Regarding timescales, the questions of:
(A) "When will the main phase of the WAIS collapse begin?" and
(B) "How fast will the main phase WAIS collapse contribute to SLR?"
Are two issues that currently experts (including Alley and Rignot) are not yet able to answer with sufficient precision for policy makers to revise their SLR guidelines in order to address such recent findings about abrupt collapse as those provided by Pollard et al 2015. While I cannot provide a precise analysis of the WAIS contribution to SLR (any more than more qualified experts); nevertheless, I can provide the following brief points as to why a relatively early and active WAIS collapse scenario cannot be ruled-out. Indeed, in the "WAIS Collapse Main Period from 2060 to 2100" thread, I have stated:
http://forum.arctic-sea-ice.net/index.php/topic,85.0.htmlQuote: "… my hazard analysis for RCP 8.5 50% CL forcing is quite aggressive, projecting a eustatic SLR of about 0.5m for the period from 2000 to 2060 (and about 1m of SLR by 2060 for the 95% CL scenario); my SLR projections for my "collapse main period" are more aggressive still, estimating about 3m of eustatic SLR by 2100 for the RCP 8.5 50% CL case, and just over 5m of SLR by 2100 for the RCP 8.5 95% CL case."
Regarding question (A): Rignot has publically commented that the main collapse phase for the WAIS could begin as early as 100-years from now, or as late as 200-years from now; while as noted above (and separate from what the GIS does) I believe that following a BAU the main collapse phase for the WAIS will begin by 2060 for reasons including:
(a) I believe that between now and 2060 the GIS will contribute sufficient surface ice melting and sufficient calving from marine terminating glaciers to help destabilize some of the key West Antarctic marine glaciers and key ice shelves.
(b) I believe that El Nino-like behavior will dominate the Tropical Pacific for the next 15 to 30 years; which will teleconnect energy directly to the West Antarctic, which will promote early collapse of key ice shelves and relatively rapid retreat of key grounding lines.
(c) I believe that the Southwest Tributary glacier to the PIIS will be triggered to accelerate in the next one to several years, due to major calving events of the PIIS; which in turn will likely active the Thwaites eastern shear margin resulting in an acceleration of the main Thwaites ice stream.
(d) Computer models have projected a major reduction in Antarctic sea ice no later than 2060-2070; which in my mind will trigger the Pollard et al 2015 hydrofracturing mechanism that will rapidly accelerate the cliff failures that I believe will begin occurring in portions of the WAIS by 2060.
Regarding question (B): I believe that Pollard et al 2015's main collapse phase SLR contribution from the WAIS of about 1m per decade may be less that the peak SLR contribution during the 2060 to 2100 period, for reasons including:
(a) Alley has publically stated that climatic conditions by the main WAIS collapse phase may exceed the Pliocene-like conditions that Pollard et al 2015 imposed on their Antarctic model.
(b) The first attached figure from Bassis & Jacobs (2013) in panel "a" shows the maximum water depth for a given ice thickness in order to prevent a cliff failure. However, during the main collapse phase (but not necessarily before, due to the GIS fingerprint effect and the density of local meltwater) the fingerprint effect will likely reduce local sea level by several meters, and also isostatic rebound will raise the local seafloor by several meters. This reduction in local water depth will accelerate cliff failure events, probably beyond that assumed by Pollard et al 2015.
Bassis, J.N., and Jacobs,S., (2013), "Diverse calving patterns linked to glacier geometry", Nature Geoscience, 6, 833–836, doi:10.1038/ngeo1887.
(c) Bassis & Jacobs (2013) do not include the basal drag or the basal ice viscosity considerations evaluated by Van der Veen et al (2011) for the Jakobshaven glacier. The second attached image (and associate extract below), indicates that for increased basal drag (such as local bed roughness) and lower basal ice viscosity (such as due to the known geothermal energy in the bed of the Byrd Subglacial Basin, BSB, that the upstream ice flow rate will accelerate, which will result in more crevasses that will facilitate more hydrofracturing and cliff failures.
C.J. VAN DER VEEN, J.C. PLUMMER, L.A. STEARNS, (2011), "Controls on the recent speed-up of Jakobshavn Isbræ, West Greenland", Journal of Glaciology, Vol. 57, No. 204.
Extract related to the second attached image: "The contour plots in Figure 8 show that an increase in glacier speed can be achieved by increasing basal drag or driving stress or by lowering the viscosity parameter."
(d) I cannot believe that if the WAIS is contributing 1m/decade, or more, to SLR that there will not be significant regional seismic and volcanic activity that will both promote calving and surface melting (from volcanic ash). Furthermore, I believe that combined hydrofacturing and cliff failure events in the BSB will be episodically dynamic which will raise local hydrodynamic water pressures that will also contribute to more calving than Pollard et al 2015's model indicate.
Other threads in this Antarctic folder contain discussion on many other factors (such as the crevasses shown in the third attached image from a Bassis analysis of cracking in key Antarctic ice shelves that would then be subject to hydrofracturing) that may contribute to an early and rapid main collapse phase for the WAIS; which would then lead to collapse of key EAIS marine glaciers.