Worst Case, How Fast Could Sea Level Rise?

“We might see a six foot tidal surge come in and stay there” a recent article about rising sea level stated.

Nonsense. Even with the most catastrophic case, it is just not possible for the global ocean to rise rapidly like a tsunami or tidal bore, which happen in minutes.

Given the accelerating melt rates in the polar regions and growing concern about flooding, it is important to have a realistic idea of what the worst-case for sea level rise could be. For those not familiar with the topic, here are a few points as context, before looking at the maximum rate of rise.

• Over millions of years of geologic time, sea level has gone through natural cycles, moving up and down 300 – 400 feet, roughly every hundred thousand years. We have now clearly broken out of that natural pattern, usually described as the ice ages.
• More than two hundred feet of potential sea level rise are still locked up in the ice sheets and glaciers on Greenland and Antarctica. Even the worst case scenarios show that full meltdown would take perhaps five hundred years, or even thousands of years on our current path. If we reverse global warming in time, we can prevent the full meltdown.
• Because the oceans have already been warmed by approximately1.5 degrees Fahrenheit (~ one degree Celsius) some sea level rise is now unavoidable, despite efforts to curb CO2 emissions and slow the warming.
• Estimates for potential sea level rise this century now range from about three to ten feet (1-3 meters), depending somewhat on how warm the planet gets in the coming decades. In recent years the upper bounds have been raised several times, as the Arctic melting has been exceeding the models.
• Geologic records indicate that in the past that sea level rose an average of a foot and a half (50 cm) a decade, some 14,000 years ago. In four centuries, global sea level rose some sixty five feet (20 m).

It is not possible to accurately predict the rate of sea level rise, as it depends on exactly how the ice sheets and glaciers on Antarctica collapse. That defies accurate prediction for reasons similar to why we cannot predict the specific timing and magnitude of earthquakes, avalanches or mudslides. (For an explanation, see Sea Level Rise Will Rise in “Steps” Not Smoothly.)

There is only one scenario for really rapid sea level rise. One or more of the large “marine glaciers” in Antarctica need to slide into the sea. Glaciers are those slowly moving rivers of ice, that work their way to the ocean simply due to gravity. Speeds of movement might be a few miles per year, but in the last few decades some have doubled, tripled, and even quadrupled in speed as the overall warming causes melting. As glaciers move from land and calve into icebergs, they add to sea level in the same way that adding an ice cube raises the level of liquid in a glass. Marine glaciers are special in that warmer ocean water can erode them from below. Though not floating like icebergs, think of them as resting in a valley or fjord. They melt much faster underwater than on the top side, causing them to move faster and faster, leading to potential cascading collapse.

While there are many models and studies, it is simply not possible to accurately predict how fast these massive structures will enter the ocean. For scale, the large ones are many miles wide, tens of miles in length, and can be over a mile in depth. As they move down valleys and around submerged mountains, the actual speed is impossible to predict accurately, particularly since we do not know how warm the planet will get. (See “The Elephants of Antarctica and Greenland“.)

The dominant marine glaciers of Antarctica are the Thwaites and Pine Island on the West and the Totten on the East, holding on the order of twenty feet of sea level between them. Most experts totally doubt that they could slide into the sea in the next few decades. But the changes to a similar glacier in Greenland have stunned even the experts. Jacobshavn (“Yak-OBS-Have-En”) glacier in Greenland was observed to go through catastrophic collapse in 2008 and in 2015 in a matter of an hour. In 2016 Antarctica even had rainfall and pooling water on the glaciers, both of which are very ominous.

Some have suggested that rapid disintegration of these glaciers could occur on the scale of decades, which could theoretically mean sea level rise as much as ten feet (3 meters) within a decade, a rate that would be catastrophic. Based on the present conditions however, it does not seem realistic for these glaciers to possibly slide into the sea within the next decade or two. Still, given the recent observations, the situation could change rather quickly in the next several years. Needless to say, monitoring changes to those glaciers is the highest priority to forecast rapid changes in the rate of sea level rise.

That leaves the question of how abruptly could sea level rise in the next few decades? Could it arrive like a six foot tidal surge, but that comes in and stays? The answer is simply NO. With a large enough calving of ice, there could be a tidal surge in a limited region, similar to a tsunami. But that water would recede quickly as it is driven by an energy pulse. For the high water level to remain, the volume of the ocean has to increase. That means melt water or huge chunks of ice from land entering the ocean. There is no realistic mechanism for that to happen rapidly enough to cause a sudden six feet of rise in base sea level.

That would require an amount of ice on the order of the size of a state to suddenly enter the sea. The only places poised to do that are the glaciers named just above, but even their physical configuration does not support their entire volume instantly going into the sea and raising global sea level. Because of the ocean volume there is simply no mechanism for sudden sea level rise, as in the course of a day. Nonetheless as those glaciers melt, slither, and collapse there could be dramatic increases in sea level over, possibly noticeable over the course of weeks and months.

While there is no absolute number, an upper limit for abrupt sea level rise is probably on the scale of one foot in a year (30 cm), more or less. At that rate it would almost certainly not be just a one year event. The cumulative rise over several years could then well be several times that, certainly causing global catastrophe. 

For now I believe we need to do the following:

• Work to slow the warming ASAP, which means reducing carbon dioxide levels and other greenhouse gases, which can slow the rise, but not stop it.
• Plan for the first 3 feet (~ meter) of higher sea level ASAP. Regardless of how soon it happens, it will happen. Raising our building codes and adapting coastal infrastructure needs to start now. We cannot wait for the water to rise, or for certainty about the speed of the rise. Fortunately the clarity that global sea level will rise at least several feet, means that all investments in that scenario will be rewarded.
• Invest in the best monitoring possible of the glaciers on Greenland and Antarctica, to get as much notice as possible, about abrupt changes, which will foretell catastrophic rise in sea level.

Catastrophic sea level rise is outside the human experience. It might be seen as a “black swan event” –– something extraordinary that we deny until it has been experienced first hand. The future of our civilization, economies, and communities depends on our ability to deal with this challenge. The burden of inaction will fall hardest on the poor worldwide, but will impact everyone, even those living far inland.

The sooner we get educated, plan, and adapt the better. But let’s do it based on what is realistically possible, not some fantastical descriptions that step beyond the reality of the ice sheets and glaciers. The realistic scenarios are scary enough.