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Sea Level Experts Concerned About ‘High-End’ Scenarios

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A survey of nearly 100 experts on sea level rise reveals that scientists think there is a good chance the global average sea level rise can be limited to less than 3.3 feet by 2100 if stringent reductions in planet-warming greenhouse gases are rapidly instituted. However, the survey, which is the largest such study of the views of the most active sea level researchers ever conducted, found that if manmade global warming were to be on the high end of the scale — 8°F by 2100 — the global average sea level is likely to jump by between 2.3 and 3.9 feet by the end of this century.

Worse yet, such a temperature increase could boost sea levels by up to 9.9 feet by 2300, the study found. Such a drastic increase in sea level would not just put heavily populated coastal cities at risk of flooding, but could also jeopardize the existence of low-lying island nations, the study found. 

Projections of global mean sea level rise over the 21st century relative to 1986–2005 from the combination of the computer models with process-based models, for greenhouse gas concentration scenarios. The assessed likely range is shown as a shaded band.
Click image to enlarge. Credit: IPCC Working Group I.

The study, led by Ben Horton of Rutgers University and published in Quaternary Science Reviews, solicited sea level rise projections from the most active researchers in the field, scientists who had published at least six papers on the subject during the previous 5 years. Since computer modeling approaches and other methods have yielded a wide range of projections, a poll of expert opinions provided Horton and his colleagues a different way of estimating the odds of particular sea level rise scenarios.

The study’s projections overlap, but are generally higher than, the most recent report from the U.N. Intergovernmental Panel on Climate Change (IPCC), which raised its sea level rise projections by about 60 percent between its 2007 report and the assessment released in September.

Horton said there is broad agreement among sea level rise experts that if emissions are significantly curtailed beginning in the next few years, the amount of sea level rise could be limited. “If we can have mitigation measures to reduce GHG emissions we can keep sea level below a meter which is a rate that the vast majority of our coastlines can withstand,” Horton said in an interview.

However, the study found that scientists are especially concerned about the consequences of high-end warming scenarios.

“You get a group of scientists together and you find that they’re projecting a lot higher level of sea level rise,” Horton said in an interview. ”... The ranges now overlap with the IPCC, but they’re higher.”

For example, 13 experts who responded to the survey estimated a 17 percent chance that sea level rise would exceed 6.6 feet by 2100, which would have potentially catastrophic consequences for coastal cities like Manila in the Philippines, and U.S. cities such as Miami, New York, and New Orleans.

“While the results for the scenario with climate mitigation suggest a good chance of limiting future sea-level rise to one meter, the high emissions scenario would threaten the future survival of some coastal cities and low-lying islands”, said co-author Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research.

This type of study peels back the curtain on sea level specialists, allowing the public to get a better idea of their views and giving policy makers the chance to assess various risk management approaches, the study said.

Sea level rise exacerbates storm surge-related flooding.
Credit: NOAA.

The IPCC projected that global mean sea level rise for 2081-2100 will likely be in the range of 10.2 to 32 inches, depending on greenhouse gas emissions. However, the report notes, as other studies have found, that local amounts of sea level rise could be much higher in some coastal areas. The scenario with the highest amounts of greenhouse gases in the atmosphere shows a mean sea level rise range between 21 and 38.2 inches, which would be devastating for many highly populated coastal cities at or near current sea levels.

During the 1901-2010 period, the report said, global averaged sea level rise was 0.07 inches per year, which accelerated to .13 inches per year between 1993 and 2010.

The IPCC’s four scenarios of the amount of greenhouse gases in the atmosphere through 2100 all show faster rates of sea level rise compared to that observed during 1971-2010, the report said.

The sea level rise scenarios in the recent report were considerably higher than those in the 2007 report, when the IPCC projected a global mean sea level rise of just 7.1 to 23.2 inches by 2100, and did not take into account the probable influence of the melting Greenland ice sheet and parts of Antarctica.

Because long-lived greenhouse gases like carbon dioxide (CO2) remains in the atmosphere at least for several hundred years, and because the oceans and ice sheets respond relatively slowly to global warming, multiple studies have pointed out that sea level rise will continue long after the amount of greenhouse gases in the air have stabilized.

This study also showed that some sea level rise experts are concerned about the possibility of extremely high long-term increases in sea level, on the order of up to 46 feet by the year 2300. Such massive fluxes in global mean sea level have been documented in Earth's history, but never when so many people were living along the shoreline. For example, the last time that the Earth had as much CO2 in the air as it does now was before humans existed, when global sea level was up to 100 feet higher than they are today.

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By Dave (Basking Ridge, NJ 07920)
on November 22nd, 2013

This way of estimation (direct poling of specialists via an independent study) is appealing in that it has the advantage that it more likely eliminates dilution-by-committee aspects that by human nature alone invariably encroaches upon institutional estimates by such as the IPCC.

The straightforward statement that there is a 17% chance of more than 2m (6.6feet) of global mean sea level rise by 2100 is unsurprising and has a ring of authenticity.

2m by 2100 has been the upper limit of the likely range distilled from the literature, often reported as 0.8 – 2m for some time now. In view of that a 17% estimated chance of SLR greater than 2m does not seem out of line or such an extraordinary revelation. Yet I think that it is still likely to be played down or ignored by the broader media which in the US instead seems to prefer precise simple middle of the road single numbers – always in feet and inches - for SLR as if it’s some kind of politically moderate position between extremes. The reality is that there is just so much net uncertainty convolved from uncertainties in future emissions, warming rates and the response of the cryosphere that the only projection for SLR certainly by 2100 (and beyond) that makes any logical sense IS a range.

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By Trucker Mark (Broomfield)
on November 23rd, 2013

The June, 2013 climate change study from the International Energy Agency forecasts a planetary temperature rise of between 3.6 and 5.3 degrees Celsius (or 6.5 and 9.5 degrees Fahrenheit) by 2100 if we can’t rapidly reduce climate emissions within 60 years, due to the 30-year lag in climate emissions fully circulating through the atmosphere. 

In addition, new calculations out of the Arctic on melting permafrost and melting methyl hydrates, not included in the recent IPCC study, raise fears that warming could be worse due to recent record Arctic methane emissions, as the recent IPCC study revised the negative effect of methane emissions higher than previously thought. 

If we take the average forecast from the IEA and throw-in a 25% rise due to record methane emissions in the Arctic, we are looking at a global temperature rise of 10 degrees Fahrenheit by 2100 if we can’t greatly reduce climate emissions, which would come to a rise of 5.6 degrees Celsius.

Other recent researchers have already forecast a sea-level rise of between 4 and 6 meters if IEA’s forecast is correct.  If we throw surging Arctic methane releases on top of the findings made by IEA, a sea-level rise of 5-7 meters by 2100 is possible in a worst-case scenario involving a failure to rapidly reduce climate emissions before 2065-70.

“Annex 1 [industrialised] countries need a 70% reduction in emissions consumption in 10 years to give us an outside chance of holding temperatures to a 2C rise. They need to cut emissions by 10% annually. We need to be fully de-carbonised in the 2020-30s, and that means planes, fridges, everything [must emit far less] to give a bit of an opportunity for poorer parts of the world to develop.[end quote]

Anyone here see as possible a 70% cut in climate emissions out of all first-world countries by 2023?

Not a chance, and as such, it doesn’t look too good for the value of oceanfront or low-lying real estate either!

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By Eric Peterson (Front Royal, VA 22630)
on November 24th, 2013

“During the 1901-2010 period, the report said, global averaged sea level rise was 0.07 inches per year, which accelerated to .13 inches per year between 1993 and 2010.”

There was a 20 year period ending in 1950 with a rise of 0.13 inches per year.  Therefore the current rise is not an “acceleration” but a “fluctuation” just like prior fluctuations.  The relevant graph:,_1870-2008_(US_EPA).png shows there is no acceleration over the long run.

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By Michael Howell (Melbourne village)
on November 24th, 2013

In all advanced science degrees you are expected to have taken calculus.  The major ad vantage of this is you should be to know if you keep increasing by a certain percentage by 10% per given amount of time you begin to approach infinity as you continue to advance.  Thisw results in catastrophic collapse.

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By Michael Howell (Melbourne village)
on November 24th, 2013

If you increase the multplier CO2 You increase the acceleration.

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By Dave (Basking Ridge, NJ 07920)
on November 24th, 2013

Given even the most optimistic take on the Warsaw talks that ended yesterday, the international inertia over doing anything significant to curb the growth in global emissions continues to be evidenced by the fact of rapidly increasing global emissions. Increasing concern about much greater amounts of sea level rise due to the increasing likelihood of the high end emissions scenarios actually playing out is therefore unsurprising. In view of that a 17% estimated chance of SLR greater than 2m (6.6 feet) does not seem out of line or such an extraordinary revelation. I think it is very appropriate to conduct such a study in that manner and report the results to serve as an impromptu and well qualified adjunct to existing SLR projections. This is clearly a moving target.


The satellite altimetry data depicts a clear monotonic trend in global mean SLR over the past 20 years. These data also provide our most accurate global mean SLR data compared with tidal gauge data which of course are instead coastal and relative to local land height and have to be corrected and correlated to provide an estimated global mean value. The current 20 year trend from satellite data could of course also be legitimately compared with a similarly accurate dataset over a similar but different period. However, unfortunately that dataset does not exist because satellites were not around. All data prior to 1993 is from tidal gauges. Therefore comparison with a longer period of the far less accurate (for global mean estimation purposes) tidal gauge data – such as over about a century – is the next best reasonable thing to do. For this reason it is technically legitimate to declare that the current rate of SLR is significantly greater than that measured over the past century or so. There is also nothing that so far indicates that the current 20 year trend in the rate of global mean sea level rise as determined via satellite altimetry is a an asystematic fluctuation as opposed to a systematic inflection.

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By Eric Peterson (Front Royal, VA 22630)
on November 27th, 2013

Dave said “There is also nothing that so far indicates that the current 20 year trend in the rate of global mean sea level rise as determined via satellite altimetry is a an asystematic fluctuation as opposed to a systematic inflection.”

There is nothing to indicate inflection either.  Also satellite altimetry is corrected for net rises in coastal land whereas tidal gauges show the actual sea level change with its real world effects.  Some areas in the Pacific ocean vary as much as a few feet with ENSO which puts the inch per decade rise in perspective.

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