to narrow the typical East Coast beach by about 50 feet (15 metres).
Since the mid-1990s, places like Miami have seen an additional five inches (12 centimetres) of sea level rise. Seas rise faster in some places than others, due to ocean currents and the effects of gravity.
Several factors contribute to global sea level rise. As the world has warmed due to the burning of fossil fuels, oceans have absorbed the majority of the heat. Warmer water expands, which takes up more space.
Glaciers are also melting, adding more water to the system. The final factor is melting from ice sheets in Antarctica and Greenland that are still protected by glaciers.
Propping up the ice
According to the recent study, we could try to build up support structures or even walls underneath the ice sheets to prevent them from breaking apart or being weakened by the influx of warm water from below.
Wolovick and Moore have been researching this concept for several years now. In their new study, they calculated how likely it would be that ice sheet engineering could avoid the collapse of the Thwaites Glacier.
It's an especially challenging glacier because of its extreme width, which means it would need a large support structure.
The first method of propping up the glacier modeled by the study authors is sticking a series of thousand-foot mounds underneath it.
While this wouldn't block warm water from flowing beneath the ice, the mounds would help support the glacier, making collapse less likely and giving it a chance to regrow.
Even this "simpler" method would be a massive undertaking - "comparable to the largest civil engineering projects that humanity has ever attempted," the authors wrote, like digging the Suez Canal but in a much harsher environment.
The authors calculated that this approach would have a 30 percent chance of preventing the collapse of the Thwaites Glacier - which could trigger the loss of the West Antarctic Ice Sheet - for the next 1,000 years.
The more complex approach would be to build an actual wall underneath the glacier, potentially stopping the influx of half of the warm flowing water. Such a project would have a 70 percent chance of success over the next 1,000 years, the authors wrote.
But it would be far more difficult to pull off.
Not a total solution
Aside from the technical difficulties involved with conducting the biggest engineering project the world has ever seen, there are other concerns about attempting this.
The study authors worry that fossil fuel interests could use the idea that there is an engineering solution to sea level rise as an argument in favor of continuing to pump greenhouse gases into the atmosphere.
But unlike other proposed geoengineering solutions, such as the idea that we could block out some of the sun's light to stave off warming, this solution is focused only on one of the consequences of climate change.
In other words, it might help lessen some of the negative effects of sea level rise, but the other effects of climate change would continue unabated. These include drought, ocean acidification, intense storms, and searing heat waves.
Plus, as the world warms, these glaciers will continue to melt and proceed towards collapse, even if they are propped up from below.
"The more carbon we emit, the less likely it becomes that the ice sheets will survive in the long term at anything close to their present volume," Wolovick said.
This article was originally published by Business Insider.
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