2011-09-13 14:10:43Abrupt Climate Change - Its Causes
Agnostic

mikepope_9@hotmail...
118.208.187.37

Introduction

That we are well down the road to abrupt climate change is evidenced by rapidly rising temperature, particularly in polar regions.  Hansen and Sato 2011 have warned that average global temperature has reached a level which is now only a few tenths of a degree below the Eemian maximum when conditions were very different from what they are now.  Loss of land-based ice was rapid, sea level was 5-9m higher than now and severe climate events were more common.

What concerns Dr Hansen is the target of limiting anthropogenic CO2 emissions to 450ppm adopted in Copenhagen.  He has repeatedly warned, this will not only result in our exceeding temperatures of the Eemian interglacial but risk average global temperatures of not 2°C but 4°C - 6°C above the pre-industrial by 2100 because of slow feedbacks.  Arctic temperatures could be expected to rise by a factor of at least 2.  Hence his call for greenhouse gas emissions to be curbed and their presence in the atmosphere to be reduced to 350ppm by 2100.

The orbital parameters which probably initiated the Holocene interglacial about 10,000 years ago have since returned to a state where global temperature would be expected to have reached a maximum or possibly be in decline.  The earth’s eccentricity is approaching zero and obliquity of 23.5° is decreasing, reducing exposure of polar regions to sunlight, which should be contributing to their cooling.

In the absence of anthropogenic global warming, ice coverage of the Arctic ocean would be expected to grow in area and duration, increasing albedo and further promoting cooling.  This should have been enhanced by recent decades of quiescent solar activity.  Yet what we now observe is Arctic warming at over 2 times the rate of average global warming, amplification which may increase toa factor of 3 due to the effects of slow feedbacks 

Anthropogenic CO2 concentration in the atmosphere will continue rising and is likely to reach 550ppm within 50 years.  However CH4 releases in the Arctic, now ~1 million tonnes CO2-e per annum will increase rapidly each year and add to this concentration with the likely result that by 2100 atmospheric CO2-e could be well in excess of 800ppm with average global temperature >4°C-6C above the 1750 level.  In the Arctic, temperatures are likely to exceed 2-3 times that level, increasing the risk of massive CH4 emissions in excess of 3 gigatonnes per annum this century, prompting abrupt climate change and speeding-up melting of land based ice globally.

Slow Feedbacks

There are several slow feedbacks, all of which have been initiated by anthropogenic CO2 emissions causing a rise in global temperature and consequential atmospheric and ocean warming.  Once initiated, slow feedbacks are beyond human control.  Most contribute to further global warming, collectively culminating in abrupt climate change.  The more important include:

  • Albedo loss
  • Methane release
  • Ice Sheets loss
  • Sea level rise

Albedo or capacity of the earth to reflect solar energy back into space, depends on the area of the planet covered by the reflective white of snow and ice during periods of longest exposure to sunlight.  The larger this area, the greater the amount of solar energy reflected back into space, lowering average global temperature.  Loss of albedo arises from melting of snow and ice or its covering with dark aerosol particles (soot), reduces reflectivity and causes average global temperature to rise faster.

Most of the solar energy reaching the earth is stored in the oceans and transported both vertically and horizontally by oceans currents to cooler parts, particularly polar regions.  Arctic amplification has primarily been initiated by ocean warming and transport of that warmth from tropical to polar regions by ocean currents, notably the Gulf Stream.  This continues to cause rapid loss of sea ice both in terms of volume and coverage of the Arctic ocean.

These losses are no longer seasonally limited since even in winter warmer ocean waters under the sea-ice continue erosion from beneath.  Each year the area of Arctic ocean covered by sea ice contracts further and is now doing so at a rate which indicates that within 20 years it may be ice free in summer.  Exposure of open seawater to solar radiation in the Arctic is causing further warming of both seawater and the atmosphere.  In turn, these temperature increases, evident for at least a decade, are having an erosive effect on land-based snow, ice and permafrost.

Methane:  Increased ocean warming of 1°C per decade has been occurring along the shallow continental shelf bordering the Arctic Ocean since at least 1980.  It has started thawing permafrost covering sediments on and beneath the seabed of the shelf.  On and under it are large and extensive deposits of clathrate and yedoma estimated to contain not less than 1,400 billion tonnes of methane (CH4). 

Dr Shakhova estimates CH4 deposits of 700 gigatonnes in gaseous form in ocean sediment, a further 500 gigatonnes contained in permafrost on and beneath the continental shelf and at least 1,000 gigatonnes in clathrate.  It can be expected that initial emissions will come from gaseous deposits and permafrost nearest the surface. 

On the East Siberian continental shelf water depth covering 75% of its 2.1 million km2 area is less than 50 metres deep.  CH4 percolating through this shallow water column has no time to oxidise to CO2 but enters the atmosphere as CH4, with ~72 times the global warming potential (GWP) of CO2 over the period it takes to oxidize in the atmosphere.  In 2005 it was found that 8 million tonnes per annum of CH4, with the GWP equivalent of ~570 million tonnes of CO2, was entering the atmosphere.

Clathrate deposits under the continental shelf bordering the Arctic Ocean are likely to have formed at relatively shallow depth of <200 metres below the surface, under an impervious permafrost cap forming the stability zone, prior to flooding of the shelf.  It is likely that they have remained stable because of low temperature and relatively high subsurface pressure caused by methane release from clathrate or yedoma, due in part to geothermal heat associated with seismicity to which much of the shelf is prone.

Shakhova and Semiletov have shown that deterioration of permafrost covering the continental shelf enables increased formation of talik subsidence channels through which gaseous methane is escaping, lowering subsurface pressure.  This pressure reduction may cause destablisation and decay of shallower clathrate deposits.   To date the result has been growing CH4 emissions and increasing likelihood of relatively sudden massive emissions.   The real danger of climate change is posed by relatively sudden venting of CH4 likely to abruptly increase the radiative forcing of anthropogenic emissions and increasingly do so thereafter

Annual venting of a small fraction, just 0.12% per annum of Arctic CH4 deposits, in addition to accumulated anthropogenic CO2 emissions (in 2010 ~30 billion tonnes per annum) would be sufficient to trigger abrupt climate change.  CH4 Emissions are predicted to exceed 1.5 gigatonnes tonnes per annum by 2030, rising to ~3.5 gigatonnes tonnes per annum, the equivalent of 250 gigatonnes tonnes of CO2, 8 times current anthropogenic emissions, by 2100.  Quantities of this magnitude may remain in the atmosphere far longer than the 10-15 years which CH4 normally takes to oxidise to CO2 because of possible insufficiency of OH radicals in the atmosphere needed to oxidise such large volumes.

 

Fig 1.  Decadal doubling of ice mass loss from the GIS and WAIS is predicted to exceed 163,000 gigatonnes by 2100. WAIS ice loss could be much greater.

Ice sheets.  In the Arctic, surface temperature is likely to rise 9°C -10°C by 2060 because of the effects of methane release, ocean warming, albedo loss and doubling of atmospheric CO2.  Hitherto unheard of summer temperatures in excess of +15°C have already been reported from northern Greenland.  Slow feedbacks will further contribute to Arctic warming and that warming is likely to have a significant affect on ice loss from the Greenland Ice Sheet (GIS).

It is estimated that in 2000 the GIS was loosing mass at a rate of ~100 gigatonnes per annum.  As a result of on-going warming of the Arctic Ocean and atmosphere, that rate is predicted to double every decade for the rest of the 21st century, reaching a mass loss rate of ~102,000 gigatonnes per annum by the year 2100, contributing the equivalent of 25.6cm SLR/annum.  Current estimates are that in 2010 GIS mass loss was >300 gigatonnes per annum, more than trebling in that decade.  This acceleration should be treated with caution until confirmed over a longer period.

Methane deposits in the Antarctic are covered by ice over 3 km deep so heat transported to this region by equatorial currents and high wind events is unlikely to result in its release this century.  However Antarctica is warming and, like the Arctic, doing so more rapidly than tropical and temperate regions.  Atmospheric and ocean warming caused collapse of Larsen Ice Shelves A and B, enabling most “Peninsula” glaciers to discharge and retreat faster.  The Ross, Ronner and other ice shelves could break up before 2100, enabling glaciers to flow at much higher rates, rapidly increasing ice mass loss.  Discharge of ice from the Thwaits and Pine Island glaciers has increased by 50% since 1994 and they  drain over 30% of the WAIS.

Most of the West Antarctic Ice Sheet (WAIS) covers a far-flung archipelago to the west of the Trans-Antarctic Mountains.  It largely rests on the seabed so it is directly exposed to equatorial currents warming the Southern Ocean and melting sub-surface ice making it very vulnerable to sudden catastrophic collapse and potential sea level rise of 6-7 metres from this source alone.  In 2006 mass loss was measured by the GRACE satellites to be 132 gigatonnes per annum.

Rignot et al 2011 estimate the net rate of loss to be 14.5 gigatonnes per annum.  Using this rate it is estimated that ice mass loss from the WAIS was ~60     gigatonnes per annum in 2000.  As with Greenland, Antarctic ice loss is non linear, accelerating rapidly and, as a result of decadal doubling, could reach >61,000 gigatonnes per annum, contributing 15.4cm per annum to SLR by 2100.

Rising ocean and atmospheric temperature are continuing and will result in ice loss from the East Antarctic Ice Sheet (EAIS) which is 10-11 times larger than the WAIS.  In 2000 the EAIS was regarded as being in balance, gaining ice from precipitation which off-set losses occurring along the coastline.  By 2006 net ice loss was estimated as 57 gigatonnes per annum and accelerating, though unlikely to double per decade before 2100. 

 

 

Fig 2.  Since 100 gigatonnes of ice contributes 0.25mm to SLR, sea level is predicted to rise ~0.5 metre by 2065, 1 metre by 2085 and a total of 4 metres by 2100.

Sea level rise (SLR) is caused by ice loss from the polar ice sheets and to a lesser extent from thawing of other land based snow and ice and thermal expansion.  The rate of thawing is increasing annually and more than doubling per decade.  This loss is not linear but, starting from a low base, is not expected to make a major contribution to SLR until the latter half of the 21st century.  It follows that average global SLR is also non-linear and, although relatively slow, 3.2mm per annum in 2000, the rate of rise will increase and, by 2050, is likely to be measured in centimeters rather than millimeters per annum.

As sea level and ocean temperatures continue to rise, warmer water could be forced under EAIS, WAIS and GIS glaciers, increasing their rate of discharge, the rate of ice loss and hastening ice sheet collapse.  By 2075, SLR of 1 metre is possible given present and predicted rates of polar ice loss.  Hansen and Sato 2011 predicts that polar ice loss will gain momentum after 2070 and increase very rapidly after 2085 producing a catastrophic SLR of ~4 metres in the last decades of this century.  There is no reason to doubt this prediction, particularly if Arctic warming increases significantly as a result of slow feedbacks, inducing decadal doubling of ice loss.

Conclusion

Trends and projections point towards abrupt climate change being likely to occur in the latter part of this century prior to 2100, possibly before 2070.  This is expected to be accompanied by:

  • Increased venting of CH4 – 1,500 million tonnes by 2030 and >3,000 million tonnes per annum before 2100.
  • A rise of 2°C - 3°C in average global temperature by 2060 followed by a further relatively sudden rise of up to 4°C by 2100.
  • Loss of polar sea ice and major ice shelves, loss of mountain glaciers and permafrost to a depth of 3 metres by 2100.
  • Passing tipping points for collapse of the GIS and WAIS ice sheets with significant losses from the EAIS and most other land based ice by 2100.
  • A rise in average global sea level of 1m by 2085 and a further 3m by 2100 and,
  • Accompanied by an increase in the incidence and severity of extreme climate events.

These changes satisfy the definition of abrupt climate change.  Trends indicating their emergence are unmistakable now.  All will be evident by 2050.  Collectively, by any measure, they constitute abrupt climate change in the making.

2011-09-13 18:15:50Nice Post
Glenn Tamblyn

glenn@thefoodgallery.com...
58.168.68.38

Agnostic. Nice summary. Several additional points you might like to consider adding wrt slow feedbacks and or abrupt changes.

Major collapse ( and burning) of the Amazon forest and possibly others.

Temperature induced outgassing of soil carbon.

Thermohaline circulation slowdown. Most concerns are about increased freshwater drainage into the Arctic/Atlantic system. But what about the possible impact of Arctic ice decline and Arctic warming on the extent of sea ice freezing - the central driver of T/H circulation.

2011-09-14 09:37:23
Agnostic

mikepope_9@hotmail...
118.208.187.37

Thanks Glenn - All valid points.

You are quite right but the reason I haven’t included these is, 1. I wanted to keep the essay to 2,500 words and 2. I am still unsure that the matters you raise are precursors to sudden climate change or the consequences of its occurrence?

I have begun drafting Abrupt Climate Change – Its Effects and I will definitely include the issues you raise.

2011-09-15 04:59:24
dana1981
Dana Nuccitelli
dana1981@yahoo...
64.129.227.4

I don't know about defining abrupt as one decade.  From a climate perspective even a century is abrupt, certainly several decades.  And that's the sort of timeframe we're looking at.

The post is rather on the long side and could use some trimming.  I'm not sure the definition of abrupt at the beginning is really necessary.  You can explain that in the body of the text as you discuss the science.

James has some relevant posts you may want to check out and link to: Slam on the Brakes, Climate Sensitivity: Feedbacks Anyone?

Got those three figures somehwere?

2011-09-15 05:23:52
Agnostic

mikepope_9@hotmail...
118.208.187.37

dana1981 - right on both points and thanks for the refs

You know my history (abysmal!) with inserts.  The graphics were produced from spread sheet data.  I saved each graph on an xls spread sheet, then uploaded them - but I suspect the surface area (the spread-sheet) is too large but I don't know how to shrink it.  Perhaps I should save graphics differently?

Anyway, HELP

Also I may need help with graph 1 which is supposed to show likely emission of CH4  expressed as (3.5) gigatonnes of CO2-e (x72 gigatonnes) during the 21st Century.  The magnitude of CO2-e is so great that it can not be compared with relatively tiny anthropogenic CO2 tonnage.  Got flu at present.  Will have another go later.

2011-09-17 06:28:48
Agnostic

mikepope_9@hotmail...
118.208.81.161

OK guys, more help needed.

I have data on a spread sheet from which I have produced 3 graphs on the same spread sheet.

How do I upload each graph please?

2011-09-17 13:18:56
Agnostic

mikepope_9@hotmail...
118.208.81.161

A problem I have is making a sensible guess about the way in which CH4 will be released by thawing permafrost in the seabed of the East Siberian continental shelf and the Arctic Tundra.

In this essay I hAve assumed major venting from the continental shelf occurring 2035-45 (1.5 G/t) and from the Tundra (thawing of yedoma) after 2080 (2 G/t).

However, gut feeling (hardly scientific!) tell me that a more likely scenario is for venting of >3 G/t, almost entirely from the continental shelf, to occur in the 2030's.

What do you think O Learned Ones - and why?

2011-09-17 14:20:26What about
jyyh
Otto Lehikoinen
otanle@hotmail...
193.199.143.214

n/t, already mentioned.

2011-09-17 15:11:31
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
97.83.150.37

Email me the graphics: I'll upload them for you.

yooper49855@hotmail.com

2011-09-18 13:30:28
Agnostic

mikepope_9@hotmail...
118.208.81.161
2011-09-18 14:18:56Uploaded and added graphics
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
97.83.150.37

Links to originals:

http://www.skepticalscience.com/pics/CC-Fig1.png

http://www.skepticalscience.com/pics/CC-Fig2.png

http://www.skepticalscience.com/pics/CC-Fig3.png

2011-09-18 14:47:46
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
97.83.150.37

Agnostic, nice (sobering and scary) post.

I do have some thoughts, but will save them for morning (my brain is numb and I'm having trouble typing as well).

2011-09-19 09:57:11
Agnostic

mikepope_9@hotmail...
118.208.81.161

Dana1981 – I have edited and corrected along the lines you suggested.  

2011-09-19 15:47:02
Daniel Bailey
Daniel Bailey
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24.213.17.130

Ok, got a second read in.  Nice post, again.  Very well documented, with supporting links embedded.  You've obviously put a lot of time into it and are to be congratulated.

Notes:

  • The source of the information you used to generate your 3 graphics should be noted and explained
  • Hansen et al 2011 should probably be referenced as Hansen and Sato 2011.  Et al usually means 3 or more authors, with the primary author listed in the title
  • You should probably do a "References/Further Reading" section at the end where you link to the Wakening the Kraken post (it is not shameless self-promotion to do so)
  • Perhaps give some context to the potential CH4 emissions.  For example, you mention that CH4 (in CO2 equivalent measures) could reach 250,000 megatons by 2100 vs current CO2 emissions of 30 gigatons.  Changing that to "250 gigatons by 2100, more than 8 times the current annual output of CO2 injected by mankind".  I.e., keep the units the same (not million tons vs gigatons) and do it each time the scale of comparison is large.
  • Perhaps some mention of the warming Arctic warmers contributing to the thinning of the mixing layer under the ice cap would be in order.  The mixing layer is that layer immediately beneath the ice insulating the ice from the warmer waters underneath.  The layer, stable for millennia, has now thinned considerably.  One of the results of that is that the fresher melt waters are increasingly transported deep into the ocean with warmer waters from the deep being upwelled to the inderside of the ice, further increasing the melt in a vicious circle of feedbacks.
  • I'm having a little trouble agreeing with the conclusions shown in Figure 2.  Given the morphology of Greenland (a bowl-shaped structure with the middle below sea level ringed by mountains with but few outlets below sea level) vs that of the WAIS (a marine terminating glacial ice sheet sitting on bedrock which itself is well below sea level), my expectation is that the WAIS will dwarf the melt contribution of the GIS by 2100.  Consider the WAIS a big ice cube sitting in a pan of warm water on a very large stove.  The GIS is also an ice cube, but it sits in a collander on the stove.  Turning up the heat of the stove equally on both yield the following observation:  the GIS in the collander will melt more quickly initially due to convection of the collander, but once the water in the pot begins to boil the WAIS will simply disappear...

We know the currents around Anarctica have been warming.  These currents are penetrating far underneath the landfast ice shelves, speeding their demise.  The PIG & Thwaites glaciers, long considered the linchpins of the WAIS, have begun to collapse.  The decadal doublings of mass-loss of the GIS will likely occur as Hansen surmises, but once the WAIS really gets going, all bets are off.  For it's a really big ice cube sitting on a slippery slope being held in place by the buttressing effects of the ice shelves and basal pressures at the grounding lines.  And those grounding lines have already begun to recede back into much deeper waters...

As we all know, gravity is an unforgiving b*tch.

2011-09-20 02:50:43
Steve Brown

brownsg@gmail...
91.220.25.25

I discuss climate impacts relating to the risk of thermohaline circulation slowdown and a comparison with what we believe happened to it during the Eemian in the next part of the Last Interglacial series: http://www.skepticalscience.com/LIG4-3008.html

2011-09-20 10:17:58
Agnostic

mikepope_9@hotmail...
118.208.81.161
2011-09-20 15:30:41
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
66.96.105.248

Agnostic:

Hope I'm legible tonight. I'm at a sales conference and I'm 6 0r 7 beers into the night already ( I lost count, which rarely happens).

As far as the GIS and the WAIS and decadal doubligs of mass loss:  I understand where you're coming from.  Indeed, one has to start somewhere.  But my reluctance to accept the results of the analysis WRT the WAIS stems from the geomorphology of the underlying physical structures upon whiich both ice maasses rests. 

As previously stated, the WAIS is entirely a marine-terminating glacial sheet with its foundations laid upon a bedrock far below sea level.  It is only the combiiantion of the butressing ice shelves and the termianl grounding points which act to retard ice sheet collapse as it is.  Factor in a warming circumantarctic polar current which is penetrating underneath the butressing ice shelves and also causing the retreat of the grounding lines rearward into deeper waters off the shallowing glacial terminii points plus the downwards slopes from higher elevations towards the lower elevations and one has the perfect recipe for catastrophic collapse.  When I wrote the intro for the Where Have All the People Gone thread, I introduced the concept of the WAIS sliding into the sea "in the lifespan of a man".  Given the forcings already in place and the potentialitiies already discussed for Abrupt Climate Change, I may well have been conservative.  Can I prove this?  No.  Similarly, established science cannot prove that it CANNOT occur.

Compare and contrast this to thie situation WRT the GIS.  Yes, the central ice sheet proper has its basal points well below sea level.  But that abyssinial depth is plumbed by 3 primary outls:  Jakobshavn (the primary outlet in the SW sector) and Petermann and Zachariae in the North.  The encircling mountain ranges will act to restrict and limit the mass losses in the future.  So while the GIS may well be "limitied" to decadal doublings, and despite starting with a headstart over the WAIS, the WAIS is practically unlimited in its potential rate of collapse.  Once the demise of the WAIS truly ramps up, no force extant upon the globe can compare with its potential to change the climate of an entire hemisphere.  The SH will fluctuate wildly during the collapse, with spectacular departures of weather from climatological norms (the same will be true of the NH and the GIS, but to lesser degrees[per Hansen and Sato 2011]).

Anyway, I hope to be a little more sober and coherent tomorrow night for another reread.

Nite

2011-09-20 15:31:25One quibble
Glenn Tamblyn

glenn@thefoodgallery.com...
60.228.60.207

Having reread the post there is one area I would quibble over a bit. The exponential growth rates are taken from Hansen & Sato 2011 and as they rightly point out this will not be linear. However that does not automatically mean that it will be a consistent doubling rate over the whole century. To cover your back over the range of uncertainties associated with the fact that non-linear could be really non-linear, it might be worth trying to incorporate some error ranges on the graphs. Also perhaps a better link to the paper that leads them to the right section - Hansen & Sato are almost on a Gish Gallop of their own, covering a lot of territory.

I also recall seeing a paper somewhere - sorry, can't remember where - by some Australian researchers that did core work near the Ross Sea - not ice cores - going back over a million years. If I recall correctly it suggested that the WAIS became unstable back then and may have alternately collapsed and reformed over periods og 1000 years. I will see if I can find it

2011-09-20 15:55:26
Glenn Tamblyn

glenn@thefoodgallery.com...
60.228.60.207

Have a look at these

http://www.nature.com/nature/journal/v458/n7236/abs/nature07867.html - this is paywalled I'm afraid

http://www.eurekalert.org/pub_releases/2009-03/nsf-adc031809.php

http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1034&context=andrillrespub&sei-redir=1#search=%22ross%20west%20antarctic%20collapse%22

And looks like I was wrong, the team aren't aussies. Interesting result was that WAIS collapse speed was of the order of 1000 years when CO2 was 400ppm. Couldn't find any suggestion of rates at higher CO2 levels. Main finding is that collapse is driven far more by ocean temps than surface temps.

2011-09-21 13:02:54
Agnostic

mikepope_9@hotmail...
118.208.81.161

Glenn - thanks for references

2011-09-27 15:35:48
Agnostic

mikepope_9@hotmail...
118.208.110.198

Steve Brown – Thanks for the reference

I think the Gulf Stream is transporting heat north-east of Spitzbergen (S) into the EurAsian basin and that this is responsible for ocean warming, retreat of sea ice and erosion of permafrost on the continental shelf.  I can’t substantiate that view – yet – but the only other cause of Arctic Ocean warming along the shelf (1°C/decade) is seismic geothermal heat which seems less likely.  It may have contributed regionally - but ocean-wide??

It seems likely that the Gulf Stream is also transporting heat to the north west of S before sinking.  Evidence of this is destabilization of seabed clathrate west of S with venting of CH4 – mostly reaching the surface as CO2 after passing through a 400 m water column, though CH4 at surface may be increasing.

The Arctic Ocean is definitely warming as evidenced by retreat of sea ice, very rapid coastline erosion and loss of seabed permafrost.

Could it be that ocean warming to greater depth has resulted in the Gulf Stream penetrating further north before overturning?

Your article (well researched) points to orbital factors contributing to the Eemian maximum but those are not the driving factors now.  I argue that anthropogenic C02 emissions have triggered slow feedbacks, the most significant of which (Albedo, CH4 emissions) will bring about significant loss from the GIS.  This will result in freshening of Arctic and North Atlantic water and its cooling – at least of surface water.

Would this be likely to reduce or stop heat transport to the Arctic (over what period?) or merely push it deeper?

 

 

 

2011-09-27 15:37:48
Agnostic

mikepope_9@hotmail...
118.208.110.198

Glenn Tamblyn – I hear what you are saying and I am not arguing.  But the graph is only intended to indicate the effect of polar ice loss doubling per decade.  I do appreciate that over a decade – or more – ice loss could well have spikes and troughs – there is no way I can predict/quantify these – which is why I have selected a scale which doesn’t (hopefully) make it necessary to show them.  An indicative graph seems better than none at all.

Seems to me that greater accuracy is not called for to illustrate expected loss of polar ice as part of a broad-brush picture showing the causes of abrupt climate change.  Yes – I know its probably intellectual laziness but is it really necessary to be more precise in an area where I just don’t know – not even enough to produce reasonable error bars?

2011-09-27 15:40:13
Agnostic

mikepope_9@hotmail...
118.208.110.198

Daniel – Thanks for you points and comments.  Both appreciated.

I have thought about the matters you raise in the context of what I want to say about Abrupt Climate Change and what I do say about it.  As a result I have made amendments to put CH4 venting in context and corrected reference to H&S2011.  I have also made amendments to note the possibility of WAIS collapse this century but not amended Fig 2 to show that collapse. 

Fig 2 is meant to show the effects of ice sheet loss doubling per decade and does just that, nothing more.  I agree that collapse of the WAIS by 2100 will become increasingly possible – an event which would push projected SLR even higher but, is it likely to occur prior to 2100?  Andrill suggests it would take >1,000 years to occur.  I think it could occur before 2200 and think it may contribute to sharp SLR by 2100.  The essay does mention the possibility of WAIS collapse but it isn’t the main theme.  The main theme is conclusions reached by Hansen and Shakhova.

The comparison you make between GIS and WAIS is valid but may be more so were it recognised that Antarctic air temps are likely to remain colder than those of the Arctic where they are already rapidly warming.  And they are likely to get a boost from CH4 emissions.  This may contribute to GIS surface melting evidenced by falling altitude of the ice sheet, lake formation on its surface and their melting through ice to bedrock. Does this produce structural damage to the ice sheet?  Will the effects of gravity on 2,000-3,000 m high weakened ice hasten its collapse?

Another consideration is that air temps in the Antarctic are cold because of the influence of an immense ice covered continent and are likely to warm much more slowly than they will (are doing) in the Arctic.  “So what?” you might argue, “The southern Ocean is warming and that is what will bring WAIS undone”.  Very true but in the Arctic both the ocean and the atmosphere are warming, attacking the GIS on 2 fronts.  Worse the atmosphere is going to warm much more rapidly thanks to loss of sea ice facilitating sea/air heat exchange and, let us not forget CH4 emissions adding hugely to Arctic warming by mid century. 

You say “but once the WAIS really gets going, all bets are off”.  Quite right!  I fully agree but the danger from WAIS is not that it will slide into the sea.  Most of it is already in the sea – the danger is that large parts of it will loose their seabed footing and float, displacing their mass in water and producing rapid, very sudden SLR.  By contrast PIG and Thwaites drain some 30% of land based ice which, like the GIS, do not pose that danger.

You rightly draw attention to decline of the mixing layer.  Again, this is an important matter but maybe not one which is central to an essay trying to draw a broad picture showing why abrupt climate change is likely to occur prior to 2100 and what it might look like.

Your comments did make me think again about the rate and magnitude of CH4 venting this century and conclude that it is more likely that venting will increase rapidly and continuously throughout the century.  Arctic CH4 deposits nearest the surface are immense (> 1,700 G/t – and that is East Siberian shelf deposits only!) and, I agree with you, will be the first to be affected by permafrost erosion. 

I am assuming that over the next 90 years ~7% of these deposits are vented into the atmosphere - only 121G/t of CH4 but >8,000 G/t of CO2-e – more than enough for very abrupt climate change and Arctic warming sufficient to thaw enough GIS to raise SLR 2 or 3m – or more this century.

Larger near surface CH4 deposits undoubtedly exist on/under other parts of the vast Arctic continental shelf but get only passing mention in the essay, though I am concerned about the CH4 potential of the shelf under the Chukchi, Kara and Barents seas.  They may easily surpass those of the East Siberian shelf.  And then there is near surface yedoma in continuous permafrost covering 1 million km2 of Tundra (500 G/t?) plus what is held in deeper clathrate (>1,000 G/t).

In the essay I am trying to explain some of the things Hansen and Shakhova have tp say about global warming and slow feedbacks, drawing a reasonable conclusion that they are likely to result in abrupt climate change before 2100.  Makes me look a bit alarmist but so be it.  What I have written is what I think.  If what I think is wrong, I will change my mind and the essay and I will add detail to a broad picture if it makes the picture much clearer.

So, Daniel and everbody, is the broadbrush picture wrong?  Are we really looking at 5 or 6 m. SLR by 2100 or, what highly respected experts tell us, will be 1 or 2m at most?  Does the essay need more detail and clarification - probably does? – eg. I have not addressed the effects of CH4 accumulation in the atmosphere and how this effects oxidation, or the likelihood of EAIS ice mass loss – not subjects to be glossed over - as I have.  And, finally, I have cut out  another 807 words and added 251.  Do I need to shorten the essay further?  If so, what else should I cut out?

2011-09-27 22:41:30
Steve Brown

brownsg@gmail...
91.220.25.25

Agnostic - interesting questions, which I can't give you a definitive answer.  The Arctic is warming due to a number of contributing factors.  As you mentioned, there is additional warm water feeding from the Gulf Stream via the Norwegian Atlantic Current.  The Norwegian current has been observed to have undergone a rapid warming in recent years (http://www.cicero.uio.no/fulltext/index_e.aspx?id=3914).  If ocean water warms up you would expect a more vigorous thermohaline circulation as evaporation will lead to more brine formation and more overturning.  However, more evaporation also means more precipitation and more ice-melt freshwater flux, which would supress overturning - all very tricky to unpick.  Albedo-Ice feedbacks are certainly a major contribution on land and surface waters.  There is also the role of the Arctic Oscillation on determining the configuration of atmospheric Rossby Waves and the position of the polar jet stream, which can play a role in sucking up warm air from the South to the far North.   How the Arctic Oscillation works is again intertwined with how heat is transfered by ocean currents.  It's like a bloody great Gordian Knot.

2011-09-29 14:38:17
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
97.83.150.37

Thanks for taking my (somewhat alcohol-laced) comments in the spirit in which they were intended: constructively.

"The comparison you make between GIS and WAIS is valid but may be more so were it recognised that Antarctic air temps are likely to remain colder than those of the Arctic where they are already rapidly warming. "

Point taken, but consider that the primary threat to the WAIS and the EAIS is not from air temperatures alone.  The primary threat for those is from the warming circumpolar current surrounding the icy continent and gravity itself.

"So, Daniel and everbody, is the broadbrush picture wrong?  Are we really looking at 5 or 6 m. SLR by 2100 or, what highly respected experts tell us, will be 1 or 2m at most?"

Mike, I do not disagree with you on these points.  The primary issue I have is the threat from the GIS relative to the WAIS.  The GIS has a tremendous head start, but the completely unquantifiable (we are in unknown territory here due to the rates of change occurring globally) "capacity" inherent in the WAIS (due to the surrounding oceans and gravity) means that any head start currently in place could be "short-lived".  By "short-lived" I mean that possibly well-before 2100 the mass-loss of the WAIS could dwarf that of Greenland.  And by well-before I mean as early as mid-century...

"Does the essay need more detail and clarification - probably does? – eg. I have not addressed the effects of CH4 accumulation in the atmosphere and how this effects oxidation, or the likelihood of EAIS ice mass loss – not subjects to be glossed over - as I have.  And, finally, I have cut out  another 807 words and added 251.  Do I need to shorten the essay further?"

No shorter (Trenberth's was over 2,000 and I've done several almost that long).  CH4 accumulation & its attendant effects:  a brief (100-120 words) chapter or two should suffice (this could be the subject of a fine future article in itself).

As I write this I've been watching the baseball pennant races here in the US on TV.  ExxonMobil has had commercials in almost every break, indoctrinating the viewership with the need to develop oil sands to achieve lasting energy independance...

2011-09-29 16:32:16
Agnostic

mikepope_9@hotmail...
118.208.9.149

.... achieve lasting energy dependence .... would be more accurate.  There is only one thing more dangerous than stepping between a politician and the prospect of unbridled power and that is stepping between a fossil fuel business giant and the prospect of mega profits.