Sea level rise is a slow process. Other consequences of climate change are generally felt much sooner. But there is something odd about the forecasts. They seem to be catching up with us, bringing a distant future closer to our doorsteps.

In part 2 of our special short series about global sea level rise we thought we’d try to create an overview of leading reports and studies published in the last ten years – all of them offering a different forecast for the extent of sea level rise by 2100:

The above scatter plot compares sea level rise forecasts of ten different reports and studies, plotted for the year they were published (between 2006 and 2016). Red values are ‘high end forecasts’ (which correspond to either high emissions/high warming scenarios or to high ice sheet dynamics scenarios), whereas blue values are ‘low end values’ corresponding to opposite scenarios. [Important to note a slight apple/orange element – for instance the fact that one publication (IPCC AR4) uses 2090-2099 as target (instead of 2100), and that different publications use different uncertainty margins (for instance 90% for KNMI’06, 95% for IPCC AR5 & KNMI’14 and ‘high(er) risk’ for Dutch Delta Committee). Also note the difference between metastudies & reports versus individual studies (Science, Nature, Atmospheric Chemistry and Physics).] Graph made by Stephan Okhuijsen (Datagraver.com) for Bitsofscience.org.

Sea level rise forecasts [low end – high end] for 2100, in chronological order: KNMI’06, 2006 (Dutch/NE Atlantic coast): [35 – 85cm]; IPCC AR4, 2007: [18 – 59cm]; Rahmstorf, 2007: [50 – 140cm]; Dutch Delta Committee, 2008: [… – 120cm]; Pfeffer, 2008: [80 – 200cm]; US National Research Council, 2010: [55 – 200cm]; IPCC AR5, 2013: [44 (28-61) – 74 (52-98)cm]; KNMI’14, 2014 (Dutch/NE Atlantic coast): [… – 100 cm]; KNMI update following DeConto, 2016: [… – ±200cm]; Hansen, 2016: [200 – 500cm]. Full references at bottom of article.

Short introduction to 2100 sea level forecasts – and this article:

In this second piece of our sea level rise series we would first like to take a moment to explain why we think it’s silly the (political) world seems to think the future ends in the year 2100 [although of course if you base all your policies on that assumption, it may indeed become a self-fulfilling prophecy].

Having made that disclaimer we’ll have a look at established sea level rise forecasts for the year 2100 – to note that these forecasts are increasing over the last decade, as slowly scientific progress (most importantly improving understanding of ice sheet dynamics & feedbacks) seems to indicate that even within our current century we should no longer be counting in mere decimetres but perhaps in (full) metres instead. (Let’s hope there’s not causal connection – if you’d extrapolate the trend line in our scatter plot you would not want to hear the sea level rise forecasts we’d make by the end of the century… [Or if you do, just for the fun of it (to illustrate the speed at which forecasts are rising), we also drew that line.])

And bit of additional background about sea level rise

Sea level rise is a very slow process. The greenhouse gases we emit today lead to a disturbance in the energy balance of the atmosphere and the oceans – and a warmer planet for centuries, at least. Sea level rise, being a consequence of this warming, lags behind even further, creating its own form of climate inertia (caused by the depths of the oceans –see our piece about thermal expansion sea level rise we added two days ago– and the size and weight of large ice sheets, although the speed at which these can melt is one of the largest uncertainties in climate science).

It’s safe to say that the greenhouse gases that we emit on a decennium scale (for instance a doubling of fossil-fuel CO2 emissions over the 40-year time frame between 1970 and 2010) create a sea level rise that will continue for millennia – and one that is largely irreversible once that it’s set in motion.

‘Somewhere’ after the year 2100 this sea level rise is set to gravely escalate – as paleoclimate comparisons indicate that even if we manage to limit the warming to for instance 2 degrees Celsius (bare in mind that nations’ current Paris 2025/2030 emission targets bring us en route to 3+ degrees – and natural climate-carbon feedbacks (biosphere CO2, Arctic & ocean methane) may make things far worse!) the new equilibrium state will be a world with a sea level 10+, 20+, possibly 30+ metres higher than the one of today – indeed wiping entire nations from the map, and many coastal megacities across the globe.

Yet oddly, when it comes to sea level rise assessments for climate policy the period beyond the year 2100 is often largely ignored. We think this is no base for climate policy and will therefore add a special article about the extent of ‘beyond 2100’ sea level rise as part of this series. Today though we’ll play by the rules of established climate politics – and just look at the state of the Earth’s oceans and coasts at the end of this century:

Comparing sea level publications: ‘average forecast’ doubled in ten years, uncertainty range from 18 to 500 centimetres(!)

Now if you want to make sense (by reading the scientific literature) of sea level rise forecasts for the year 2100 you can only be confused. Judging from the second-last IPCC report (2007) it could be as low as 18 centimetres. Judging by the newest peer-reviewed research (2016) it could be twenty-eight times as high – multiple metres. We’ve tried to create an overview in the chatter plot at the top of this article, citing ten influential studies and reports that were all published in the last decade (2006-2016).

The big difference in studies that show a relatively low outcome for 2100 sea level rise and those that show rapid sea level rise is in assumed values for ice sheet dynamics. Studies that (try to) incorporate calving and other melting & disintegrating feedbacks show much more rapid melting and therefore sea level rise – especially a much larger contribution from Antarctic ice masses.

How to handle a transitional phase in the science around sea level rise? Precaution would be good(!)

We conclude ‘the science’ around this specific subject is in a transitional phase. It will be very interesting to wait for a bit to see how the ‘consensus publications’ (IPCC) and other metastudy reports pick up on this – most notably the upcoming special IPCC report on sea level change, that is scheduled for publication in 2018.

Meanwhile – if you’re a politician – you should (by now) realise there is clearly a growing pile of evidence that shows it’s becoming fully irresponsible to base policies (both mitigation and adaption policies, coastal management to name one) on the conservative sea level rise estimates of a decade ago.

We’d like to note that even established climate research institutes like the Royal Netherlands Meteorological Institute (KNMI) now speak of ‘disastrous’ sea level rise by/from 2100 (in an official statement reporting on new insights that are currently shared within the scientific community (DeConto, Nature), while adding ‘another metre’ compared to their own 2100 forecast published just two years earlier).

And with an abundance of new studies indicating possible accelerated ice sheet melting it is also becoming more and more likely that the future IPCC reports will also acknowledge the possibility of much more rapid sea level rise within this century. Consider that our own little forecast. (But if risk management is your political responsibility, waiting (till the IPCC update of 2018) is clearly your worst option…)

References:

© Rolf Schuttenhelm | www.bitsofscience.org