Six Feet Under

“Sometimes, 1 plus 1 can equal 3.”

This quote from Columbia University climatologist Radley Horton isn’t likely to convert any climate deniers, but it is a succinct way of summarizing an intriguing new study about how global warming will increase flooding in U.S. coastal cities.

If you have even a passing familiarity with climate change science, you know that global warming raises the risk that coastal cities will be inundated. Dozens of studies have made that point. But there’s an element of the research that few pick up on: There are two entirely different mechanisms that place coastal communities at risk.

First, sea levels are rising. Over the past century, global mean sea level has risen between four and eight inches. In certain areas, like the northeastern United States, this number has topped a foot. By the end of the century, those areas will likely see between two and six feet of additional rise. This is due to both the melting of ice sheets and glaciers, which add water to the world’s oceans, and the heating of the existing water. When water gets warmer, it expands. This “thermal expansion” is responsible for about one-half of observed sea-level rise.

So the steadily rising sea is one way in which climate change threatens to drown coastal cities. The other mechanism is less predictable and more cataclysmic: extreme weather. Although we can’t yet connect any particular mega-storm to climate change, warmer oceans do provide more energy to fuel hurricanes and other big storms. According to the National Oceanic and Atmospheric Administration, human-induced climate change will increase storm intensity by between 2 percent and 11 percent by the end of the century. The agency also predicts that tropical cyclones will produce 10 percent to 15 percent more rain by 2100. Bigger storms cause bigger floods.

When you read about research on how climate change threatens the coasts, it’s typically about one of these two mechanisms. In today’s study, a multi-institutional team of climatologists fed both effects—sea-level rise and increasing storm intensity—into a model to see what might happen.

They found an alarming synergy. When considered together, larger storms plus higher sea levels don’t add up arithmetically—instead, they more or less multiply each other’s effects. So when a hurricane makes landfall on a stretch of coast already dealing with rising seas, the impact is greater than the sum of its parts. The authors found that even if we address runaway carbon emissions, there will be a 4- to 75-fold increase in the flood index, a measure that combines the height and duration of flooding. If we continue to combust fossil fuels with no restraints, the flood index could increase by a factor of 350. And this wouldn’t only occur in some distant land but also in U.S. cities like Atlantic City, New Jersey, and Pensacola, Florida.

So, when it comes to rising waters, 1 plus 1 sometimes equals 3. It may not sound like good math, but it is good science.