Sea-level rise and hurricanes: In it for the long-haul

Sea-level rise and hurricanes: In it for the long-haul
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Climate change is not irreversible. Whether by design or by running out of supply, we’ll eventually dramatically reduce our fossil fuel use. Reduce it enough, and carbon dioxide levels in the atmosphere will also start dropping. Maybe a few decades after that, the global average temperature will begin a downward trajectory. Once that happens, most of the climate change impacts on extreme weather will likewise begin tapering off. The whole process is going to take a while.

It will take much longer, though, to reverse sea-level rise. Even under a very aggressive mitigation scenario, most models project sea level in the year 2300 to be 1 to 10 feet above present-day levels. Without aggressive mitigation, the projections are much higher. So, we’ll have to deal with higher sea levels no matter what.

Climate change is likely to continue to affect many aspects of hurricanes. In Hurricane Ida, we saw lots of ways that sea-level rise in particular makes hurricanes worse.

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Along the Louisiana coast, where Ida made landfall, the combination of local land subsidence and global sea-level rise have combined to produce seas that are rising at the rate of about a third of a foot per decade, maybe more. In some ways, this makes Louisiana a bellwether of what will happen elsewhere as the inexorable rise of sea level continues. So, what did sea-level rise do to Ida and her impacts?

Most obviously, sea-level rise makes storm surges higher. If the sea level is starting off a few feet higher, then storm surge will automatically be a few feet higher too. 

This is a big deal. Coastal infrastructure is designed to withstand a certain amount of storm surge. As a storm surge gets higher, those design limits get exceeded. There’s a huge difference in impacts between “withstand” and “not withstand.”

Inland from the immediate coast, the impacts are even greater. Higher sea level makes barrier islands that much shallower, reducing the first line of defense for storm surges farther inland.  It’s like lowering the levees: storm surge can move inland more easily, and the amount of water moving inland is greater. So unless you go in and manually raise the barrier islands, the sea-level rise effect is amplified once you get past the barrier islands and coastal marshes.

If sea level rises slowly enough, the barrier islands can migrate inland, and marshes can continually regenerate themselves. But accelerating sea-level rise, combined with local subsidence, already appears to be happening too rapidly in Louisiana for the marshes to keep up.

In Louisiana, as elsewhere, not only has the ocean risen and the land sunk, but the natural landforms have been altered. In Katrina, now-dismantled levees concentrated the storm surge until it eventually broke through in New Orleans' Ninth Ward. Meanwhile, canals have been carved through the marshes, often for the sake of easier oil and gas exploration, but with the unintended side effect of allowing saltwater to intrude inland and kill off freshwater marsh plants. 

In addition to having the most rapid relative sea-level rise in the United States, Louisiana also has an exceptionally flat coast. As marshes die back or drown, the ocean expands substantially, bringing the Gulf of Mexico closer to inland cities. This means that any storm like Ida making landfall will survive longer and retain its hurricane-force winds longer, causing damage and taking down power lines. Inland cities are at increased risk of wind damage due to sea-level rise even if they are well above sea level.

Scientists are still teasing out the many ways that climate change is affecting hurricanes.  Probably more intense rain. Storms that are probably more likely to become major hurricanes.  Maybe an overall increase in the number of major hurricanes, if the total number of storms doesn't decline much. Possibly decreases in the motion of storms in many areas. Changes in rapid intensification that go along with changes in major hurricanes, since most major hurricanes intensify rapidly. Probably a few other things we haven't even considered.

The one that may be most certain, sea-level rise making storm surges worse is also the one that will most likely be the longest-lasting. Long after we've brought carbon dioxide emissions under control, long after global temperatures have started falling, long after rainfall intensity has settled down — the sea will still be rising, and the storms will keep coming.

John W. Nielsen-Gammon is a professor in the Department of Atmospheric Sciences at Texas A&M University.