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Each degree of global warming might ultimately raise global sea levels by more than 2 meters

Posted on 27 July 2013 by John Hartz

The following article is a reprint of a press release posted by the Potsdam Institute for Climate Impact Research (PIK) on July 15, 2013

Greenhouse gases emitted today will cause sea level to rise for centuries to come. Each degree of global warming is likely to raise sea level by more than 2 meters in the future, a study now published in the Proceedings of the National Academy of Sciences shows. While thermal expansion of the ocean and melting mountain glaciers are the most important factors causing sea-level change today, the Greenland and Antarctic ice sheets will be the dominant contributors within the next two millennia, according to the findings. Half of that rise might come from ice-loss in Antarctica which is currently contributing less than 10 percent to global sea-level rise.

Photo of sea shore at dusk

“CO2, once emitted by burning fossil fuels, stays an awful long time in the atmosphere,” says Anders Levermann, lead author of the study and research domain co-chair at the Potsdam Institute for Climate Impact Research. “Consequently, the warming it causes also persists.” The oceans and ice sheets are slow in responding, simply because of their enormous mass, which is why observed sea-level rise is now measured in millimeters per year. “The problem is: once heated out of balance, they simply don’t stop,” says Levermann. “We’re confident that our estimate is robust because of the combination of physics and data that we use.”

The study is the first to combine evidence from early Earth’s climate history with comprehensive computer simulations using physical models of all four major contributors to long-term global sea-level rise. During the 20th century, sea level rose by about 0.2 meters, and it is projected to rise by significantly less than two meters by 2100 even for the strongest scenarios considered. At the same time, past climate records, which average sea-level and temperature changes over a long time, suggest much higher sea levels during periods of Earth history that were warmer than present.

For the study now published, the international team of scientists used data from sediments from the bottom of the sea and ancient raised shorelines found on various coastlines around the world. All the models are based on fundamental physical laws. “The Antarctic computer simulations were able to simulate the past five million years of ice history, and the other two ice models were directly calibrated against observational data – which in combination makes the scientists confident that these models are correctly estimating the future evolution of long-term sea-level rise,” says Peter Clark, a paleo-climatologist at Oregon State University and co-author on the study. While it remains a challenge to simulate rapid ice-loss from Greenland and Antarctica, the models are able to capture ice loss that occurs on long time scales where a lot of the small rapid motion averages out.

If global mean temperature rises by 4 degrees compared to pre-industrial times, which in a business-as-usual scenario is projected to happen within less than a century, the Antarctic ice sheet will contribute about 50 percent of sea-level rise over the next two millennia. Greenland will add another 25 percent to the total sea-level rise, while the thermal expansion of the oceans’ water, currently the largest component of sea-level rise, will contribute about 20 percent, and the contribution from mountain glaciers will decline to less than 5 percent, mostly because many of them will shrink to a minimum. 

“Continuous sea-level rise is something we cannot avoid unless global temperatures go down again,” concludes Levermann. “Thus we can be absolutely certain that we need to adapt. Sea-level rise might be slow on time scales on which we elect governments, but it is inevitable and therefore highly relevant for almost everything we build along our coastlines, for many generations to come.”

Subject Article:

Levermann, A., Clark, P., Marzeion, B., Milne, G., Pollard, D., Radic, V., Robinson, A. (2013): The multimillennial sea-level commitment of global warming. Proceedings of the National Academy of Sciences (early online edition) [DOI: 10.1073/pnas.1219414110 ]

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Comments 1 to 10:

  1. Considering temps and sea level by the Pliocene, I'd say in the long run it's significantly more than 2 m per degree.

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  2. Indeed. The idea that it takes 30 degrees of warming for the 60m of Antarctic ice to melt is too absurd to take serious. So, lets take a look at what the paper really says. In the abstract it is stated:

    we are committed to a sea-level rise of approximately 2.3 m °C−1within the next 2,000 y

    I'd guess the melting continues after 2000y. Perhaps, if you´d consider the first 4000y it would be 4m °C−1. So it seems to me the 2.3m number is rather arbitrary, dependent on how long you define ´ultimately´.

    Although, In defense of the paper, they did call it ´multimillennnial´ sea level rise, and 2000y is the shortest timeframe that can be called so.

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  3. The Potsdam press release is full of generalities and too imprecise for critical review but it may be worth noting that it refers to global warming – average or surface is unstated – rather than Arctic amplification combined with increasing penetration of warm ocean currents. Both have an effect on the rate of mass loss from ice sheets, the degradation of permafrost and, in the Arctic, the release of carbon from onshore and continental shelf deposits. Obviously the latter will increase temperature amplification, speeding up the loss of land based ice in the Arctic.

    That effect is not likely to occur in the Antarctic where methane deposits lie beneath massive ice sheets and lower temperatures are maintained by stratospheric ozone depletion and insulating circumpolar winds. Even time scales referred to, “several millennia”, are too broad to be meaningful or permit comment on the finding that an increase of 1°C in global temperature = 1 metre sea level rise. That broadly accords with the findings of others, as does an estimated 2100 average global surface temperature 4°C-6°C above the preindustrial, assuming business as usual. However, the admonition that we must prepare to “adapt” is nonsense.

    Average global temperature is very likely to exceed 2°C by 2050 and at least twice that by 2100, even if we make strenuous efforts (very unlikely) to reduce CO2 emissions to zero by 2080, as currently proposed. If the findings of Levermann et al are largely correct (a big if), try “adapting” to 4 metres sea level rise and extreme wind events causing tide surges which take actual sea level rise to 6 metres or more.

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  4. If each degree of global warming is likely to raise sea level by more than 2 metres in the future, why did the 0.75º C global temperature rise of the last 100 years only cause 21 centimetres of sea level rise, less than one tenth of 2 metres?

    And why do so many people mention "Business as usual," when no such thing has ever existed in the history of our ever changing human society?

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  5. Earthling, the question asked in your first paragraph is essentially answered in the second paragraph of the article (the use of the word "ultimately" in the title of the post should also be a hint).  The oceans have a massive thermal inertia, which means that it takes a long time for the oceans to warm sufficiently to come back into equilibrium with the surface.  Thus the full sea level rise due to thermal expansion of the oceans will only been fully evidient after a delay of many decades.

    The second of your paragraphs is just pedantry.  "business as usual" is just the terminology used for the course of progress where no real attempt is made to curb fossil fuel use and instead exploit fossil fuels in the interests of rapid economic growth.  "business as usual" seems as good a name for that as any.

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  6. @Earthling #4 Because it wasn't a "global temperature rise", was global temperature. Takes 3.8 ZettaJoules to heat all land by 0.75º C, 4.1 Zj for all air, 110 Zj for all freshwater and 4,400 Zj for the oceans. The oceans are holding back the surface temperature by absorbing 250 Zj to date and taking it down into the deeps by currents. Their average temperature is 3.1 degrees, they'll keep taking heat down until they've added same temperature amount as the surface, this takes thousands of years. Needs 11,000 Zj to melt all ice on Earth. 100 years is the blink of an eye, this thing is an ocean liner with no brakes.  

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  7. @Me #6 typo: "global temperature" should be "global surface temperature".  

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  8. 2.3 meter rise per degree of warming is a ficticious number pulled out of a hat and now declared as Fact. There is no Evidence, Data, Facts, Past History to support that conclusion.

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    Moderator Response:

    [DB] Please refer to this site's Comments Policy.  Comments constructed such as this one of yours fall under the heading of Sloganeering, as scaddenp helpfully notes.  Please comport all future comments to comply with the Comments Policy.

    Thanks in advance for your compliance; have a nice day.

  9. ajkuiper55- I assume you followed the link to the paper that this article discusses? Can you please explain how the evidence, data, facts and past history presented in this paper doesn't exist?

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  10. ajkuiper - perhaps you should read the paper on which the article was based. The input and uncertainities are discussed both there and in the referenced papers. The number is good agreement with estimates from historical data (eg and in particular this referenced by the paper). 

    "Declared as Fact" is simply sloganeering on your part. It is simply the best estimate from science so far. If you wish to object to the conclusion, it would be better to discuss the actual paper and what difficulties you might have their methodology, assumptions or way of reaching a conclusion.

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