TIL: Why Sea Level Rise is not the Same Everywhere

Source: NASA/JPL-Caltech.

Years ago, I saw a photo of a buoy in a harbor somewhere, with a caption saying the sea level was the same as it was 100 years ago. "Proof" was provided in a photo of the same buoy from 100 years ago. The sea level was indeed the same in the two photos. The photos were offered as proof that global warming and sea level rise were all a great hoax. What gives, I thought?

That was years ago, but that "proof" always bothered me more than it ever seemed to bother climatologists. This month, Wired magazine published an article that answers the question that's bothered me all these years. What gives? Read on.

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Between now and the year 2100, coastlines may average an extra 2 feet of rise, thanks to emissions already in the atmosphere, and up to 5 feet more if humanity fails to cut its emissions between now and then. Emphasis on average. The reality is that different stretches of the coasts will see wildly different rates of oceanic creep. By 2050, the average rise will be 4 to 8 inches along the Pacific, 10 to 14 inches along the Atlantic, and 14 to 18 inches along the Gulf. So what gives? If melting glaciers are loading all the oceans with extra water, shouldn’t all the coasts experience sea level rise equally?

Source: Wired.

The key to why that buoy seems to be unaffected by sea level rise is because sea level rise affects different places on Earth differently. Even different neighborhoods in a single city can experience sea level rise differently. It turns out it's "relative" sea level rise that's most important locally. The buoy's harbor is apparently rising at the same rate sea level is.

That map above is color coded by how much the sea level is changing on different coasts. Alaska is blue because the land is rising and has been for thousands of years as glaciers melt and their weight leaves the land. The US Gulf coast is red because the land is sinking, some of it because humans are extracting water and oil. Even with the same sea level rise everywhere, "relative" sea level rise will be different.

In growing coastal cities, poor people tend to be crowded into marshy land that's sinking, whereas wealthy people tend to live on land over bedrock. Such land suffers less subsidence. Over time, the observed sea level rise in the marshy area is relatively higher than sea level rise in the area with bedrock to build on. But wealth doesn't always save you.

In Galveston in particular, the land has been collapsing at an astonishing rate. “The numbers I'm going to give you are going to be hard to believe, but there's an area in Baytown, where there's a big Exxon Mobil industrial plant, that sank about 11 feet in a period of 50 or 60 years, because they were just unsustainably pulling water out of there,” says Bob Stokes, president of the Galveston Bay Foundation, a conservation nonprofit...“There was a nice upper-middle-class subdivision where all the Exxon executives lived that ultimately had to be condemned, because water was lapping up the foundations of these houses.” (Whether or not the irony was lost on them isn’t clear.)

Source: Wired.

So, thank you Wired for answering a question I've had for a very long time.