We've already seen Manhattan swallowed by surging waves as glaciers collapse and drive sea levels sky-high—on video screens and in nightmarish daydreams about human-driven climate change. But what will sea level rise really be like for a coastal metropolis of the future? It's actually easy to answer that question yourself, not with a ride in a time machine but simply with a trip in a car, boat, or plane. Visit any of more than a dozen coastal cities around the world and you'll soon get a first-hand taste of what's coming. But be forewarned; it's probably not as thrilling as what you've seen on TV.
We often hear about how high the sea could rise as ice sheets thaw and crumble. If we switch quickly to non-fossil fuels, the sea level should rise about as much as it did during the naturally warm period before the last ice age, roughly 20 feet or so. If we burn all of our remaining coal, then all land-based ice will vanish and the sea level will stand over 200 feet higher than today.
We rarely hear about the slow pace of such changes while gaping at disaster flicks, or at the so-called "flood maps" that show America with the thumb of Florida bitten off as if by sharks. If the thought of catastrophic climate change finally makes you so depressed that you chuck everything and simply wait on your favorite beach for the Great Flood, you'll certainly perish there. But not from drowning; you'll die of old age long before the ocean overtakes the trees or buildings around you.
Sea level is currently rising by about three fingernail thicknesses per year, and it climbed about seven inches during the entire 20th century. Nobody knows exactly how much it could speed up during this century; the high end of the prediction spectrum suggests another 15 feet or so by 2100 AD. But most of the estimates coming from experts who study this topic for a living put the range closer to two or three feet by century's end. That averages out to about two or three inches per year.
In other words, if you're going to sit on the beach and wait for the 10-foot-high roof of the hot dog stand behind you to disappear, you're going to have to wait between 300 and 1,000 years, so you'd better bring plenty of cash to pay for all of those hot dogs you'll be eating in the meantime.
Now, don't get me wrong. Sea level rise is a horrible outcome of human-driven warming. But not in the way you may be thinking of it. It's not so much a crashing apocalypse as it is a chronic problem, more like a slow cancer than a sudden heart attack. We still need to do all we can in order to prevent as much of it as possible, but we also need to realize that it's going to take a long time. This is not to remove all concern about the issue, but rather to help us recognize that the slow pace of these changes can mask their seriousness. And it also means that this is going to bother us and our descendants for a very long time—probably for thousands of years.
So what's it really going to be like to live in a coastal city from here on out? You might ask residents of the American Gulf Coast, much of which has been sinking faster than the ocean rose during the last century, thanks to groundwater extraction and removal of oil and gas. Some sites around Houston, for example, have recently started sinking by more than an inch per year, though you wouldn't know it from the scarcity of media coverage about that long-term "slow-pocalypse" (PDF).
Or consider Tokyo, which sank about 10 feet during the last century. Some sites near the harbor are subsiding by more than four inches per year. China's largest coastal city, Shanghai, sank about 9 feet deeper into the Yangtze delta over the last hundred years. And Bangkok is sagging by four or five inches per year, which is over twice as fast as the inundation in a worst-case future and forty times the rate of today's sea level rise. In these and a surprisingly large number of similar places, the sinking of the ground has much the same effect as a rising of the sea surface, and the magnitude is often greater than what we worry about from climate change. It's been costing some cities billions of dollars in dikes, pumps, and repairs for many decades already. It has become such a chronic problem in Texas that the state legislature has officially designated a "Coastal Subsidence District" around Galveston.
Clearly, this is a severe drain on resources that most towns would surely prefer to do without. Unless, perhaps, you raise the topic with residents of Venice. The soft sediments beneath the city are dropping twice as rapidly as the sea is rising, and the streets have been flooded for centuries. But urban life in Venice is not paralyzed by this, and tourists don't flock there in order to gawk at the devastation of a city gone under. By now, Venetians are used to it and I suspect that, given a choice, most of them would keep things much as they are. Of course, that might change when even faster sea level rise kicks in and piles on top of their existing problems.
Or consider Amsterdam. Roughly two thousand years ago it was a nondescript bit of rural landscape isolated from the ocean by miles of low-lying coastal plains. When naturally rising sea levels eventually formed what is now the Zuiderzee harbor, they also opened a route to the shipping lanes of the North Atlantic and Amsterdam was transformed into a bustling hub of commerce and culture. Even as we lose today's Big Easies and Shanghais, tomorrow's nascent Amsterdams will spring to life in the "zone of anticipation" that precedes the waves. Centuries later, they too will succumb to submergence, and so on it will go.
This is not a future that most nations would choose, nor should they have to. Fortunately, we still have time to keep most of today's seaside settlements above water, albeit not without somewhat of a dunking in the lowest areas—if we switch to carbon-free fuels as soon as possible. Our distant descendants, if they think of us at all, might thank us as they twirl their pasta in the canal-side cafes of an even-more-ancient Venice or only-partially-waterlogged Manhattan in 5000 AD.
Curt Stager is an ecologist, paleoclimatologist, and science journalist with a Ph.D. in biology and geology from Duke University. His new book is DEEP FUTURE: The Next 100,000 Years of Life on Earth (St. Martin's Press, March 2011).