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Thanks for the response and the link to Pfeffer (2011); very nice overview. To me as a lay person it gives the impression that ice sheet behaviour is not yet well enough understood to make confident projections for 2100 and further.

So I'm wondering to what extent Hansen and Sato have a point when they say:
"The kinematic constraint may have relevance to the Greenland ice sheet, although the assumptions of Pfeffer at al. (2008) are questionable even for Greenland. They assume that ice streams this century will disgorge ice no faster than the fastest rate observed in recent decades. That assumption is dubious, given the huge climate change that will occur under BAU scenarios, which have a positive (warming) climate forcing that is increasing at a rate dwarfing any known natural forcing. BAU scenarios lead to CO2 levels higher than any since 32 My ago, when Antarctica glaciated. By mid-century most of Greenland would be experiencing summer melting in a longer melt season. Also some Greenland ice stream outlets are in valleys with bedrock below sea level. As the terminus of an ice stream retreats inland, glacier sidewalls can collapse, creating a wider pathway for disgorging ice."

Are they simply wrong, or could they be right that ice streams could disgorge ice faster than the fastest rate observed in recent decades?

You say:
"To date all of the changes we have seen in our glaciers and ice sheets fit our understanding of their dynamics. The timing of the onset of changes has been faster and more widespread than any of us anticipated, but the actual dynamics fit very well."

So even if the dynamics so far have been understood, how certain can we be that changes this century (and later) won't be faster than Pfeffer et al (2008) think possible?

And what about those processes that Pfeffer (2011) says are not yet well enough understood to model them? If Arctic sea ice is able to surprise the scientific models as it is doing now, why not the ice sheets as well? Especially since apparently many relevant feedbacks/processes have not been incorporated in the sea ice models, nor in other climate models.

To me this seems not only relevant for adaptation, but even more so for mitigation. A rate of SLR of say 1-2 m/century or 3-4 m/century could make a big difference for adaptation chances, so would be also very relevant for mitigation decisions. Pfeffer (2011) speaks about the 'fat tail' of the PDF of potential SLR and the difficulty in estimating the probabilities. So I suppose for now we'll have to be satisfied with expert judgement.

So as an expert, how serious do you think the risk is of rates of SLR of 4-5 m/century in coming centuries, as Hansen & Sato seem to think possible/probable under BAU?

Thanks for any further reply,

Lennart van der Linde


Seke Rob,

Tell Hansen and Schellnhuber not to worry any longer, since they seem to think a runaway 'Venus Effect' on Earth is not entirely impossible. Or maybe they changed their minds recently?

Climate Changes

Well, 40 years ago we couldn't begin to imagine that the Arctic Ice would be gone by 20..? :P


The questions I have relating to the GIS are related to possible alteration of the topography by recent glacial changes.

Are any fjords presently being widened, straightened or deepened by glacial outflow. and is this a problem that we would expect to increase in the near term.

My understanding is that the surfaces on which the ice is presently grounded may be either bedrock, or a rock/gravel surface that might be much more easily cut through than bedrock. Are there any studies that indicate which surfaces are more prevalent, or whether there are areas that could be breached allowing oceanic waters to penetrate into the below sea level regions far inland from the present shores.



A paper from January this year on the (remote) possibility of the runaway 'Venus Syndrome':

As noted Hansen and Schellnhuber have also speculated on this possibility and seem to think it's not even as remote as this paper concludes.


Hansen wrote on the 'Venus Syndrome' in his Storms of my Grandchildren' and spoke about it in this talk in 2008 (pp.22-24):

Schellnhuber talked about this risk in his talk in Oxford in 2009 (pp.16-18):

For audio/video of this talk see:

Would be nice if they wrote a paper about it.

Climate Changes

That's some great stuff. :)

"How much a forcing must be maintained to cause rgw? Our models blow up before the oceans boil, but it suggests that perhaps runaway conditions could occur with added forcing as small as 10/20 W/m2."

I'd guess that humans would've d-evolved, moved to another planet or gone extinct before the water would boil but Hansen's right in saying that we should be thinking of runaway as a possibility, if not for future generations/life.


There appears to be water showing in the most inland range of possibly the NordenSkold or Hisinger Glacier, r02c03, most visible with aqua.

Seems too far from any other open areas, and I can't seem to get arctic.io's split zoom to load right know so can't be sure.



present and for a long time, and water is still here, I'd not worry on that front for a longer while... Maybe the next incarnation of HSS will.

Otto Lehikoinen

http://obuoy.datatransport.org/monitor#buoy6/weather looks like buoy#6 has hit the waterline, or maybe it's been picked up too, does someone know?

Otto Lehikoinen

Oops, I apparently hit the midnight image, the buoy is still on ice.

Otto Lehikoinen

http://obuoy.datatransport.org/monitor#buoy8/webcam is far out above Siberia.


A nice sea level map.


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