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In my article in Tuesday’s Science Times about the risks of long-term sea level rise, and in an accompanying podcast, I reported on the link between past instances of global warming, caused by natural fluctuations in the climate, and higher shorelines.

Based on a study of these past variations, some scientists believe that even if we stopped burning fossil fuels tomorrow, we would be due for a substantial rise in sea levels over the long term as ice sheets slowly respond to the warmer temperatures brought on by the greenhouse gases that humans have already dumped into the atmosphere.

The paleoclimate record, as it is known, suggests that even a slight amount of global warming can produce a rise of 25 to 30 feet. And if scientists are anywhere close to right in their projections, the warming over the coming century due to human activity is going to be more than slight. That means a long-term rise in sea level of as much as 80 feet cannot be ruled out.

As I noted in the article, that would take quite a long time to occur, at least hundreds and more likely, thousands of years. On the time scale that people tend to worry about, the next century, the rise would be much less. A common estimate is three feet, although there are persistent fears that the situation could turn out to be worse than that.

What I did not point out is that, now matter how fast it happens, the increase of sea level will not be an even rise everywhere on the planet. Rising sea level, it turns out, is “lumpy” – the sea goes up more in some places than others.

This may seem strange, since we tend to think of the ocean as a giant bathtub filled with water. If you turn on the faucet and add water to a tub, of course, it rises evenly. Why doesn’t the ocean work that way?

Scientists say sea level rise in a given location is actually influenced by a multitude of factors. To take one of the most obvious, land can be sinking even as the ocean is rising. This is happening today across much of the East Coast of the United States, with especially high rates of sinking in the Chesapeake Bay region and in southern Louisiana.

The converse can also be true, of course – land in some places is rising fast enough to outpace the rise of the sea. That is happening today in parts of Alaska and Scandinavia.

If land is sinking, that worsens the local effects of a global increase in sea level. And it turns out that sinking land is going to be one of our biggest problems, worldwide, as climate change proceeds. That is because many of the world’s great cities are, like New Orleans, built on river deltas – which is to say, on mud.

Humans have interrupted the flow of sediment to those deltas in most places, and the land surface is sinking as the older sediment underlying the cities compacts and settles. Withdrawal of groundwater or petroleum can worsen the situation, although water withdrawals have been limited in many places specifically to slow the sinking.

If we get a global increase of sea level of several feet by the end of this century, as many scientists expect, the effects are going to be felt most strongly in the big cities with brisk rates of land subsidence. Aside from New Orleans, examples include Tokyo, Houston, Shanghai, Bangkok and Venice.

The complexities do not stop there. As water is added to the ocean basins, the basins themselves adjust to the extra weight, behaving less like a bathtub and more like a kid’s pool made of flexible plastic. The ocean floor can actually sink. The deformation is slow, but it changes the distribution of ocean water over time.

Another factor that can alter sea levels is a shift in wind patterns. The prevailing winds can cause water to pile up in some locations, as happens today off the eastern coast of Asia.

All of these factors complicate any forecast about sea level rise, and most or all of them can be important for a given locale. But to me, the strangest and most interesting variable has to do with gravity.

The ice sheets on Greenland and Antarctica are immense, and it turns out they exert enough gravitational pull to draw a substantial amount of ocean water toward them. So if you imagine the whole Greenland ice sheet melting, for instance, something quite bizarre will happen nearby: sea level will fall across an area stretching more than a thousand miles from the ice sheet.

“If Greenland should catastrophically melt tomorrow, sea level will fall in Scotland, it will fall in Newfoundland – and it will fall in Greenland, by a lot,” said Jerry X. Mitrovica, a Harvard scientist who helped to pioneer the study of such effects. Something similar would happen in the vicinity of Antarctica if the West Antarctic ice sheet, believed to be highly vulnerable to a modest global warming, were to melt.

Think about the implications of this finding: If sea level is going to fall near the melting ice sheets, that means it has to rise more than the average in other places, right?

And that is bad news for some countries. It means an average global rise in sea level of 30 feet, say, could translate to a 40- or 50-foot rise in some parts of the world.

Scientists think the exact distribution of sea level rise will depend on which of the two ice sheets melts – or, more likely, what proportion of each melts. If you live on the mid-Atlantic coast of the United States, for example, a given amount of melting in faraway Antarctica will produce a substantially larger increase of sea level than the same amount of melting from nearby Greenland. It’s counterintuitive, but true.

On top of all this, as the ice sheets melt, the redistribution of mass around the planet will be large enough, according to Dr. Mitrovica and several of his colleagues, that it will actually alter the rotation of the earth. That, in turn, will cause another modest redistribution of ocean water.

With so many moving arrows, the effort to predict exactly what will happen with sea level rise is a work in progress. But we already know that both Greenland and West Antarctica are starting to melt and dump extra ice into the ocean in response to climate change. Globally, sea level is now rising at a rate of about a foot a century and the rate appears to be accelerating.

Given the immense growth of human settlements near the ocean over the last century, the risks of this situation are pretty clear. Scientists fear a perpetual humanitarian crisis will begin before the 21st century is out, as people are forced to flee the rising sea.

At my request, the researcher Benjamin H. Strauss at the organization Climate Central, made a rough calculation of the number of people living within 10 meters, or 33 feet, of sea level: 710 million, or about 12 percent of the world’s population. And about 1.3 billion people, or 21 percent of the population, live within 82 feet of sea level. In other words, the kinds of long-term increases in sea level that scientists are talking about could wind up displacing a substantial fraction of the human population.

So far, the advice from scientists to reduce the level of risk by limiting greenhouse emissions has basically fallen on deaf ears, and global temperatures are rising.

In my article, I quoted Dr. Mitrovica as pointing out that the temperatures expected later this century have, when they occurred in the past, caused the catastrophic collapse of the ice sheets in both Greenland and West Antarctica. In an interview, he added, “I don’t think you should have any confidence that we’re going to somehow escape that if we continue with these temperatures.”