When will the Gulf Stream ‘shut down’ and how will it affect our part of the World?

Quick answer: It probably won’t shut down completely in the next century or two. It will almost certainly weaken progressively during the 21st century, and will have an effect on both UK climate and US sea level. 

 This means that the UK may experience an amount of cooling relative to the global average and the long-term local average, and the US Eastern seaboard may experience an amount of sea level rise greater than the global average and the local long-term average, both within this century.

In simple terms, even if the Gulf Stream does not shut down completely, the impact of a weakening of the system will be felt this century, and conceivably within the next 20-30 years. It has already been felt at least once (2009-2011), and the current CO2 concentration pathway suggests that this kind of thing can easily be repeated, is likely to be repeated.

Back in 2006 I spent some time trying to understand how the Gulf Stream worked and what the implications of climate changes would be for me, living in the UK. This included exchanging some emails with a few of the people who were active at the time in analysing the data and models, and a lot of reading.

In 2009, the UK Climate Impacts Programme (UKCIP09) published it conclusions about projected changes to the UK in the 21st century, which also leaned heavily on similar sources to infer likely scenarios for change to our climate.

Since that time, the issue has stayed in the scientific literature, and occasionally pops up as a matter of concern on the internet or in the media.

Most recently, the technically reliable RealClimate has drawn my attention again to the matter in a post: The Underestimated danger of a breakdown of the Gulf Stream System. 

This summarises a paper which draws attention to some of the limitations of the modelling of the Gulf Stream, or AMOC.

Whilst it is a very useful reference, including links to several recent developments in both analysis and modelling, it does not address a question which I raised in the comments section but have not yet had answered: When is this going to happen?

Even more recently, The open access Journal of Ocean Science has published a paper which, though it focuses on a different aspect (sea level rise), has a strong section on the AMOC and some interesting material on modelling: Changes in extreme regional sea level under global warming.

The Scripps paper (Wei Liu et. al.) applies a flux correction to better model the freshwater exchange in the system on which so much depends. It suggest that the Gulf Stream could lose about a third of its strength by 2100.

The Brunnabend et al  paper uses a reasonably new high resolution ocean model, POP, which is able to capture the mesoscale eddy formations in the north Atlantic and the impact this has on regional sea level rise. It also, as you might expect, provides a greater resolution for modelled AMOC flow under the scenarios used (RCP8.5). The model runs out to the end of this century, from an original baseline of 1950, and a comparison between the first and last twenty years of the model.

In that model: The maximum AMOC at 26◦ N decreases from about 20 Sv to about 5 Sv (red curve in Fig. 4a). The spatial pattern of the AMOC does not change, but the North Atlantic Deep Water shallows by about 1000m (Fig. 4b–c). The maximum strength of the AMOC at 35◦ S decreases (blue curve in Fig. 4a) by more than 60 %. The decline in the AMOC causes a rise in mean DSL of up to 0.4 m near the North American continent, mostly because of a redistribution of ocean mass towards these regions (see Fig. 1a).

*Note: Sv refers to Sverdrups: "...used almost exclusively in oceanography to measure the volumetric rate of transport of ocean currents..." This is derived from a model which assumes a high CO2 concentration pathway and associated global temperature change, so might be considered a ‘worst case’ result.

Where does this get us?

So far, I am at the stage where I’m reasonably confident that the Gulf Stream will demonstrate a severe case of Hiccups over the next 80 years, with has already begun. As in 2009-2010, there will be years when the current slows by 30% or more, resulting in seasons which are noticeably cooler than the long-term trend would otherwise suggest. 

This is the UKMO comment on the Winter of 2010, which followed the most recent hiccup:

The following represents an assessment of the weather experienced across the UK during winter 2009/10 (December 2009 to February 2010) and how it compares with the 1971 to 2000 average (the period used for the seasonal forecast). 
Mean temperatures over the UK were 2.0 °C below the 1971-2000 average during December, 2.4 °C below average during January and 1.6 °C below average during February. The UK mean temperature for the winter was 1.6 °C, which is 2.0 °C below average, making it the coldest winter since 1978/79 (1.2 °C). Over England and Wales it was also the coldest since 1978/79. Over Northern Ireland and Scotland, winter 2009/10 was comparable with 1978/79 and 1946/47, with only winter 1962/63 significantly colder in series from 1910. For northern Scotland, it was the coldest winter on record, with the highest number of frosts. A generally mild first 10 days in December was followed by a colder period. This cold spell persisted for the first half of January, with some severe frosts. After mid-month, temperatures rose to around normal before a return to colder conditions. These persisted for most of February, with only a few brief milder interludes mostly in the west and south.

For the sake of simplicity, I’ll assume that a slowdown of 30% roughly equals a drop in mean winter temperature of around 2 degrees. I am not aware of a similar ‘spike’ in the US Eastern Seaboard sea level and am interested to know if this has been researched.

This occasional and regional cooling has to be taken into the context of expected rises in global mean temperature (which will go on irrespective). As things stand, depending on the mitigation scenario, this is likely to mean a temperature rise of between 2 and 4 degrees out to 2100. Again, for simplicity, I’m going to use a median figure of 3c by 2100.

So, by about 2070, the cooling effect of the AMOC slowdown will be offset by the warming effect of the global mean, which at that stage would be between 1.5 and 2.5 degrees. This suggests that the UK will experience a number of cooler winters, comparable to 2010, for the next forty years. After that, its more likely that the cooler winters will be statistically more like recent seasonal averages, i.e., mild.

Meanwhile, back on the US Eastern seaboard, I am speculating that there is a chance that AMOC weakening events could enhance dynamic sea level rise and add 20-30 cm of extra level by the end of the century. This means that a rise of a metre in some areas is plausible by 2060-2070. Add the effect of storm surges and increased offshore cyclonic activity, and New York could experience ‘Sandy-type’ surges on several occasions, with the probability and frequency rising as time goes on.

This is a complex area of study and I have not really done it justice here; my hope is that the average lay person can get a bead on what a slowdown of the Gulf Stream actually means for us and our children.

Sea ice, sea level and permafrost

Recent updates all over the place point out that Arctic sea ice levels are pretty low considering the time of year. Historically, the rate of decline of Winter Arctic sea ice cover (area, extent) is slower than that of the Summer (and the intermediate seasons). This year, the maximum so far (as pointed out at Neven's excellent blog, the finale is not yet concluded for certain, rather like a Tchaikovsky symphony) is below two standards deviation from the long term average. (here)

I don't subscribe to the notion that one season's extent is indicative of following seasons, for the simple reason that the range of inter-seasonal variability exceeds the range of climate variability. In other words, it isn't enough that there isn't much ice now to determine how much there will be at the end of this Summer (if only it was that easy!). But here we see the fortification (or continuation) of an existing and persistent trend, a reminder that we are apparently set into a pattern for the long term.

Meantime, Down South, recent observations (not yet officially published) are that the Larsen C ice shelf, thought to be stable when assessed in 2010, is now showing a big crack comparable to the one which preceded the loss of Larsen B some years back. Unlike some media, I won't publish more till the original is published, but you can check out the discussion paper at The Cryosphere.

Add to this the recent material illustrating the instability of the Totten Glacier in East Antarctica, the latest of a run of observations done in different parts of the EAIS showing a trend towards likely destabilisation (At least 4 papers recently on different glaciers).

Looking at a couple of other papers (I'll have to go find the sources at some point) I was caught by surprise when it was noted that the potential for sea level rise from the melting of permafrost is potentially very large (over time and relative to scale) and that permafrost is indeed melting reasonably rapidly across all Arctic landmasses (and, one supposes, potential under the ocean as well).

Which is all apropos of what?

One looks at a range of things and extrapolates a pattern. The apparent pattern is that a lot of the markers which might indicate an accelerated rate of sea level rise are all pointing in the same direction, and furthermore, that the observations of the changes are imminent, not down-the-line projections. For some time I've said, along with Rasmus Benestad and others, that the current 'mainstream' estimates of SLR are on the low side. 

I'd like to see a concerted effort to assess recent developments in observations and evidence as a whole and a review of the conclusions. IMO the odds of a 1 metre rise by the end of the century are now much shorter than they would have been twelve months ago. if a reader can point me to any material on the projected contribution of permafrost melting to SLR, that would be helpful, too.

If it's Tuesday, this must be...

It's been raining quite a lot recently in the UK:

http://www.bbc.co.uk/news/uk-25898521

So, since it's the weekend, and at the weekend we come out and play, here is:

The wonderfully tacky video was filmed at Southsea beach, where I lived between 2002-2012. Southsea is part of the City of Portsmouth, one of the UK's main Naval Bases, home of Nelson's HMS Victory and, as seen on the video, a Castle, from which Henry VIII watched his flagship, the Mary Rose, sink with all hands nearly 500 years ago.

A few years ago, business took me to the office of the city's Head of Planning. On his wall was a very large map, showing expected sea level changes up to 2080. Something between 1/3 and 1/2 of the city was marked in blue. That's a city with existing flood defences and a major strategic military base, and a population of around 200,000. I had a lot more sympathy for the man and the job he had to do after the meeting than I had before.

Someone tell me honestly you don't think we need to take action on our changing climate.

For everything there is a season - tide and time

Over at the warren, Uncle Eli has posted about SLR, a subject I touched on last month, here, and here, including references to Grinsted's excellent material.

It occurs that the Average Josephine (IOW, most of the world) might look at the projections of sea level rise and, whilst registering that a change will have an impact, might reasonably ask why a meter or so of extra sea might be such a big deal. So here's an attempt to place this into a 'human' context.

On its own, a few centimetres of water 'added on' to the water level at any given beach or dockside is no big deal. This is because the other forces which have an effect on sea level at any given location are much stronger (they create more variability) than the underlying 'signal' of sea level.

First and most obvious is the effect of the tides. Whilst in some places sea level fluctuates by a few metres over the course of a day, in others the tidal range can top ten metres and more (up to around 15-16 m for some locations). In practical terms, coastal human habitation and business takes this into account, so in most places human activity takes places above the high tide level, for obvious reasons.

At certain times of year, specifically the Equinoctial Spring Tides, the various forces which interact to create tides and tidal ranges combine to create particularly high and low tides. It is not uncommon already for these to overreach the human adaptation level and to result in localised flooding, which is further worsened if these tidal periods occur in sync with strong weather conditions (in particular, depressions, often related to storms).

Now, the current range of projections for changes in sea level have to be considered in the context of historic tidal ranges and existing infrastructure and human-ocean interfaces.
For example, the 'averaged' sea level range globally hits around 79cm during Spring tides. This means that, overall, the shoreline would experience a few extra inches more or less. But in a local and regional context this 'average' is effectively meaningless, and is not reflected in the real experience of many coastal dwellers. For most people on coasts, the tides go up and down several metres.

If global average sea level rises by, say, half a metre, what does this mean for local impacts? And what effect does this have on local tidal ranges and, in particular, during the upper bound of the ranges (the Equinoctial Springs) and those occasions when these coincide with storm surges?

Well, it should not be difficult to work out that an extra 'average' SLR of 50cm is going to mean a rise of high tides, and of high springs, in the order of 2 - 5 metres of 'extra' sea. Given that a storm surge can increase sea levels (for example, in the Philippines) by another 4-5 metres, and you end up with places which are likely to experience regular (annual or more frequent) tidal surges in the range of 5 -10 metres. We have seen the horrific effects on one part of the world where a tidal surge of 5 metres, added to extreme weather conditions, has resulted in devastation and carnage. Now multiply this by all those places which are 'vulnerable' to such variability.

As an aside (because I'm not certain of the projected effects), it should be noted that a Spring tide normally produces currents twice as fast as Neap tides, but with eight times the power, or force. Anyone who has struggled to get out of the water onto a beach during an ebb in difficult conditions can have a sense of how much power is involved - it is, literally, an overwhelming force. 

It seems rational to presume that higher Springs will be associated with faster currents and therefore greater forces - resulting in more erosion, more localised damage and greater stress on infrastructure (including defences). So, the effect-multiplier of a few centimetres of extra sea level 'on average' produces impacts which can easily be seem to include, for example, the overwhelming of low-lying islands, or the inundation of coastal cites, oil refineries, nations (Bangladesh).

If all of this is the consequence of half a metre of sea level rise, what then is the consequence of a metre or more? Is it likely to be twice as bad? Or, given the 'effect multipiers, are we instead talking about a localised effect with a difference of an order of magnitude?

Finally, for the economically-minded amongst you, remember that an astonishing proportion of the world's trade is conducted across the oceans - around 90% of all trade goods is shipped at some point. Now, consider the impact on shipping of the changes outlined above. The cost of building tougher ships, the cost of building new, relocated shipping hubs (the World's three deepest 'ultratanker' and supercontainer ports are all vulnerable to rising sea level). The risks and losses, all to be paid for by someone.

A very high proportion of the Global population lives in the coastal strip - I think it's about 85% of the population. Not all of these people would be vulnerable in the way I outline above, but with sea level rise must come, inevitably, relocation and mass urban movement inland, fundamentally changing the dynamic geography of our society.

That's why sea level rise matters, and why a metre is more significant than half a metre.

NOTE also Simon Donner's eminently pragmatic contribution to the discussion of adaptation.

Reasons to be cheerful, one, two, three

Back to the AR5 on the subject of the Antarctic, and in particular the Marine Ice Sheet Instability Hypothesis. The AR5 confidently asserts that 'it is likely that abrupt and irreversible ice loss from WAIS is possible'. Why does this matter? Because if it occurred, that additional sea level rise which the AR5 does not expect, of 'several tens of centimetres', would become a reality.

For non-scientists, (usual warnings apply), a 'sudden and irreversible ice loss' refers to the possible 'collapse' of the mechanism which currently constrains the export of ice into the oceans and therefore the contribution to sea level rise. This would be signalled by rapid, non-linear changes to the Pine Island Glacier (PIG) and Thwaites Glacier (TG). These two represent a substantial proportion of the potential sea-level contribution of the WAIS, which is why they are important. One of the keys to recognising that a sudden process  is under way is the retreat of the Grounding Line (GL) of the glaciers. Recent research suggests that Pine Island is more risky than Thwaites because of the relative basal geography of the two.

The process follows a theoretical sequence: The glaciers at the sea end thin, meltwater seeps down from above, the glacier fractures deep into the outlet section, then collapses; the glaciers behind speed up, pushing more ice seawards, and the grounding line (where the sea can no longer get 'under' the glacier, retreats inland. 

In Chapter 13, The AR5 discusses the condition under which such an event might occur and the consequences:

"Two processes that could trigger GL retreat are particularly relevant to contemporary polar climate change. The first is the presence of warmer ocean water under ice shelves, which leads to enhanced submarine iceshelf melt (Jacobs et al., 2011). The second is the presence of melt water ponds on the surface of the ice shelf, which can cause stress concentrations allowing fractures to penetrate the full ice-shelf thickness. This process appears to have been a primary factor in the collapse of the Larsen B Ice Shelf (LBIS) over the course of two months in 2002 (MacAyeal et al., 2003). The collapse of the LBIS provided a natural demonstration of the linkage between the structural integrity of an ice shelf and the flow of grounded ice draining into it. Following the breakup of LBIS, the speeds of the glaciers feeding the collapsed portion of the shelf increased two-to-eightfold, while the flow of glaciers draining into a surviving sector was unaltered (Rignot et al., 2004; Scambos et al., 2004; Rott et al., 2011). This indicates that a mechanical link does indeed exist between shelf and sheet, and has important implications for the future evolution of the far more significant PIG and TG systems of the WAIS."

In addition to any other, earlier material, a new paper in The Cryosphere describes Condition One:

Dutrieux, P., Vaughan, D. G., Corr, H. F. J., Jenkins, A., Holland, P. R., Joughin, I., and Fleming, A. H.: Pine Island glacier ice shelf melt distributed at kilometre scales, The Cryosphere, 7, 1543-1555, doi:10.5194/tc-7-1543-2013, 2013. There are other observations and calculations substantiating that there is water under the PIG, that it is relatively warm, that it is progressively undermining the glacier. There seems little reason to doubt that this condition is currently being met. 

Condition two requires the presence of meltwater at the top of the glacier. I can't find the source (maybe someone can help), but I am certain that I have very recently seen a record of observation of this phenomenon - I believe it was at the Thwaites Glacier, but there is no reason to suppose that the phenomenon would exist at one glacier and not the other.Forthcoming papers (not yet published) look at the 'condition' of PIG and seem to offer further evidence that the conditions exist for a full-depth fracture of the PIG.
So; it would appear that the required conditions for the fracture and subsequent collapse of the Pine Island Glacier are being met. If this is correct, the next 'critical' stage would be the appearance of a glacier-wide fracture, probably during the Austral Summer. On the surface, it very much feels like this is no longer a matter of 'if', but of 'when'.

Prediction a non-linear shift is, of course impossible. It has to be a sophisticated form of educated guesswork as to when this could occur. To me, given that other indicators suggest that the transformational process in the cryosphere are systematically understated in the 'official' lines, it could as easily be this Summer, or ten years, or twenty, thirty years away.

If any of my readers can give guidance as to the plausible timeline in which we are working here, it would be much appreciated. Likewise, if someone thinks that I am misrepresenting or overstating the case. But from the viewpoint of this blogger, that 'likely that it is possible' statement of the AR5 goes nowhere near capturing the reality of the situation or the risk. As a consequence, the decision to eliminate sea level rise estimates which incorporate a collapse in the 21st century look at best misguided, at worst, much too conservative.

I'll look out for other connections on the subject: for example, Aslak Grinsted's excellent summary of the estimates, and RealClimate's ongoing discussions.

I think this is a real, substantive risk which has a significant timeline for the current generation, and that serious and urgent investment in Antarctic research is now essential.

Sea Oddity - it's a godawful small affair

Well, RC went and stole a march by posting about chapter 13, and deservedly their material will get more attention because the contributors are specialists in the area. But whilst they discuss some technical matters, they don't seem to have covered some of the 'oddities' which, to my eye, point towards an innate conservatism in the Sea level rise assessment. Since RC has done it I will also, after all, use some of the draft, but will try to do so sparingly. Interested readers can read it for themselves.

As usual this is the 'outside' view, so I expect (and hope) for feedback.

First up is the paragraph about the two model intercomparison papers, Bindschadler et. al. (2013) and Nowicki et, al (2013), which concludes:

"The resultant projection included contributions from lubrication, marine melt and SMB-coupling and generated a mean SLR at 2100 of 162 mm over five models, or 53 mm if an outlier with anomalously high response is removed (including SMB results in SLR at 2100 of 223 and 114 mm for five- and four-model means, respectively). This comparison lends further weight to our confidence."

I don't have a problem with the concept of removing outliers as such, and it is clear from the very large differences in results that the outlier must be well out, but if they are included in the model intercomparisons, should they not be included in the assessment of the range? My beef with this is that they seem to be saying that once they have fiddled a bit with the original papers (and I'll admit, I might also be inclined to discount an outrageous outlier if there was no sensible reason to consider its' plausibility), they end up with a number which adds confidence to their 'chosen' assessment range. This doesn't seem, on the surface, to be entirely transparent.

Next up is the 'introduction' of the Marine Ice Sheet Instability material:

"There is an underlying concern that observations presage the onset of large-scale grounding line retreat in what is termed the Marine Ice Sheet Instability (MISI; Box 13.2), and much of the research assessed here attempts to understand the applicability of this theoretical concept to projected SLR from Antarctica."

Now call me a pedant (it's a reasonable charge), but the language chosen: 'this theoretical concept' at this point, whilst it may be technically correct, might predispose a reader to imagine that MISI is an implausible scenario. Later on, the Chapter includes the memorable:

"In summary, based on ice-dynamics theory, numerical simulations, and paleo records, it is likely that abrupt and irreversible ice loss from WAIS is possible. However, theoretical considerations, current observations, numerical models, and paleo records currently do not allow a quantification of the perturbation that is necessary to destabilize the ice sheet."

Can someone tell me what that first sentence means? It's a cracker.

Last up for this post is what appears to my eyes to be mildly remarkable:

"More detailed regional modelling using scenario A1B illustrates the potential for warm water to invade the ocean cavity underlying the Filchner-Ronne ice shelf in the second half of the 21st century, with an associated 20-fold increase in melt (Hellmer et al., 2012). Based on the limited literature, there is medium confidence that oceanic processes may potentially trigger further dynamical change particularly in the latter part of the 21st century, while there is also medium confidence that atmospheric change will not

affect dynamics outside of the Antarctic Peninsula."

Wow. So there is another hypothetical process that indicates a possibility of a larger potential sea level rise, which includes local changes in orders of magnitude. Does this not indicate that the reasonable range of SLR might be upped a bit? After all, we are talking about a range, aren't we?

I'm going to finish this off in another post because I am tired.

Comments, please.

Little by Little

After the last IPCC (AR4) I spent some time on the available literature as well as conversing with several qualified people (scientists) and concluded (purely on the basis of how it all seemed to add up), that the AR4 estimates of sea level rise (SLR) probably were conservative and that a rise of around a metre by 2100 was a reasonable estimate, that 1.5 metres was plausible, but that much more seemed unlikely in the timescale, given the range of evidence.

Having gone through chapter 13 of the new AR5, I am steadily moving towards the view that a metre by 2100 is possibly the lower bound of SLR, that 1.5 metres is now the reasonable estimate, and that a larger rise is not so much unlikely as uncertain, which is a very different thing.

Of course I'm not allowed to print the stuff (well..) in the draft itself, but I extracted some parts, particularly in relation to the contribution to Sea Level Rise of the Antarctic Ice Sheet and the Greenland Ice Sheet, thinking to comment as a whole. But there is too much - even reduced to the bits which seemed most relevant to the dialogues here and on other websites.

In addition, there are plenty of better qualified commentators already working on this in the public domain. Thanks to Steve Bloom, who provided these sample links (you will need to expand the comments). This is also thematically connected to the discussion of the cryosphere (Chapter 4) here.

What I wanted to draw out of the AR5 were the potential reasons why others might think that their assessment is conservative. I have come across 9 slight oddities which will be dealt with a bit at a time (otherwise it's just too much). The first one concerns the basis for estimating the SLR contribution from the Antarctic.

It looks to me as if the core statement of estimated likely SLR from this source is based on Little et. al. (2013). [NOTE - the Little paper in question is behind a paywall at Nature Climate Change, but available as a pdf via Google Scholar] Actually, that's what the report says. But then it goes on to say, apparently, that the reason for choosing this as the reference point is that it's somewhere in the middle of the available ( process model-based) assessments, not including outliers (!) , and not taking into considering semi-empirical model estimates. But this is undermined in the selfsame explanatory paragraph by stating that it falls at the lower end of the estimates from the other process model-based assessments which were available. [NOTE - the Little paper in question is behind a paywall at Nature Climate Change, but available as a pdf via Google Scholar]

It's important to remember that the people who put this stuff together for the IPCC are experienced professional scientists and they work very hard to present what is the representative view in a balanced way. Yet, throughout the chapter, there are side-comments and minor details which, put together, strongly suggest that their chosen rubric is a consciously conservative one. 

The other point which is frequently being made is that this is an example of over-dependence on the process models. There is enough evidence in the chapter to support this view, some of it overt. As things stand, I am inclined to think that not enough weight has been given to either the semi-empirical models, which consistently produce SLR rates higher than the process models, or to the people on the ground, who also seem consistently to question the AR5 estimates and suggest they are understating things.

As was pointed out to me, the most important source of difference between estimates of sea level rise is the Antarctic. Why? In broad terms, the contribution from other sources, such as the Greenland Ice Sheet, can be imagined as adding to SLR in a way comparable to the way a tap adds water to a bath, steadily. With Antarctica, this is also the case, but there also is an identifiable (but uncertain) risk that someone is planning to jump into the bath - via a destabilisation of the West Antarctic Ice Sheet (definitely more on this in a later post). The analogy also works when we think about the proposed timescales for change. There's a huge difference between adapting to a slightly overfilled bath by being careful, and clearing up after the idiot went and jumped in.

There is a lot more to be said on this subject, suffice it to say for know that I am now more concerned, after reading the report, than I was before, about the ability to adapt to some of the possible changes in the next forty-odd years, and even more worried about the following half century.

On the rocks, no ice

A swift trawl through some of the Open Access Journals has brought up this gem from The Cryosphere:

Global glacier changes: a revised assessment of committed mass losses and sampling uncertainties

S. H. Mernild^1,2, W. H. Lipscomb³, D. B. Bahr^4,5, V. Radić⁶, and M. Zemp⁷

Abstract. Most glaciers and ice caps (GIC) are out of balance with the current climate. To return to equilibrium, GIC must thin and retreat, losing additional mass and raising sea level. Because glacier observations are sparse and geographically biased, there is an undersampling problem common to all global assessments. Here, we further develop an assessment approach based on accumulation-area ratios (AAR) to estimate committed mass losses and analyze the undersampling problem. We compiled all available AAR observations for 144 GIC from 1971 to 2010, and found that most glaciers and ice caps are farther from balance than previously believed. Accounting for regional and global undersampling errors, our model suggests that GIC are committed to additional losses of 32 ± 12% of their area and 38 ± 16% of their volume if the future climate resembles the climate of the past decade. These losses imply global mean sea-level rise of 163 ± 69 mm, assuming total glacier volume of 430 mm sea-level equivalent. To reduce the large uncertainties in these projections, more long-term glacier measurements are needed in poorly sampled regions.

In layman's terms, the authors are suggesting that (accepting high uncertainty) the World's glaciers and ice caps are shrinking faster more out of balance than previously estimated, with a consequential increase in their contribution to global sea level rise. This conclusion is premised on a future climate resembling that of the last ten years (so no projection included for increasing mean annual temperatures).

I'd be interested to get Stoat's response - it's his kind of thing, as well as those of similar inclination. My first thoughts are that here is more suggestive evidence that the synthesis estimates of future change represented by the AR5 tend to underestimate likely future climate-related trends. 

The niggling worry is that there may be a cumulative imbalance - for each metric a conservative estimate, each conclusion/estimate slightly understated - and, if it proves otherwise, the gross cumulative change is far more substantial than the projections. In this case, the paper draws our attention to something I have previously researched and have an interest in - global seal level rise. There are plenty of voices in the scientific community already saying that the AR5 estimates are conservative - here is another potential string to their bow. At it, friends...

(Indeed, Stoat responds here: http://scienceblogs.com/stoat/2013/09/30/ar5-cursory-review-of-chapter-4-cryosphere-mass-balance-of-antarctica/ see comments 8&9.) Thanks, William.

Further comment: since it is a well established and credible blog, the discussion continues at WC's site - no point in duplicating effort. Please go there and read on...