Yeah, don’t know how I missed that your paper was only examining Antarctic instability. Thanks for the response. I see now that your results are broadly aligned with IPCC projections.

Sorry for the delay in responding, but I was busy elsewhere. I don’t do drive-bys.

ALLEY
Yeah, warming melts ice. We face sea level rise in a warming world. The numbers that you often see for what is most likely are on the optimistic end of what is possible. So the less you trust us the more worried you might be.
http://thedianerehmshow.org/shows/2015-12-09/environmental-outlook-the-earths-melting-ice-sheets

Guests
Chris Mooney energy and environment reporter, Washington Post
Richard Alley professor of geoscience, Penn State University
Eric Rignot professor of Earth system science, University of California, Irvine; principal scientist for the Radar Science and Engineering Section at NASA’s Jet Propulsion Laboratory
Dr. Ben Strauss Chief Operating Officer and Director of the Program on Sea Level Rise, Climate Central.

We just saw the rate of SLR spike to 1 centimeter per year in 2015:

(http://robertscribbler.com/2015/10/06/were-gonna-need-a-bigger-graph-global-sea-level-rise-just-went-off-the-chart/)

Is it just El Nino and we can expect a return to the mean, or are now just able to descry a true exponential acceleration? We have seen the rate of SLR go from 0.8mm to 1.9mm to 3.1mm to above 4mm, and now we see 10mm per year.

In the next few years, we will see an ice-free summer Arctic. It’s acceleration turtles all the way down.

And this, especially, gives pause for thought about the implications of a century or so of SLR – Mengel & Levermann (2014):

Changes in ice discharge from Antarctica constitute the largest uncertainty in future sea-level projections, mainly because of the unknown response of its marine basins. Most of West Antarctica’s marine ice sheet lies on an inland-sloping bed and is thereby prone to a marine ice sheet instability. A similar topographic configuration is found in large parts of East Antarctica, which holds marine ice equivalent to 19 m of global sea-level rise, that is, more than five times that of West Antarctica. Within East Antarctica, the Wilkes Basin holds the largest volume of marine ice that is fully connected by subglacial troughs. This ice body was significantly reduced during the Pliocene epoch. Strong melting underneath adjacent ice shelves with similar bathymetry indicates the ice sheet’s sensitivity to climatic perturbations. The stability of the Wilkes marine ice sheet has not been the subject of any comprehensive assessment of future sea level. Using recently improved topographic data in combination with ice-dynamic simulations, we show here that the removal of a specific coastal ice volume equivalent to less than 80 mm of global sea-level rise at the margin of the Wilkes Basin destabilizes the regional ice flow and leads to a self-sustained discharge of the entire basin and a global sea-level rise of 3–4 m. Our results are robust with respect to variation in ice parameters, forcing details and model resolution as well as increased surface mass balance, indicating that East Antarctica may become a large contributor to future sea-level rise on timescales beyond a century.

2) is it possible that the high range of variation in sea level response to 500 +/- CO2 and the relatively high uncertainties of polar ice sheet collapse scenarios one and the same thing?

It certainly makes you wonder. From Greenbaum et al. (2015):

Totten Glacier, the primary outlet of the Aurora Subglacial Basin, has the largest thinning rate in East Antarctica. Thinning may be driven by enhanced basal melting due to ocean processes, modulated by polynya activity. Warm modified Circumpolar Deep Water, which has been linked to glacier retreat in West Antarctica, has been observed in summer and winter on the nearby continental shelf beneath 400 to 500 m of cool Antarctic Surface Water. Here we derive the bathymetry of the sea floor in the region from gravity and magnetics data as well as ice-thickness measurements. We identify entrances to the ice-shelf cavity below depths of 400 to 500 m that could allow intrusions of warm water if the vertical structure of inflow is similar to nearby observations. Radar sounding reveals a previously unknown inland trough that connects the main ice-shelf cavity to the ocean. If thinning trends continue, a larger water body over the trough could potentially allow more warm water into the cavity, which may, eventually, lead to destabilization of the low-lying region between Totten Glacier and the similarly deep glacier flowing into the Reynolds Trough. We estimate that at least 3.5m of eustatic sea level potential drains through Totten Glacier, so coastal processes in this area could have global consequences.

However, looking at all the paleodata together, it looks like around 500ppm there is a fairly high amount of variation from instance to instance.

Still, a multi-meter ultimate rise in sea level may be inevitable given current CO2ppm plus unavoidable with maximum effort to limit emissions, plus the actual extra CO2 because we will not attain maximum effort.

This leaves two questions open to me. 1) How long will it take? A century here or a century there is lost in the paleorecord; and 2) is it possible that the high range of variation in sea level response to 500 +/- CO2 and the relatively high uncertainties of polar ice sheet collapse scenarios one and the same thing?

You were very clear. It is very clear that you are only talking about the effect from Antarctic instability from your piece.

So putting it into the global context: the IPCC estimated there is a 2 in 3 chance that total sea level rise in 2100 for the A1B scenario will be between 42 and 80 cm. Our results would bump that up by a few centimetres.

Tamsin