The Inevitability of Sea-Level Rise

Sea-level rise and major U.S. coastal cities along the Atlantic seaboard: The challenges are inevitable, and commitments soon taking shape mean ‘society needs to decide what we want to give up.’

Small numbers can imply big things. Global sea level rose by a little less than 0.2 metres during the 20th century – mainly in response to the 0.8 °C of warming humans have caused through greenhouse gas emissions. That might not look like something to worry about. But there is no doubt that for the next century, sea level will continue to rise substantially. The multi-billion-dollar question is: by how much?

An aerial view of Miami, considered among the major U.S. cities likely most at risk from rising sea levels.

The upper limit of two metres that is currently available in the scientific literature would be extremely difficult and costly to adapt to for many coastal regions. But the sea level will not stop rising at the end of the 21st century. Historical climate records show that sea levels have been higher whenever Earth’s climate was warmer – and not by a couple of centimetres, but by several metres. This inevitability is due to the inertia in the ocean and ice masses on the planet. There are two major reasons for the perpetual response of sea level to human perturbations.

One is due to the long lifetime and warming effect of carbon dioxide in the atmosphere. Once emitted carbon dioxide causes warming in the atmosphere over many centuries which can only be reduced significantly by actively taking the greenhouse gas out again. This is because both the amount of heat and carbon dioxide the ocean can absorb is reduced, and so the temperature stays up for centuries or even millennia. Of course, not cutting emissions would exacerbate the problem even further.

The other reason is that both the ocean and the ice masses are very big and a warming of the surrounding atmosphere will only penetrate slowly, but inevitably, into them. As a consequence their sea level contribution continues even if the warming does not increase. Sea level rise over the last century has been dominated by ocean warming and loss of glaciers. Our recent study indicates that the future sea level rise will be dominated by ice loss from the two major ice sheets on Greenland and Antarctica – slumbering giants that we’re about to wake.

Sea level rise contributions over 2000 years from: ocean warming (a), mountain glaciers (b), Greenland © and Antarctic (d) ice sheets. The total sea level commitment (e) is about 2.3m per degree of warming above pre-industrial.

It is easier to understand a future world that has adjusted to a new equilibrium of higher temperatures than it is to understand the dynamic (perhaps rapid) transition from today’s world to a warmer one. That is why we used physical models for the ocean, the mountain glaciers and the big ice sheets to compute how the systems would be different if the world was warmer.

What we found was that for each degree of global warming above pre-industrial levels the ocean warming will contribute about 0.4 metres to global mean sea-level rise while Antarctica will contribute about 1.2 metres. The mountain glaciers have a limited amount of water stored and thus their contribution levels off with higher temperatures. This is over-compensated for by the ice loss from Greenland, so that in total sea level rises quasi-linearly by about 2.3 metres for each degree of global warming (see figure).

How fast this will come about, we do not know. All we can say is that it will take no longer than 2,000 years. Thus the 2.3 metres per degree of warming are not for this century. They need to be considered as our sea level commitment – the sea level rise that cannot be avoided after we have elevated global temperatures to a certain level.

Ben Strauss of Climate Central has considered the different possible future pathways that society might take and computed which US cities are at risk in the long-term. He poses the question as to what year, if we continue with greenhouse emissions at current rates, we will have caused an inevitable sea level rise that puts certain cities at risk.

According to his analysis, within the next few years Miami in Florida will be committed to eventually lie below sea level, while our future actions can still decide on whether we want to one day give up cities such as Virginia Beach, Sacramento, Boston, Jacksonville or New York City.

This is a decision society has to take for future generations. We will need to adapt to climate change in any case, but some things we will not be able to adapt to. Society needs to decide whether we want to give up, for example, the Tower of London, or to put the breaks on climate change so that we don’t have to.

This article was originally published at The Conversation. Reposted with permission.

The Conversation
Anders Levermann, Professor of Dynamics of the Climate System at Potsdam Institute for Climate Impact Research. Anders Levermann does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.

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16 Responses to The Inevitability of Sea-Level Rise

  1. Greg says:

    Thanks for sharing – scary – need to initiate mitigation actions such as putting a price on carbon ASAP

  2. Peter Capen says:

    The inevitability of sea level rise is a given as the planet continues to grow hotter as we pump CO2 into the atmosphere. Putting a price on carbon may help in the long run to slow down COs emissions but will do little to help respond to the sea level rise that has already been built into the system from thermal expansion and glacier melting. While sea barriers, such as those in Holland may help for a while, the only solution that offers hope ultimately is to relocate homes, businesses, and infrastructure back from the sea, which will be both incredibly costly and politically divisive. Nonetheless, the sooner we admit to the reality of sea level rise, the sooner we can begin planning for the future. As the recent FEMA maps show, many coastal areas are now threatened, not the least of which is New York, Miami, and New Orleans. The time for climate denial is at an end, and the longer we continue in stasis the greater will be the costs of any meaningful response to it and the more painful will be the realities of what we face.

  3. Dan Rogers says:

    Rather than putting a tax on carbon, why don’t we make room for the additional water from rising sea levels? There are plenty of places in the world which lie below sea level and which could be flooded quite easily.

    Cutting a channel from the Mediterranean Sea to the Dead Sea basin would be rather easy to do from an engineering standpoint, and we could make a very nice sea water lake where the Dead Sea and the Jordan River valley are located. The water drop of about a thousand feet would also carry enormous hydroelectric potential enabling all sorts of industry to move into that area which now experiences high and troublesome unemployment. Desalination projects for agricultural purposes could turn that area into a veritable garden.

    Other areas which could be flooded include Death Valley in California and the Qatarah Depression in the eastern Sahara. There are others. Flooding these places would put millions of people to work in a cause that is universally recognized as worthy — saving our existing coastal areas from flooding — and rid us of dreadful arid land that really could and should be put to better uses.

  4. Nullius in Verba says:

    The unanswered question here is how fast will the *land* rise?

    And yes, land does rise/fall, too. Erosion/deposition/subsidence change the land all the time. Rivers carry silt downstream, but as the water reaches the sea it stops and drops its load. The result is alluvial plains and river deltas whose level is precisely matched to sea level. Coral islands are built of corals that grow up to the sea surface but no further, and match rapid erosion with rapid growth right on the boundary.

    Those parts of the land that are notable for being very close to sea level are generally so for a reason, and it is by no means obvious that the rate of rise of sea level cannot be matched or exceeded by a consequent rise in the level of the land.

  5. Please explain why sea levels during the last interglacial were 31 feet higher than the present, and Greenland 8C warmer than the present, without anthropogenic forcing.

    What evidence suggests the current interglacial is any different?

    • Dan Rogers says:

      We are still in the “last” ice age. It isn’t over yet. All available evidence, as cited by James Lovelock in his Gaia books, indicates that the Arctic Ocean has been ice-free during each of the previous four inter-glaciation periods the planet has experienced over the past two million years or so. So just be patient Mr. Schtick. If history is any guide, there will be further warming before the present ice age finally comes to a close, and we humans will have next to nothing to do with causing it.

  6. Peter Capen says:

    There are currently and estimated 7 billion humans plus, the bulk of which are living in coastal areas that are already vulnerable to sea level rise and storm surges. Moreover, many of the world’s most populated areas are subsiding, not rising, which puts humans, their property, and public infrastructure at increasing risk from rising sea levels and growing flooding events. The huge naval base at Norfolk promises to be the nation’s first “sub-marine” bases, while Miami, its residents, and the city’s billions of dollars in real estate are “doomed.” No wonder the big private insurers have fled the state. In fact, much of Florida is likely to be submerged in the coming decades, according to new FEMA flood maps. So, given the fact that so much of modern civilization is vulnerable to the very real impacts of climate change now and even more so in the coming decades, it is an utter waste of time to throw out what happened in the geologic past over millions of years, or babble on about the potential of carbon pricing as anything but a tool to reduce future CO2 emissions. What is critical now is that we stop denying the reality of global warming and its potentially catastrophic implications for modern civilization and embark on a comprehensive plan to confront it before it is too late to avoid massive social disruption and costs we may simply may be no longer able to afford. The longer we raise spurious issues, the greater the pain from our inaction and denial will be. We are facing a challenge that is greater than any civilization has faced in the last 10,000 years. And the public better start waking up to that.

    • Nullius in Verba says:

      Most of the places where land subsidence is a problem, it’s because of flood prevention, not sea level rise. It’s not a new problem, and there are long-standing and well-developed technological solutions.

      What would be a waste of resources would be to spend time and money on global warming, and its relatively trivial consequences, when we ought to be developing proper flood management/land reclamation schemes to address the actual problem.

      That’s what a lot of engineers and geologists think, anyway. You might disagree. Views differ, and in a free and democratic society everybody gets a say.

    • DoctorDave says:

      You did not answer Hockey Schtick’s question yet.

  7. Peter Capen says:

    Those who cannot see the future are condemned to live it.

    • 1200intell says:

      See the future? Apparently the evidence from the past isn’t showing up on your climate crystal ball.

  8. DoctorDave says:

    You did not answer Hockey Schtick’s question yet.

  9. Peter Capen says:

    It is clear from tenor of the vacuous and uninformed responses that have been posted to the sea level rise article that large segments of the public continue to be in steadfast denial of what the climate models show, or what researchers have observed firsthand is already taking place around the world. As a society, we continue to be in denial about the realities of a rapidly changing climate at our peril. As Nicholas Coch, who is a professor of coastal geology at Queens College, observed in his editorial letter in the September 2nd “New York Times, “You cannot build fixed structures on a moving shoreline. The only realistic answer is to limit development in the vulnerable areas using restrictive zoning or government buyouts of threatened properties. The only alternative is for inland taxpayers to repeatedly pay for our failed coastal management policies.”

    As I said previously, “Those who cannot see the future are condemned to live it.”