How much can the sea level go up with global warming and how fast will it happen?

According to some estimates, if sea levels rose one meter, Boston would lose 3% of it’s land surface, Washington DC a mere 1%. Tampa and Miami would lose 18% and 15% respectively. New Orleans would lose 91%.

A six meter rise would result in much larger losses. Norfolk, Virginia and Miami Florida would be essentially gone.

These estimates use the assumption that the sea level rises in those areas vertically, and the corresponding topographical level in the coastal city becomes the shoreline. They don’t account for the fact that the ocean does not work that way. (see Sea Level Rise…Extreme History, Uncertain Future.)The shore of the ocean normally consists of a relatively flat zone covered by sea (perhaps exposed ~2 times a day at low tide), a steeper zone where the sea intercepts the land (and generally goes up and down a certain amount with the tides) which was carved out by erosion, then inland, whatever topography would have been present prior to the incursion of the sea. The original shorline first contacted by the sea is gone, and the strandline has moved, or transgressed (that’s the term we use), some distance across the landscape. In a place like Miami, the sea may transgress many miles across relatively easily eroded sediment. In a place like Boston, filled land (which makes up a huge amount of that city’s land surface) might be easily eroded away, glacial sediments that make up much of the city’s substrate would erode fairly quickly. Rock conglomerates that make up much of the southern part of the city would erode slowly while weathered argilite underneath Cambridge would be eroded away quickly. The North Shore communities, sitting on hard rhyolite, would make nice islands for a long time. In other words, it would be complicated.

But can sea levels rise this much, and if so, over what period of time?

If the entirety of the Greenland Ice Sheet melted into the sea, the sea would rise by over 7 meters. At the moment, Greenland’s ice is melting at a rate higher than previously expected, and there are factors as yet unquantified that may make this happen more quickly than at present. Two of them, to be precise. First, the surface of Greenland is experiencing “total melt” now and then, which may be normal, or may be increasing in frequency. When the surface of the glacier melts even a little bit, the ability of that ice to retain rather than reflect sunlight increases, causing more melting. Second, global warming enhanced drought together with global warming enhanced heat has caused increasing wildfires in the Northern Hemisphere of the Americas, which causes the deposition of sooty stuff on the surface of Greenland’s ice, thus enhancing melting. (See this for more information on this.) So yes, Greenland’s ice can certainly contribute to sea level rise, and may be doing so at an accelerating rate.

But will all of Greenland’s ice melt? Probably not, but during the Eemian, the last climate phase that was very similar to the present, a good chunk of Greenland’s ice (and other ice) did melt off under conditions that were globally probably about the same as today or even a bit cooler. During the Eemian, however, the Northern Hemisphere may have been warmer than it is today, or at least, the weather patterns that prevailed may have caused Greenland to be warmer, and thus, have less ice. If the current trends in climate change are causing a situation like this to emerge, then we can expect Greenland to become less icy and the sea levels to rise as a result.

But how fast? Whenever you read about continental glaciers like Greenland’s (or Antarctica’s) melting, or about sea level rise, these changes are described in “geological” time scales. Of course, it would take many decades or even centuries for such a large scale change to happen, right? But the truth is, we have no idea what the full range of time frames might be for this to occur. Some of the estimates of sea level rise caused by Greenland ice melting, using current or extrapolated near future conditions, suggest a rate of a several centimeters over a few decades of sea level rise, which would contribute roughly half of the total sea level rise if we add in thermal expansion of the oceans at the same time. However, none of the current estimates that I’ve seen include catastrophic demise of the ice sheet, where large portions of the glacier physically break apart, some sliding into the sea. Consider how fast an intact ice cube melts if you put it on your kitchen counter, vs. the same ice after you’ve broken the cube into dozens of small pieces. A deteriorating Greenland ice sheet could melt much faster than any of these estimates suggest.

Meanwhile, the Antarctic Glaciers are melting as well, especially the West Antarctic Ice Sheet, which represents about 10% of the total ice volume of the frozen southern continent. In total, there is about 52 meters of potential sea level rise stored up in the Antarctic, so the West Antarctic sheet, all melted, would contribute directly about 5 meters or so to sea level, added to thermal expansion.

The thing about the West Antarctic ice sheet is that it is not really on land, but rather, in the sea; it rests in a giant “inland sea” where the Antarctic continent is below sea level, having been depressed by having this giant glacier weighing it down. If this ice sheet melted and deteriorated enough, the much of the ice could become free(ish) and float off into the ocean, causing sea level rise without even complete melting. That would, of course, allow the depressed part of the continent to (slowly) rise up again. Which would displace more of the sea, causing some rising. It’s all very complicated.

The key point is this: The rate of melting of various parts of the West Antarctic ice sheet is increasing, the rate having increased by about 50% over the last 15 years. The Pine Island Glacier has been sliding into the sea at a rate of about 25 miles a year. The situation there is changing very quickly and the things that are happening involve processes that are not well enough understood to model, and thus, processes that are pretty much unpredictable.

So, we are left with the same questions we started with: How much can the sea level go up with global warming and how fast will it happen?

Another way to get at this is to ask, how much higher was the sea level during the Eemian, when conditions were roughly like they are now but, for some reason, lots of what is now ice was melted into the sea contributing to a higher sea level? And, we can also look at the rate of sea level rise the last time it occurred quickly, following the Last Glacial Maximum (about 18,000 years ago).

Regarding Eemian sea level stands, the ocean at that time was 5–7 meters higher than it is now. So, forget about those coastal cities mentioned above. Forget about Boston, New York. The name New Orleans will refer to a place to go fishing. Kiribati, the country, will no longer exist. School children will learn about Venice but never be sure if it really existed.

But how quickly can this happen? From the time of the Last Glacial Maximum to the present, sea levels rose more than 120 meters. During the earlier part of that melting the sea levels went up much more quickly than during the latter part, and during that phase of rapid rise, there were “events” during which the rate was quite high. The most extreme of these is known as Meltwater Pulse 1A, a 500 year time period during which sea levels went up about 20 meters. However, since sea level rise tends to wipe out detailed evidence of itself, we can not say that the rate of rise was uniform over that time. Meltwater Phase 1A could have involved a number of very rapid rises. But if we assume that the rate is constant, and pick a 50 year time period to be concerned about (since our children or grandchildren would observe this so it is something we can relate to) that’s about 2 meters.

Franklin Roosevelt ushered in social and economic policies that form part of our system of government today more than 50 years ago. Fifty years ago, we started the space program and everything ran on tubes. We had just left behind legal segregation in most states and there was no Civil Rights Act. Inventions, innovations, social and political changes work in this time scale. Things we do now will matter in 50 years and people living then will know who did those things…or, perhaps, failed to do them. If the ice of continental glaciers does what it looks like it is going to do, and if the rate of melting is at the higher end, we could have a 1 or 2 meter rise in sea level in time for our children to know that our failure to act today caused it. Go find a toddler and look it that toddler’s eyes and imagine that person as an adult wondering what you were doing back in the day, when we knew we were in trouble but did nothing effective.

And that assumes, again, that events like Meltwater Phase 1A were steady, ponderous if somewhat high rate events. If increasing sea levels really does involve a handful of catastrophic changes amid a steady rate of melting, than we could experience real jumps of, say, 10 or 20 centimeters in one year, then a few years later, another 10 or 20, taking everyone by surprise. Superstorm Sandy flooded the New York Subway because sea levels had gone up over previous decades by just enough to cause the storm surge to overtop the NY equivalent of levies. So, we’ve already managed to fail to act and then suffer the consequences, in that case on a very small scale. Do we want to do that on a large scale?

Judging by how we seem to be addressing the climate change crisis, apparently so. Shame on us.

Much of the information referred to here comes from the new book, Rising Sea Levels: An Introduction to Cause and Impact, which I’ve reviewed here.

See also: Sea Level and the Limits of the Bathtub Analogy