Past, current and future sea level rise

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What’s there to worry about sea level rise; it’s going very slowly, right? Let’s put current sea level rise in a historical perspective.

Here’s a graph of sea level since the last ice age. As the ice from the last ice age was melting, sea levels rose by some 120 metres over the course of about 8000 years, before it flattened out ~6000 years ago. On the top right, I drew a black line with an approximate slope of 3 mm/year, which is the current rate of sea level rise (over the past 20 years or so). This is much faster than the relatively stable sea level during the ~6000 years before, though not as fast as the sea level rise at the end of the last ice age.

Let’s zoom in on the last 9000 years (covering most of the Holocene epoch). The strong sea level rise at the end of the last ice age is still visible on the left hand side, slowing down 7000 years ago and even more so 4000 years ago. Until recently: Current sea level rise represents a clear increase. For the future, most recent estimates of sea level rise fall between 0.5 and 1.5 metres in 2100. It won’t stop thereafter, since there’s a lot of inertia involved in warming up the oceans and in melting (parts of) the large ice sheets (Greenland and Antarctica).

 

Under a business as usual scenario sea level will likely rise by multiple metres over the course of the next few centuries (see e.g. the Dutch DeltaCommittee and German WBGU reports). The vertical red line attempts to give an indication of these long term projections (which are uncomfortably uncertain). With increased warming, sea level rise will likely also increase, so the black line denoting 3 mm/yr can be seen as a lower bound (until the temperature goes down again and the climate system re-equilibrates). An upper bound is harder to predict, because that depends e.g. on tipping points being surpassed, such as parts of the great ice sheets becoming unstable. The fact that in the past sea level has risen with multiple metres per century means that such rates are physically possible. Whether that will happen again in the next millennium depends partly on nature’s forces (uncertainty in the physics, i.e. the response of ice sheets) and partly on what we do to poke nature around (uncertainty in our future emissions).

Quoting from a  recent book “Climate Change: Global Risks, Challenges and Decisions” (eds. Richardson, Steffen and Liverman), the chapter on sea-level rise and ice-sheet dynamics:

The period of relative stability of sea level over the past 6000–7000 years (Harvey and Goodwin, 2004 ) has now ended, and sea level is undoubtedly rising in the post-industrial period (IPCC, 2007a ). Given the massive heat capacity of the ocean, the Earth is already committed to many more centuries of sea-level rise due to thermal expansion alone. The dynamics of the large polar ice sheets and the rapid retreat of glaciers and ice caps will significantly add to the magnitude of sea-level rise. The critical questions are: how much and how fast?

(…) 

Several lines of evidence point towards a sea-level rise by 2100 of perhaps a metre higher than 1990, which is at the upper limit of the IPCC ( 2007a ) range of projections. First, the mid-range of projections of semi-empirical models is centred around 1 m (Rahmstorf, 2007; Grinsted et al ., 2009 ). Second, observed sea-level rise is currently tracking at or near the upper limit of the IPCC projections (Rahmstorf et al ., 2007 ; Domingues et al ., 2008 ). Third, recent observations show an increasing rate of mass loss from both the Greenland and West Antarctic ice sheets (Cazenave, 2006 ; Rignot and Kanagaratnam, 2006 ). Finally, an analysis of the kinematic constraints on dynamical ice loss suggests a plausible increase in sea level of 0.8 m by 2100 (Pfeffer et al ., 2008 ).

Nevertheless, considerable uncertainty surrounds many of the processes that contribute to sea-level rise, especially concerning the dynamics of the large polar ice sheets. All of these uncertainties point in the same direction – towards more rapid and severe sea-level rise. Thus, rises higher than 1 m by 2100 cannot be ruled out.

(Graphs based on Robert Rodhe’s Global Warming Art)

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25 Responses to “Past, current and future sea level rise”

  1. Sou Says:

    Thanks, Bart. It’s good to see this presented clearly and in historical perspective. I remember visiting a church in Germany that was built over a period of 300 years. We don’t do much long term planning. Some businesses plan for 20 or 30 years ahead, governments plan mostly for 3 to 5 years ahead (up to 20 years or so for some projects).

    Given that it seems increasingly less likely that we’ll prevent sea level rises of magnitude, maybe it’s time for the world to do some longer term planning and start thinking about how to accommodate emigrants from countries in your part of the world, parts of Asia eg Bangladesh and Pakistan, coastal USA and Australia, Egypt, island countries and other nations from where large populations will need to move so they can remain on solid ground.

  2. Bart Says:

    You have a spare room? ;-)

  3. NevenA Says:

    I discussed this with some Dutch skeptics a while back, based on an interview with NZCSC founder Gerrit van der Lingen.

  4. Sou Says:

    Lol – I’ll be six foot under by 2100, but yes, you’re very welcome to stay in the spare room – umm… how many are you? :)

    The state where I live (Victoria) is 1/4 or more under water at the moment, but the current trend is hotter and drier, with very high temps, drought and summer fires becoming the the norm. I’m not sure that we’ll have enough potable water to support lots more people.

    I see parts of the UK and Europe are getting their feet wet now. We’re heading for interesting times.

  5. Dan Olner Says:

    “Given the massive heat capacity of the ocean, the Earth is already committed to many more centuries of sea-level rise due to thermal expansion alone.”

    I was a bit confused about what this meant, could you help? Are they saying that the existing heat stored in the ocean takes time to translate into thermal expansion? I had it in my head that heat would immediately = the exact amount of thermal expansion. Err… but that’s not right, is it? So are they saying the heat translates over time into lower pressure water and thus higher volume? If so, what’s the timescale on that for a given heating? p.s. I haven’t even googled, I’m just asking you, so feel free to tell me to go google it!

  6. Bart Says:

    Dan, from IPCC TAR:

    The long time-scale (of the order of 1,000 years) on which thermal expansion approaches its eventual level is characteristic of the weak diffusion and slow circulation processes that transport heat to the deep ocean.

  7. Dan Olner Says:

    1000 YEARS!? Jesus. Thanks for that. Wow, future generations are going to be really grumpy at us, aren’t they?

  8. isaacschumann Says:

    Thanks Bart, great post!

    I am of the understanding that sea level varies from place to place by, at times, tens of meters based on temperature differences as well as gravitational pull of continents. I would assume this to mean that individual regions could experience much higher (and some lower) sea level rise than the averages discussed above. Given estimates of average future rise, what is a theoretical maximum range that an individual area could experience, or is it too uncertain to say?

    Sou,

    That’s terrible to hear, so sorry. You can come live in Indiana (midwest U.S.) where I live, I think were expected to fare quite well (I hope). We have plenty of space and are conveniently located next a disproportionate share of the worlds fresh water in the form of the great lakes, just don’t expect much in the way of topography, it’s quite boring;)

  9. J Bowers Says:

    @ isaacschumann

    Some interesting work by Jerry Mitrovica:

    http://harvardmagazine.com/2010/05/gravity-of-glacial-melt

    http://www.theglobeandmail.com/news/technology/science/article9752.ece

  10. isaacschumann Says:

    J Bowers,

    Thanks! this was exactly what I was looking for.

  11. Sou Says:

    @isaacschumann and @J Bowers – thanks for the link to sea level variations – about to check it out.

    Isaac, I’ve cheated and live at the start of a valley that feeds into the headwaters of the Murray River. Though we get more drought and higher temps and a lot bigger fires than we used to get, we also have the mountains that catch some of the water coming across, so the micro-climate at this end of the valley is more benign than most places and the water, when it falls, is clean and soft (though much more intense so we’ve had to cut holes in the guttering to keep the rain out of the house!). And it’s very picturesque.

    Today is glorious and mild for summer, a sunny 30 degrees C, 40% humidity and a gentle breeze. Perfect :)

  12. Eli Rabett Says:

    It is interesting that the sea levels have remained steady for so long. As Eli understands it, we should be on the downswing due to the Milankovitch cycles, so perhaps this points to Ruddiman’s position of early human effects? Interesting to think about.

  13. Bart Says:

    Eli,

    Not sure about that. Sea level has risen very slowly over the past 4000 years (~0.7 mm/yr according to this ppt by Anderson and Wallace, see slide 16), before it picked up speed in the 20th century. I guess the reason is the same as what I wrote in my previous comment: The long timescales involved with temperature equilibration of the deep ocean, which are still responding to the warming leading up the interglacial max, and they haven’t noticed under there yet that this max has already passed and that the glacial cycle has entered a cooling phase. But I’m merely speculating here. It’s an interesting question to ponder.

  14. isaacschumann Says:

    Sou,

    That does sound glorious, right now its winter and -5C here. That AGW must be a hoax!;)

  15. cassandraclub Says:

    since there’s a lot of inertia involved in warming up the oceans

    I wonder where you got that idea. Only direct sunlight can warm the oceans and the oceans dissipate that heat through evaporation. On earth energy from the oceans to the atmosphere and not the other way around.

    Why don’t you add the Jason-satellite sealevel data in this post?
    http://cassandraclub.files.wordpress.com/2011/01/sl_noib_ns_global.jpg

  16. Bart Says:

    “there’s a lot of inertia involved in warming up the oceans”
    I wonder where you got that idea.

    Heat capacity of water; plenty of literature as well as assessments of such.

    Sea level has increased by a bit over 3 mm/year indeed, as shown on the plot you linked to, and as I’ve mentioned in my post. Thanks for confirming.

  17. PJB Says:

    During the 6,000 years of “stasis” with that 0.7mm average rise in sea level, what were the rates of rise (and fall) of corresponding 100 year periods to the present acceleration of 3.0mm?
    Is it not possible that the current variation is part of the normal fluctuations of the vagaries of planetary climate during an inter-glacial?

  18. Bart Says:

    PJB,
    Everything is possible, but not everything is equally likely. If you put the whole picture together (increasing air and ocean temperature leading to thermal expansion of seawater and thus sea level rise. decreased melting of ice leading to increased sea level, etc), increased greenhouse effect (i.e. more energy kept within the earth system because of increased GHG concentrations) by far the most likely explanation is that sea level rise is a consequence of anthropogenic climate change.

  19. PJB Says:

    Thanks. In looking at that long term temperature graph as well as (or perhaps in comparison to the Vostok graphs for the last several interglacials) it seems that the current inter-glacial has “over-stayed its welcome” in terms of the duration of the warmth before the fall, as it were.

    As far as the GHG go, certainly an important part of the equation notwithstanding the rather contested effect of the main one (water vapor) regarding climate system response.

    In such a case, transparency and accuracy as opposed to subterfuge and obfuscation are, for me, the deciding factor at present. I was a confirmed CAGW advocate until I saw the Vostok T/[CO2] correlation re: timing. At present, the more I see of the advocacy and agendizing from the Warmist community, the less I like it. Those few individuals have converted me, for the time being, but as a scientist, I am always open to new information and analyses.

  20. Bart Says:

    PJB,

    The time lag between temperature rise and CO2 increase during a transition from glacial to interglacial was supposedly predicted by Hansen before it was observed. Temp and CO2 influence each other both ways: 1) CO2 is a greenhouse gas, so it causes warming 2) ocean take up of CO2 is temperature dependent.

    At the end of the ice ages, temperature rose first, followed by CO2 ~800 years later (and thus CO2 is an effect of temperature rise rather than a cause.) However, both temperature and CO2 continued to rise for another ~4000 years. Thus, the initial temperature increase at the end of an ice age is not caused by CO2, but CO2 did contribute to the remaining temperature increase in the final ~4000 years. It acted as a positive feedback, similar as water vapor. Temperature and CO2 influence each other in both directions; it is a bit of a chicken-egg discussion. We cannot explain the temperature observations [difference between glacial and interglacial] without [the warming effect from] CO2.

    However, the current situation is clearly different from that at the end of the ice ages, since now we know that the extra CO2 is brought into the atmosphere by human activity. Currently, CO2 is not increasing in response to the warming, but rather due to human emissions, and as such it is now one of the driving forces of the warming. The very different relation (and timescales) between temperature and CO2 now as compared to ice ages shows that the situations are indeed different.

  21. Robert In New Orleans Says:

    Bart,

    How much do you believe will the sea level rise in the next 10-20 years. I would like to hear your “scientific answer” and what your “gut instinct” tells you?

  22. Bart Says:

    Robert,

    Looking at most recent projections of SLR, its rate will slowly increase over the next few decades, i.e. my hunch is between 3 and 5 cm/decade. It’s in the second half of this century where different projections start to really diverge, i.e. that’s where the projections become increasingly uncertain (due to uncertainty in the ice sheet contribution that pushes the upper limit up).

    David Vaughan gives some “personal projections of global SLR” that make a lot of sense to me:

    Coming decades
    • Current rate plus a bit
    • 4-6 mm per year [I would put the lower limit at 3, see above]
    Next 90 years
    • Uncertainty is high (with reason)
    • Low-end = current rate
    • High-end = plausible contribution from Antarctica and Greenland
    • 40 – 140 cm
    Next 190 years
    • Uncertainty is much higher
    • Low-end = current rate
    • High-end = rate similar to MWP1A
    • 80 – 400 cm

  23. Accelerated Sea Level Rise as Global Warming Accelerates! (j/k) | Not all alleged is apparent… Says:

    [...] “appears to have“, as all the past 132 years can be compared to are proxy data (like these for the past 9,000 to 24,000 years, or these for the past 2,000 years), as no global direct [...]

  24. Miriam English Says:

    Nice, informative page. Thanks Bart.

    What worries me is our tendency to project linearly onto changes that are non-linear, thereby badly underestimating them.

    Here is an example of what I mean: The ice on Greenland is melting at an accelerating rate which means that most of the melt will be dumped into the north Atlantic in a rush near the end. All that cold fresh water doesn’t just raise sea levels. It also carries the danger of destabilising the currents that bring warm water to the north Atlantic. If they stop (as they are known to have done before in the deep past) then a lot of the northern hemisphere (and probably the south too) might experience rapid freeze-over, possibly putting Europe and much of North America under a kilometer or more of ice within mere decades of the sudden melt.

    Linear processes are easy to plan for, but what we are seeing is non-linear. Will the heating simply continue to accelerate, feeding more storms and drowning our coastlines at a quickening rate? Or will it switch-back into something far more dangerous? Nobody really knows. This is what makes it so worrying.

  25. Sea Level Rise. Complicated. « The Septic Sceptic Says:

    [...] above the background rise due to the current interglacial (if they are alarmists) will point to the 3mm rise per year over the last 20 years which if extrapolated gives a somewhat alarming trend – how satisfying for them. On the other [...]

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