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Posted on 11 October 2012 by Rob Painting

Jerry Mitrovica is a Professor of Geophysics at Harvard University in the USA. He is one of a group of scientists who have, in the last few decades, dramatically increased our understanding of sea level rise from the last ice age to present-day. As his lengthy list of peer-reviewed scientific publications will attest, he is one of the world's foremost experts on this topic.

The video below is a Washington DC talk he gave in 2011, which covers an excellent overview of sea level rise since the last ice age. In the lecture he takes contrarian talking points and uses them as teachable moments, demonstrating the following key points:

  • 2mm of sea level rise per year, which is roughly the rate during the 20th Century, was anomalous, and is something the Earth has not seen for about 10,000 years - when it was in the midst of the last ice age deglaciation.
  • The current rate of sea level rise varies from place to place, however this is to be expected due to the location and presence of land-based ice sheets, and gravitational changes brought about by the disintegration of these present-day glaciers and ice sheets.
  • The rate of sea level rise has increased from the 20th century average of 2-2.5mm per year to over 3mm per year during the record of satellite-based observations (1993-present).
  • The future sea level rise projections of the 2007 IPCC report were too low. Current sea level rates are already at the uppermost range of projections, which reinforces this view. 

For any reader interested in sea level change, it is well worth the half-hour investment of time. 


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Comments 1 to 22:

  1. So what do we conclude from this? Is global see level of less interest and what we instead should keep track of is the local sea levels?
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  2. Interestingly enough according to the presentation0 there is a positive 2nd derivative in the past two decades for sea level rise and that make me wonder what the explanation for this is?
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  3. Nice video, thanks!
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  4. "So what do we conclude from this?"

    Actually, I'm very surprised that no commenter has noticed that these observations scuttle a well-worn contrarian myth. They very broadly support the conclusions of a very well-known paleoclimatologist from Penn State University.
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  5. Global sea level is very much of interest..

    From geological records, sea level rises of meters per century (max between 3 and 10m) are established; however these were in response to considerably slower warming. My impression is that both the WAIS and southern portion of the GIS are effectively unstable right now - as in, if we stablised CO2 concentrations at current values they would probably vanish over time.

    The only real question is how fast this can happen. So far the answer seems to be 'not catastrophic', but there again that's what we thought about the sea ice last decade..
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  6. batsvensson@1: If I understand correctly you are right. Using tide gauges we need to track local sea levels, because only by tracking local changes and the reconstructing the global change from the fingerprints can you get an accurate picture of the volume of water in the oceans (which is what changes when ice melts). I presume satellites give a more direct indication of volume.

    For any future ice melt, you need to project where the ice will melt and then apply the corresponding fingerprints to gain an estimate of local sea level rise. Global sea level rise is a useful metric to give a big picture, but doesn't tell you what will happen locally.

    Rob's excellent 'Sea level Isn't Level' series goes into more detail on this.
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  7. that was a truly fascinating lecture!

    i was left wondering about the duration of the 'european problem': when the ice on greenland melts the gravitational pull diminishes, and adjacent sea levels drop... i understand that mechanism, it makes perfect sense. but, once the weight of the ice sheet on greenland disappears, wouldn't this cause greenland to rise, and won't the gravitational pull then be re-established? and would that not affect sea level in the northern hemisphere (increase)?
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  8. anja247, isostatic rebound is a slow process. Here in North America crustal rocks are still rebounding from the demise of the Laurentide Ice Sheet and its associated overburden more than 10,0000 years ago.

    Expect SLR on the decadal-to-millennial timescale with crustal rebound lasting for many, many millennia more until a new equilibria is reached.
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  9. Very interesting lecture - thanks for the link. I thought: Maybe they can use the geographical fingerprint to add useful data to our understanding of ice mass melt over time (e.g. maybe the tide gauge data would show that Greenland started to melt before the WAIS)... but wouldn't that be confounded if there was also non-uniform warming of the oceans? Given that a component of sea level rise is thermosteric (I hope that's the right word), presumably some parts of the world's oceans are warming faster than others and therefore contributing more to sea level rise in those regions. If the portion due to ice melt was not geographically uniform then that would be another complication to have to take into account, and might limit the usefulness of the 'fingerprints'.
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  10. Jerry does explain the SLR variability maily with icesheet gravitational pull. He shows the map (at 16:10) of modelled GIA melt that explain the "European problem". He mentions the isostatic rebound be also a part of that map, but does not explain the relative impact of ISR vs. gravitational pull. We know that i.e. Scandinavian Peninsula is affected by ISR.

    Further, I don't see ISA from Laurentide Sheet on Jerry's map. In particular the sinking of the East US coast is not visible at all. I would like to know at least the ballpark figure how those two effects relate: which one is larger at the current SLR rate. Anyone knows?
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  11. Rob@4,

    Yeah, let's say that: Jerry did not draw the complete picture of SL in last 10ka but suggested it would be a hockey stick shape with a blade angle 1.5-2mm/y in XX century and accellerating to 3.5mm/y in satelite era. So there is no doubt which paleo guy from PSU you are talking about.

    It's so obvious that I don't even think about it, and I'm not even sure if SLR hockey stick can be technically compared with the original from tree rings but the PSU guy deserves every piece of credit, so let's add SLR hockey stick to his "league".
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  12. the gravitational pull effect occurs likely with Antarctica too, so what does that do for sea levels in NH?
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  13. Icarus @9

    "He [Mitrovica] and others working on the same problem start with the equivalent of four "fingerprints" all laid down atop each other: they know how much sea levels have risen during the past century - or in some cases, several centuries - at various inhabited locations. From these observed changes, their work is to isolate the distinct impact - of the melting of the West Antarctic Ice Sheet; the Greenland Ice Sheet; other glacier systems including that in Alaska; and thermal expansion...In this way they are able to make an indirect observation of precisely how much volume and mass each ice sheet has lost." (Harvard Magazine)

    "Localization" effects can be determined from the difference between the video's Greenland effects map based on a uniform 1mm equivalent (of meltwater to cover the world's oceans) and another Greenland effects map with the same amount of meltwater located where the melting actually took place, based on best estimates. (Oceanography, pp32-34, Figure 4)

    Conceivably this might also be done with separate oceans. I bet Jerry Mitrovica could solve it, if he had enough grad students.
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    Moderator Response: [DB] Fixed Link.
  14. jyyh@12,

    Jerry explains that in great details starting from 25:00. Re-watch it.
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  15. AndrewDoddsUk@5,

    Would be lovely to know what ice was available to melt when we had meters-per-century rise. That's one part of the puzzle I'm unclear on; was this large chunks of the northern ice caps running off into the ocean? (that is, would the max rate be proportional to the length of the glacier faces onto the oceans?) Did sea level rise trigger some large positive feedback? Was it just plain *melt*? (wouldn't that require an extraordinary amount of heat delivered to white ice?)
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  16. dr2chase - See the USGS reference on this topic.

    Potential (m) / Location
    64.80 / East Antarctic ice sheet
    8.06 / West Antarctic ice sheet
    0.46 / Antarctic Peninsula
    6.55 / Greenland
    0.45 / All other ice caps, ice fields, and valley glaciers

    80.32m Total

    In other words, Greenland and Antarctica are the primary repositories of land ice, and the source of past multi-meter changes in sea level. I expect that Mitrovica and company will be putting out a number of papers on specific attributions based upon relative sea level change fingerprints in the near future.
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  17. KR@16, yes, that's the state of the ice now. But in the paleoclimate record, what was the state of the ice just before sea level started rising multiple meters per century?
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  18. dr2chase - As I indicated, I would expect more information as isostatic and gravitational effects are disentangled with regional sea level proxies. That should include the possibility of determining mass distributions in both glacial and interglacial periods, depending on the number and certainty of those sea level proxies.

    I am looking forward to those papers.
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  19. dr2chase - You might also research geological records of ice sheet extent; there is a lot of data available on extents, although the thickness of the ice sheets is still a point of discussion.
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  20. I largely agree with Dr Mitrovica’s analysis of the effects of ice sheet melting and gravity on sea level but it does provoke questions. I note that he leaves to the very end of his lecture, a prediction making Hansen’s prediction on sea level rise look conservative.

    Hansen warns that present average global temperature is only a few tenths of a degree below the Eamian maximum when sea level was 5-6 metres above existing levels. Metrovica’s analysis predicts sea level rise of 6-8 metres by 2100 making both Hansen’s prediction and my own (4 metres) look somewhat conservative.

    I also agree with his conclusion that both USA coasts are very likely to prove particularly vulnerable to sea level rise caused attributable to ice loss from the WAIS but my view is that GIS will also contribute to rising sea levels on the east coast of the USA and that major cities and infrastructure on and near that coast will be inundated as a result.
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  21. dr2chase, to add to KR's points, research the Laurentide, Scandinavian and British Ice Sheets, all present at the Last Glacial Maximum, along with other hefty mid-latitude ice caps in the Alps, Patagonia and elsewhere. The extent of many of these has kept geomorphologists busy for decades (early work was 19th Century), with estimates of thickness (e.g.research on nunataks), available ice volumes have become better understood in recent decades. These can be compared to the palaeo sea level record of water volume locked up in all ice sheets, as the water has few other places to go!

    So we have some pretty good knowledge of the size and shape of individual large ice sheets in past times. It was the big mid-latitude sheets that were most vulnerable to rapid change under pre-Industrial forcing, but modern forcing places the Antarctic (esp. WAIS) and Greenland into the "vulnerable" category.

    But... It's a much more complicated question to determine how fast the sheets might melt under a given forcing, as there are issues of feedbacks, dynamical responses, surface elevation changes, moisture availability amongst many other things. You can't simply look at the "coastline", at least in part because significant ice margins were on land! That's where some modelling comes in, to give insight into key processes, constrained by the geomorphological, sea level evidence and appropriate dating of each. Events like Meltwater Pulse 1A happened, but the processes driving that rapid sea level rise are not necessarily straightforward. Something like it may not happen to Greenland or Antarctica (the glaciology may not be favourable), but as yet it can't be easily ruled out.
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  22. Agnostic@20,

    I wonder where he said it, becuase that would be an extraordinary claim. In the video, he just said that previous analysis of Eonian maximum SL was 4-6m but he revised it upwards to be 6-8m on average based on his research. He did not say anything about the timeframe in the future when such average would be achieved or I have missed this important detail.
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