Hansen’s article in the current New York Review of Books gives a worst case historical rise of one meter per 20 years. How does this relate to your one meter per century number? Are they different events or different interpretations of the same event(s)?
Thanks for a timely answer. I think you could better state the problem with uncertainty, the way Hansen does: (hope you don’t mind a long quote, but I think Hansen states the problem as well as possible)
” Time constants: the slippery slope.
Three time constants play critical roles in creating a slippery slope for human society: T1, the time required for climate, specifically ocean surface temperature, to respond to a forced change of planetary energy balance; T2, the time it would take human society to change its energy systems enough to reverse the growth of greenhouse gases; T3, the time required for ice sheets to respond substantially to a large relentless positive planetary energy imbalance.
T3, the ice sheet response time, is the time constant of issue. I argue that T3 is of the order of centuries, not millennia, as commonly assumed. Growth of ice sheets requires millennia, as growth is a dry process limited by the snowfall rate. Ice sheet disintegration, on the other hand, is a wet process that can proceed more rapidly, as evidenced by the saw-toothed shape of glacial-interglacial temperature and sea level records.
It seems inescapable to me that the time constant T3 is measured in centuries, not millennia. I would be surprised if T3 exceeded 1-3 centuries. Ice sheet models will not be capable of providing a good assessment of T3 until they are driven by all anthropogenic forcings, incorporate realistically all significant processes and feedbacks, including those discussed above, and demonstrate the ability to simulate realistically rapid nonlinear ice sheet disintegration as occurred during meltwater pulse 1A.
The likelihood that T3 is comparable to T1 + T2 has a staggering practical implication. T3 >> T1 + T2 would permit a relatively complacent “wait and see” attitude toward ice sheet health. If, in the happy situation T3 >> T1 + T2, we should confirm that human forcings were large enough to eventually alter the ice sheets, we would have plenty of time to reverse human forcings before the ice sheets responded.
Unfortunately, T3 ~ T1 + T2 implies that once ice sheet changes pass a critical point, it will be impossible to avoid substantial ice sheet disintegration. The reason for this is evident in the definition of the time constants. The comparability of these time constants, together with the planetary energy imbalance, make the ice sheets a ticking time bomb.
If, as I have argued, T3 indeed is not very much larger than T1 + T2, it becomes of high priority to detect as early as possible beginnings of ice sheet disintegration. High precision measurements of ice motion and sea level change are needed for early detection of any acceleration in the global rates of ice movement and sea level rise.”
I was happily surprised to come across this very interesting article in the press that also covers this issue, and gives a nice flavor of what earth science field research is like (100 proof peppermint schnapps is also good for a quick warmup): LA Times report on ice sheet dynamics research.
The fact that models missed the dynamics issue, instead focussing on the latent heat of melting, should be a cautionary note for policymakers, as well as for students of the earth sciences. There may be other trigger-like booby traps in the climate system; the carbon trapped in methane ice clathrates and in the northern permafrost zones is also a cause for concern. I’ve also heard reports of anomalously high CO2 emissions over the past couple years; “sink exhaustion” could also be a concern – i.e., the possibility that the mechanisms that naturally remove CO2 from the atmosphere could become “saturated” or could themselves be negatively affected by climate change or human activities: see for example Letter on tropical deforestation to Science
Along the lines of models and data, this Brevia in Science is interesting. The authors consider the effects of multidecade trends in warming on the mid-troposphere by examing the ‘following northern summer’ effects of El Nino. Maybe this is more relevant to the previous article on the Walker circulation, though a warming mid-troposphere would also have effects on the ice sheets: Enhanced Mid-Latitude Tropospheric Warming in Satellite Measurements
The poleward shift of the jet streams would lead to less shear in the atmosphere; shear impedes hurricane formation. The widening of the tropical Hadley circulation due to warming – there seems to be an issue in whether or not this is captured in climate models. Increased hurricanes and altered rainfall patterns could have societal impacts, sure.
The Anasazi culture is widely appreciated for their beautiful cliff dwellings in the American Southwest – but those cliff dwellings were the result of brutal wars of extermination brought on by climate change and the resulting limitations of essential resources. The single ladder leading up was easily defended from raiders. Wouldn’t it be nice to avoid such scenarios?
This site really seems to be having a positive informational effect on media coverage of climate science.
I think ice sheet failure is the key issue in sea level rise, along with a less rapid rise due to thermal expansion of ocean waters. I also think that the significance of latent heat for snowmelt, which was described by Dunne and Leopold (1978):
“If water from moist air condenses on a snowpack, 590 calories of heat are released by each gram of condensate. This is enough energy to melt approximately 7.5 gm of ice, which when added to the condensate yields a total of 8.5 gm of potential runoff”,
Re #1: Hansen’s New York Review of Books article (supposedly a book review although he barely mentions the books in question) is referring to an event at the end of the last ice age known as Meltwater Pulse 1a (see this summary or the Science paper). This occured when the Earth’s ice caps were much larger and extended much further south, and with a 5 degree (C) rise in global average temperature. It is unlikely that melt rate could occur with today’s smaller ice caps and two or three degress of warming. But clearly, the IPCC 2001 projections for sea level rise in the 21st century were too low. I suspect the 2007 report will be different in this area.
Re #5: The Eemian is the previous interglacial period, about 120,000 years ago, similar to the one we are currently experiencing. Global average temperature was about 1 degree C higher than today, and sea levels were 4 to 6 meters higher.
re 8: Are you thinking of Marine Isotope stages which are marked in the north atlantic by D-O (I have to abbreviate, I can’t spell the names!) events? Eemian refers to a glaciation period in Europe (ISTR it’s been called the Ipswichian in lil’ol Britain). I think there is some difficulty in precisely relation the marine isotope stages with events in europe as the former were driven by glacier advance/melt in the Laurentide ice sheet in North America.
An interesting article but will it make any difference?
“And because greenhouse gas concentrations and ice sheet loss are effectively irreversible, policy decisions may need to be made based on the information in hand, which argues that deglaciation could be triggered by a modest warming.”
This equivocation may be suitable when addressing fellow scientists on a new hypothesis, but is it likely to rouse a sleeping public, or alter the policies of an intransigent administration? The ending ought to have read:
“Greenhouse gas concentrations and ice sheet loss are irreversible. Therefore policy decisions must be made based on the information in hand, which argues that deglaciation is being triggered by a modest warming.”
But it is not just journalistic skills that scientists are lacking. They also seem to be incapable of doing the engineering calculations which would show the desperate plight we are in. The speed at which the glaciers move into the sea depends on the resolution of only two forces: gavitation and friction. Friction depends on the coefficient of friction and the weight. With water perculating to the base of the Greenland ice sheet and reducing the coeffient of friction, the speed of the glaciers towards the sea will accelerate. But as sea levels rise and the grounding line of the Ross ice shelf retreats, the weight of the WAIS is being reduced as more of it floats. This will also reduce the friction and cause the WAIS to accelerate towards the sea. In other words, if one ice sheet surges, then it can cause all the others to surge too, just as is seen in paleo record.
Steven Schneider famously said “And like most people we’d like to see the world a better place, which in this context translates into our working to reduce the risk of potentially disastrous climatic change. To do that we need to get some broadbased support, to capture the public’s imagination. That, of course, entails getting loads of media coverage. So we have to offer up scary scenarios, make simplified, dramatic statements, and make little mention of any doubts we might have.” These doubts are now receeding, but under the leadership of Sir John Houghton the media and the public have been bypassed and the scientists have directed their message to the “opinion formers”, who are though to be civil servants and politicians. Rupert Murdoch, Bob Geldorf, and Bill Gates are ignored. It is time that the scientists tell the journalists the truth, scary or not. Better late than never.
[Response: Actually, the 'decisions may need to be made' was my editing oversight. Michael's final draft was written to say 'decisions need to be made' but I messed up between versions. I have corrected it above. -gavin]
Trying to fathom the glasier issue as a layman, I came to the view that there may be three rather distinct processes (or stages) at the various altitudes, latitudes and weather circumstances.
The dry process occurs in the interior areas, where temperatures remain solidly below zero at all times. Clean and dry snow ensures that radiation absorption is minimal, heat transport is by conduction only, and cracking of the ice is due to increasing weight and the ground slopes below. Overall, the glasier is growing.
The surface melt stage introduces many changes. Energy absorption increases at the surface. Energy transport to the interior is also provided by water running into the crevasses; re-freezing in the still cold interior releases the water latent heat. Re-freezing also provides mechanical strains due to expansion at water phase change. Uneven small area heating contributes more strain due to thermal expansion of ice. Ice is broken into smaller blocks, sharp corners are rounded. Re-freezing also welds blocks together in some cases. Speed of flow increases. Some water run-off occurs on the surface, also changing the weight balance locally.
At the wet stage the glasier internal temperature is close to zero. Liquid water penetrates through the cracks to the base and provides lubrication between ice and the bedrock. Friction is minimized throughout. Collisions between blocks cause further breakage and rounding of the blocks. Surface energy absorption remains high, heat transport to the interior is less efficient through water specific heat only (no latent heat release). Water run-off occurs both on the surface and through under-surface channels out to the sea, but local internal pools may also create hydraulic pressures. A rapid collapse is possible.
Wet stage occurring in areas where the base is below sea level brings some buoyancy forces into the play as well.
Some process to model, there being very little quantative observation data available.
I think the problem of sea level rising that we are facing is due to the ice sheet failure! If this continue persistently I am sure that our earth will submerged in water. We need to do something in advance so that we can save our earth!
Re #6: Thanks for the elucidation. To my layman’s eyes, it certainly makes sense that the rate of melting is, to a first order approximation, proportional to the surface area of the ice and to the temperature above zero (with both perhaps raised to a power not quite equal to one). I wonder if Hansen is engaging in a touch of hyperbole or if he just feels it’s prudent to assume the worst given that “model failure is the key issue”. As for his book review not being a book review, I can assure you as a long-time reader of the New York Review of Books that he has captured the style exactly – it grows on you after a while.
Does the breakup of an ice sheet such as the Larsen B contribute to increased sea level (other than what I presume is more rapid melting of the ice once it’s broken up and more of it is in contact with seawater)?
Re meltwater pulse 1a in #6: This occured when the Earth’s ice caps were much larger and extended much further south, and with a 5 degree (C) rise in global average temperature
It is true there was alot more ice available for melting at that time, but it is not true that this happened in conjunction with 5oC rise, at least not from what I thought I knew or what I got from your own links. The rapid melting happened over only 500 years and temperatures were not rising 1o/century.
So, more ice then but much faster temperature rise now makes it very plausible that we will experience similar SLR.
Jud, the breakup of an ice sheet such as the Larsen B does not contribute to increased sea level by itself, but may open up places near the land/sea boundary to allow meltwater runoff from land which may have been plugged to flow into the sea.
… referring to an event at the end of the last ice age known as Meltwater Pulse 1a (see this summary or the Science paper). This occurred when the Earth’s ice caps were much larger and extended much further south, …
I understand that glaciers extended much further south during the last ice age than now, but I don’t know what you mean by Earth’s ice caps were much larger then. Interior areas of Antarctic and Greenland have likely been accumulating ice since the Eocene.
Secondly, the deglaciation of the last ice age. By eye, I make the sustained rate from 11 to 8 ka to be about 1.75 m/century. The average is more like 1 m/century over the entire deglaction, and could be considerably more during isolated events, such as the apparent Meltwater pulse 1A. Is there a reason that people prefer to talk about the 1m/century average rather than some of the demonstrated fluctuations? Admittedly we have less ice now, so maybe a slower rise is to be expected.
I’ve seen claims that the Eemian deglaciation, which occured under considerably greater Milankovitch forcing than today, allowed sea level to peak ~4-6 m above present and occured significantly faster on average than the recent deglacition. Could someone comment on what is known about the rate of sea level change during the Eemian deglaciation?
Also, many of the projected temperature scenarios seem to be suggesting that Arctic climate could end up in a regime by 2100 that has not occured for millions of years. While one should definitely be sceptical of the ice sheet models, one also has to worry about whether we have appropriate paleoclimate analogs for the changes that could be coming.
Temperature of most important ice sheets on their surface and within, is not to my knowledge being measured on a grand scale, it would be difficult to model this important variable without some extensive observations.
May I point out an interesting display? This displays the current global ocean temperature anomalies. It is in good agreement with the others like it out there, but shows the data the best. http://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP/.EMC/.CMB/.GLOBAL/.Reyn_SmithOIv2/.weekly/.ssta/figviewer.html?map.url=X+Y+fig-+colors+land+-fig
The most intriguing sustained paradox has been the area around Greenland – it has been anomalously cold (the blue area), year-around, for as long as I’ve been looking at this chart, – over a decade, I recall. The unusually cool area, which used to be just along the south-east coast, is now a wide half-ring around the bottom of the island. This is in spite of the whole North Atlantic being anomalously warm.
A dynamically stable long-term anomaly like this must have a power source. I suspect the increased flow of glaciers â?? but it would have to be a sustantial flow increase. HAve we seen this? Or is there another cause?
Just like to note an implication out that S. Molnar missed. Hansen points out (I am sure that it not original to him), both in his NYRB review and in the Climatic Change essay, (God, Alastair is right in #12, two clauses, two partenthetical remarks before I even get near the point) that increased meltwater at the top of the ice can percolate to the bottom and lubricate the flow of the glacier. What SM misses, is that this means that more ice will flow into the sea AS ICE. Of course, absent an intervening Titanic it then melts, but even if it does not, having flowed into the oceans, it will raise their level by displacement.
Thus, the rate of melting beyond that which provides enough lubrication, is irrelevant to the amount of sea level rise.
“The most intriguing sustained paradox has been the area around Greenland – it has been anomalously cold (the blue area), year-around, for as long as I’ve been looking at this chart, – over a decade, I recall.”
One hypothesis would be that the increased flow of cold meltwater from Greenland’s glaciers is displacing warmer water from the Gulf Stream that had been warming Greenland’s southern tip. It’s one mechanism by which global warming would lead to regional, temporary cooling.
In any event, the question of one cooler spot on the globe pales next to the question of how fast, and how far, sea level is going to rise.
Re #23: I didn’t so much miss the implication as assume (perhaps improperly) that since the perimeter of a glacier is roughly proportional to the square root of the surface area and the thickness of the sheet tends to increase with more area, the effect scales up (roughly speaking) with more ice. Of course, that’s only true in cases like Greenland and West Antarctica where the edge is an ocean – an ancient glacier covering the northern half of North America would not disappear the same way. Actually, I’m still quite concerned about those magnitude 5 icequakes I asked about back on the “How much future sea level rise?” thread, but people tend to treat you like a nutcase if you ask whether a sudden ice discharge from Greenland can create a tsunami that will inundate New York.
A friend of mine recently pointed out that the removal of ice by melting through to the base rock is very similar to “tunnel gully erosion” — which I’ve noticed happening after forest fires on a restoration site in N. Ca. but is only described in the literature from New Zealand as far as I know.
Take a layer of loosely consolidated or fragile material (loess, pumice, or I suspect glacial ice) on top of a sloping harder material (rock).
Water cuts notches down through the overlying material then hollows out channels at the base that enlarge over time.
Slumping of the upper material into the channels removes more and more until penetrations through to the surface occur.
Collapse back to the ‘angle of repose’ occurs over and over as the material is eroded away at the base, so long as water continues to run.
Re #22, 25: This isn’t just “one cooler spot” on the globe – it’s the critical interface point between the Gulf Stream, the start of downwelling for the THC, and Greenland itself. What happens here can influence climate far afield.
For one thing, enough freshwater (if that’s the cause of the temperature anomaly) will definately affect the THC.
More importantly, from the scientific point of view, this should be a really good way to cross-check the recent satellite observations of the Greeenland icecap; “simply” (I know this is a loaded phrase) measure the temperature, salinity, and isotope changes in the water, and with a little bit of math we can say, “yes, here it is: prediction confirmed; call the press conference” or, “it’s a no show; better fix that &!@#@$ model”
My point was simply that there could be significant sea level rise by ice moving from the Greenland ice cap without actually melting the ice cap. We appear to agree on that. Yes, there are currently only two such ice caps, but one of them appears to be a LOT more subject to the process. If this is indeed the case, everything else is second order.
If the rate discharge into the sea increases because of bottom lubrication you actually need little direct melting.
Re #27: Ice does not behave like that, at least on a small scale, until it’s warm enough to turn to slush, tending to move in discrete chunks instead. I don’t think either of these analogies would necessarily be very helpful in considering the movement of Greenland’s glaciers due to the way in which the ice there is squeezed into outlet glaciers as it moves toward the coast.
Re #17: Coby – Meltwater Pulse 1A occured after about six thousand years of sustained warming. It is not accurate to only look at the 500 years during which it took place. But it was also not accurate to count the entire 5 degress of warming that preceeded the interglacial, as I did. The best information I can find (this Science paper) suggests 2 or 3 degrees of warming before it took place.
So there was a lot more ice, it extended much further south, and there was a similar degree of warming, though not nearly as rapid warming as is happening now. I do not think either Greenland or Antartica could produce a rate of sea level rise equivalent to mwp-1A, but there could well be very significant melting events in the next few centuries.
…. In an influential paper published in Science, Zwally surmised that the ice sheets had accelerated in response to warmer temperatures, as summer meltwater lubricated the base of the ice sheet and allowed it to slide faster toward the sea.
In a way no one had detected, the warm water made its way through thousands of feet of ice to the bedrock â�� in weeks, not decades or centuries.
So much water streamed beneath the ice that in high summer the entire ice sheet near Swiss Camp briefly bulged 2 feet higher, like the crest of a subterranean wave.
“This meltwater acceleration is new,” Zwally said. “The significance of this is that it is a mechanism for climate change to get into the ice.”
….University of Texas physicist Ginny Catania pulled an ice-penetrating radar in a search pattern around the camp, seeking evidence of any melt holes or drainage crevices that could so quickly channel the hot water of global warming deep into the ice.
To her surprise, she detected a maze of tunnels, natural pipes and cracks beneath the unblemished surface.
“I have never seen anything like it, except in an area where people have been drilling bore holes,” Catania said.
No one knows how much of the ice sheet is affected.
—— end quote
Bi-Polar Outlet Glacier Acceleration and the Jakobshavns Effect
It is correct that glaciologic models do not work well enough to model the system correctly. Having used these models for 20 years I realize the uncertainties. A key problem is knowing the specific boundary conditions at the base of the ice sheet, and the temperature gradient in the lower section of the ice. These two items control much of the sliding and internal deformation. On the other hand the basic physics are understood. Thus, a rapid breakup of a marine based section of an ice sheet and its ice shelves have long been recognized as a scenario that has and can happen. The rapid breakup is borne out in the glacial geologic record for the rapid deglaciation via Hudson Strait of a Marine based ice sheet. Meanwhile land based ice sheets even with fjord connections such as the Cordilleran Ice Sheet along the BC and SE Alaskan coasts did not disappear very quickly. Nor of course did the main portion of the Laurentide Ice Sheet ending in the United States and retreating slowly north. This understanding does include the ability of a thin ice shelf such as the larsen Ice Shelf to rapidly breakup, though certainly not quite as fast as it did.
Let us be clear the Greenland Ice Sheet is not marine based, and the speedup of a few outlet glaciers albeit draining a noticeable portion of the ice sheet, does not physically lend itself to a rapid collapse of the Ice Sheet as some have suggested. This Ice sheet more resembles that of the Cordilleran Ice Sheet and the Scandanvian Ice Sheet neither of which disappeared all that quickly. It is not like the West Antarctic Ice Sheet which is more like the Innutian Ice Sheet of the previous ice age over the Canadian Arctic, which did disappear quickly.
So does the bi-polar acceleration of key ice sheet outlet glaciers that has been observed in the last decade pose similar possibilities. Pine Island and Thwaites Glacier West Antarctic Ice Sheet (WAIS) and Helheim, Kangerdlugssuaq and Jakobshavns Glacier in Greenland. Twenty years ago Terry Hughes proposed the Jakobshavns Effect (JE). The JE as explained by Hughes (1986) results from an imbalance of horizontal hydrostatic forces at the grounding line. With positive feedback mechanisms that sustain rapid ice discharge: ubiquitous surface crevassing, high summer rates of surface melting, extending creep flow, progressive basal uncoupling, lateral uncoupling, and rapid iceberg calving.
Are the recent outlet glacier accelerations indicative of the Jakobshavns Effect at work is the reduction in back stress allowing the ice to be pulled out of the ice sheets, or is reduced basal coupling solely enhancing basal sliding? The following summarizes some key recent findings.
As noted by Sterns and Hamilton (2005) Kangerdlugssuaq Glacier and Helheim Glacier, two fast-flowing tidewater glaciers in South-East Greenland, accelerated between 2001 and 2005 and retreated 3-5 km since July 2003. Together, the catchment basins of these two glaciers encompass ~10% of the area of the Greenland ice sheet. Helheim Glacier was flowing at ~8 km/yr in 1995 and 2001. In 2005, flow speeds were ~11.7 km/yr, a ~40% increase. The acceleration of Kangerdlugssuaq Glacier was more substantial. Portions of the main trunk that were flowing at ~5 km/yr in 1988, 1996 and 2001 were flowing at ~14 km/yr in summer 2005. The acceleration of these glaciers was synchronous with a rapid retreat of calving fronts, which had been stable, and a lowering of the ice surface by about 100 m. The rapid thinning, acceleration and retreat of these two relatively nearby glaciers suggests a common triggering mechanism
de Lange, de Lange, Murray, Luckman, Hanna (2005) investigated the reasons behind the speed-up of Helheim Glacier. The results showed a dramatic increase in velocity during 2002/2003 but not during the 1990′s, although the glacier thinning rate observed by NASA of around 1.5 ma-1. They conclude that the recent speed-up does not coincide with high runoff, suggesting no direct link between surface runoff and velocity during the 1990′s or the recent speed-up event.
The Jakobshavn Isbrae has long been viewed as the fastest sustained tidewater glaciers in the world. After nearly 50 years of stability (Pelto, Hughes and Brecher, 1990) a remarkable retreat of the ice front and an increasing flow velocity has been noted (Dietrich, Maas, Baessler, Ruelke, Schwalbe, 2005). In August 2004 they determined the flow velocity of the ice front and last 5 km of the glacier. They obtained velocities range up to 40 m/day, a dramatic acceleration from the 20-23 m/day that had been observed over a sustained period. (Mayer and Herzfeld, 2005) observed that in 2002, this ice stream with a 12 km long floating tongue, suddenly entered a phase of rapid retreat. The ice front started to break up, the floating tongue disintegrated, the production of icebergs increased. Thomas (2004) argues for acceleration via back force reduction. The thinning and acceleration of the glacier immediately following calving of about 4 km of its 15 km floating ice tongue, suggest that acceleration may have been initiated by the calving. He assumes that the force perturbation associated with such weakening is swiftly transmitted up-glacier, to a maximum point 10 km upglacier. The conclusion is that the initial observed changes in flow are consistent with the comparatively small perturbation associated with the calving. And would afterwards be sustained by thinning of the remaining ice tongue.
Rignot (2002) has observed recent large changes on Thwaites and Pine Island Glaciers draining the WAIS into the Amundsen Sea. These glaciers because of their size and lack of large buttressing ice shelves have been referred to as the weak underbelly of the WAIS (Hughes, 1981). Rignot (2002) documents an 18% acceleration of Pine Island Glacier during the 1992-2000 period over a 150 km reach of the glacier. The glacier has thinned at 1.6 m/a and the grounding line retreated 5 km between 1992 and 1996. For Thwaites Glacier Rignot (2001) observes a 1.4 km retreat of the grounding line from 1992 and 1996 indicating a 1.4 m/a thinning of the lower glacier. There is insignificant surface melting on these glaciers, thus basal sliding cannot be enhanced via this mechanism. An examination of the forces acting on these glaciers by Schmeltz, Rignot,DuPont and MacAyeal (2002) led them to conclude that a reduction in buttressing due to a decrease in ice shelf area could cause a 70% acceleration, while basal shear stress would only increase it 13%. Shephard and others (2001) observed an inland thinning and acceleration of Pine Island Glacier that led them to conclude this glacier was responding to enhanced glacier bed lubrication. On Thwaites Glacier there is no reported acceleration above the grounding line.
We return to the key question is the force driving these accelerations simply due to basal shear changes or to the more comprehensive change in dynamic forces of the Jakobshavns Effect? But even if we do the Greenland Ice Sheet based on glaciologic principles just is not capable of rapid collapse.
Steve, “ice doesn’t behave like that” unless covered with surface water and slush apparently — see field reports via the first link in resp. 3 above; I tried a longer excerpt post but it isn’t showing up; so this short one:
“…University of Texas physicist Ginny Catania pulled an ice-penetrating radar in a search pattern around the camp, seeking evidence of any melt holes or drainage crevices that could so quickly channel the hot water of global warming deep into the ice.
To her surprise, she detected a maze of tunnels, natural pipes and cracks beneath the unblemished surface….”
In the _New York Times_ today there was a very interesting article on geoengineering to counter the effects of climate change emissions. It is entitled “How to Cool a Planet.” In this article it was explained that there is a great of opposition/skepticism in the scientific community to studying geoengineering approaches to global warming. I suggested in a thread on RealClimate that geoengineering approaches to climate change should be explored. My suggestion was rejected out of hand. In light of the planetary emergency we are currently facing, is such opposition/skepticism not dangerous?
As pointed out in the piece that initiated this current thread, “ice sheet loss[es] are effectively irreversible.” We know that global warming is occurring and accelerating. The solution currently being embraced/championed by most scientists and the environmental lobbying community is not working (i.e., the reduction of greenhouse gas emissions). Looking at the U.S., China, and India, we know that global climate change emissions are set to increase. It should also be stressed that it is very possible that humanity has already put enough greenhouse gasses into the atmosphere to trigger catastrophic warming, and it is only a matter of time before this full warming occurs.
We have modeled business-as-usual enough! It is time to thoroughly model and explore geoengineering responses to our planetary crisis. The failure to do so will likely lead to our collective demise.
Comment by George A. Gonzalez — 27 Jun 2006 @ 11:41 PM
I read the same article. It seems to me that it is difficult enough to make projections of future climate without adding other large scale perturbations. Of the methods proposed, the only one that seemed even faintly plausible to me was the injection of aerosols in the stratosphere. This at least has some observational support from what we have seen happen from very large volcanic eruptions. But allowing greenhouse gas emissions to rise at an accelerated rate and at the same time trying to counteract their warming effect with such a mechanism seems like a hire wire act with the state of the world in balance. So, geoengineering on a large scale without first trying to control emissions seems like a bad idea. Also, if you think it is difficult to get international agreement to limit emissions, imagine how much more difficult it would be to agree on such projects. Perhaps modest measures along these lines could be tried in the future as an adjunct to emission controls, but not as a substitute for them.
Geoengineering to block sunlight won’t slow the acidification of the ocean as CO2 increases. Collapse of the ocean food chain by about 2100 is the near term problem, little discussed because it’s unthinkable.
I’d like to see Paul Creutzen invited here to discuss his suggestions.
OK. So rate of ice movement and loss is the â??criticalâ?? issue of the moment.
With present poor coverage of glacier movement and ice loss by conventional systems, is it not time for IPCC to seek a UN mandate to deploy a vast array of sensors so that we â?? the people of the earth â?? can get a clear idea of what is happening to our planet.
The design of a simple probe to deploy would be trivial.
Probe say 1.5m long in a bit of aluminium or HD PVC pipe. Three 1-wire temperature chips – bottom (for â??deepâ?? temperature) 1m up (for â??surface ice temp) and top (for ambient temperature). Add a GPS encoder for location data, a timer, a unique ID chip, a battery, and a satellite phone dialer. All these components come off the shelf. The probe would be configured to float top-up, so when the ice melts we then get sea temperatures and ocean current information until the battery dies.
Get the navies of the world to deploy these using helicopters flown of boats, or whatever. With a simple electric drill a guy stands on the skid of a helicopter, drills a 1m deep x 25mm diameter hole into the ice, and drops in the probe, and flies to the next site. A helicopter could deploy 100 probes in a day.
Within a few months we could have 10,000 to 100,000 probes monitoring ice conditions world wide.
Within a year we would have good data and a very strong story to tell.
Re: comment #34: Thank you, Mr. Pelto, for a learned review. Would you care to comment on the observations of accelerations in ice streams following the Larsen shelf collapse ? I believe these results were from the British Antarctic Survey.
The LA Times article (link in #3) mentions sea level in relation to the ice cap twice; apparently Greenland like Antarctica is actually several islands:
“Mile upon mile of the steep fjord was choked with icy rubble from the glacier’s disintegrated leading edge. More than six miles of the Jakobshavn had simply crumbled into open water.”
“… The ice is so massive that its weight presses the bedrock of Greenland below sea level, so all-concealing that not until recently did scientists discover that Greenland actually might be three islands.”
Thanks for the informative post RealClimate. I’ve read the Hansen paper and he certainly is a very charismatic writer. However, as he points out, he is no glaciologist and so I wondered if any of arguments he puts forward in the piece are patently wrong?
If not it seems to me that he makes a very strong case for framing the argument in terms of uncertainty rather than minimal 21st Century sea level rise as suggested by the IPCC.
Geoengineering of the type and scale discussed briefly in #36 was somewhat dismissed in #37 for reasons that are readily accepted when one considers trying to counterbalance increasing energy consumption and its emissions with injecting aerosols into the stratosphere. Sounds like a tail-chasing venture and one that the international community will debate forever.
However, Mr. Gonzalez has a valid, larger point, if not the details, to challenge RC contributors.
Assume base-case projections will be scrapped when positive feedback is finally detected and accepted. What then? Does the developed world pull up the ladder and focus on survival? That is a likelihoood where borders are essentially closed by wide seas. But the Ganges Delta, for example, has no borders and provides topsoil and agriculture for about 400 million people and two of its neighbor nations have nuclear arsonals.
My view of geo- and bioengineering has everything to do with rapid deployment of highly drought, pest and heat resistent crop varieties and and augmented water supplies, (please, no disucssion on nuclear powered desalination plants) new and effective vaccines stockpiled in strategic sites around the world, a new look at forestry management in a warmer, drying world and an international effort to find a truly rational substitute for gasoline and diesel powered vehicles that does not require turning our food producing topsoil into fuel refineries.
When the engine room damage report comes up to the bridge, we expect the captain to have a plan in mind to save the crew. That is not the time to experiment with a plan to flip the ship over so the water does not come in through the torpedo hole.
Adaptation is not at the cost of mitigation efforts. It is a part of getting the next generation prepared for the destruction we have brought to their lives.
Comment by John L. McCormick — 28 Jun 2006 @ 6:58 AM
Geoengineering and increasing climate change gasses are both dangerous. It needs to be stressed, however, that political and economic elites have refused to abate climate change emissions. (For a discussion of why see Gonzalez, George A. 2005. “Urban Sprawl, Global Warming, and the Limits of Ecological Modernization.” _Environmental Politics_ 14, no. 3: 344-62.) In light of this, it appears that a rational course of action for the scientific community is to explore/analyze the plausibility, effectiveness, and risks of geoengineering responses. In the current context this may be scientists only course of action.
Comment by George A. Gonzalez — 28 Jun 2006 @ 8:36 AM
I’m just curious what reception James Hansen’s recent New York Review of Books article has had (as well as earlier articles of his that argue similarly). It seems his main purpose is to get us to take seriously, by way of historical analogy, the possibility of a rapid sea level rise, such as 1m every 20 years. I have little sense of whether Hansen’s argument is representative of increasing alarm among the majority of climate researchers, or is still at the moment judged to be an outlier position among climate researchers. Any comment?
Also, can anyone recommend a book on glaciology for newcomers–e.g. an Intro to Ice 101? Ideally such a book would explain the difference between ice sheets, ice shelfs, glaciers, icebergs, ice streams, ice caps, ice domes, sea ice, ice floes, etc.–to the extent that these are different (perhaps some of these phrases are synonyms?) E.g. I’m dimly aware that it is significant that the WAIS is anchored below water, as this would make it easier for it to come unanchored and slip away (right?) But what portion of the WAIS rests on land below seal level? Just the outer edge of it? And at what depth at the deepest? To what extent does this matter (viz. portion and depth)? Are we talking about just the portion that is marine anchored breaking off and slipping away, or is there a chance it would take the rest of the WAIS with it? I find that hard to imagine. But as I said, it would be nice if there were an accessible book to introduce the WAIS, GIS, etc. to newcomers (Or even a textbook for science undergrads, which I could probably understand.)
Elaborating on #40 Ice shelves buttress the glaciers that feed them slowing them down considerably. This has long been understood. However, our first chance to observed this directly is with Larsen B. Satellite images from before, during and after the break-up of the Larsen B Ice Shelf in March 2002 illustrate the acceleration. To feeder glaciers Crane Glacier and the Hektoria-Green-Evans Glacier sped up. Crane Glacier increased from 1.7 meters/day to 3.1 meters/day in April through December of 2002, and then to 4.1 meters/day between December 2002 and February of 2003. (Source)
With respect to #41, takes me back to my first job as a scientist, mapping the ocean floor in front of Thwaites and Pine Island Glacier back in the early 1980â??s. These glaciers have relatively limited floating tongues, that are really just extensions of the land based glaciers. They have generated fairly substantial deep troughs immediately beyond the glaciers. This suggests to me a more limited circulation of water beneath these ice shelves than others. However, we have no real measurements. The sliding is being enhanced on Pine Island Glacier and there is no surface melting, but nor is there a connection to the ocean on the land based section. Under the ice shelf, warmer water would enhance melting, which would thin the tongue, which would reduce the buttressing and allow acceleration. But the bottom of the glaciers are truly a land of unknowns.
When might the collapse of these sheets will begin?
Excerpt: … The business-as-usual scenario, with five degrees Fahrenheit global warming and ten degrees Fahrenheit at the ice sheets, certainly would cause the disintegration of ice sheets. The only question is when the collapse of these sheets would begin. The business-as-usual scenario, which could lead to an eventual sea level rise of eighty feet, with twenty feet or more per century, … http://www.nybooks.com/articles/19131
There is geoengineering to remove CO2, which is a dubious proposition energetically but which would be absolutely fantastic news if someone could somehow get it to work.
And there is geoengineering to deflect solar radiation. My impression is that it’s almost certainly a bad idea. If someone could get that to work, it could prevent the mean surface temperature of the earth from rising, but it could not balance out the local forcings (nor the chemical disequilibrium in the carbon cycle, as Hank Roberts has already pointed out).
Anthropogenic global change is the problem, and global warming is only a symptom.
The idea of crudely cancelling out a growing perturbation with another one seems to miss the point, as well as being fraught with peril, like treating alcoholism with aspirin.
As symptomatic relief, the cost of colossal panteary beach umbrellas of various sorts should not be compared with the cost of reducing emissions or the cost of increasing sequestration. If a case can be made that such an action would protect the ice caps preferentially, for instance, its cost might be compared against the cost of sea level rise. But stabilizing greenhouse gas concentrations stabilizes the system. Adding a contrary perturbation does not.
We’d still be kicking the climate system with increasing perturbations, and we’d be moving farther away from any paleoclimate analogs, so we’d be increasingly less certain of what we were doing. And if people took this as a license to keep emitting greenhouse gases, the solar perturbation would have to keep increasing apace, continually increasing the risk.
Global mean temperature is a symptom, albeit an important one. Nobody lives in a global mean climate, so stabilizing global mean temperature isn’t in itself a useful goal; it in no way guarantees an end to anthropogenic climate change.
Perhaps this is another side effect of the use of the inappropriate name “global warming” to describe our problem. It would certainly be not just a tragedy an absurd tragedy if global warming in a literal sense, i.e., mean surface temperature increase, were eliminated at great expense while anthropogenic perturbation of the earth system would continue to accelerate, simply because we decided to name our problem “global warming”, confusing the symptom for the disorder.
As usual Michael says it all very well. Sequestering CO_2 is a much more serious possibility, at least as a short term strategy, if someone can figure out how to do it and our leaders were to pursue it. It is unfortunate that while sequestering CO_2 is often cited as a possibility tied to increasing use of coal, few if any actual proposed coal plants take it seriously. For the present, it seems more of a cover for increased use of coal without CO_2 emissions controls, which is actually the worst thing we could do.
I object to the notion that geoengineering would give the political and economic elite a license to continue fostering the emission of dangerous amounts of greenhouse gasses. They already have such a license, and are exercising it.
My point in invoking geoengineering is to get a discussion on it going, and to draw on the expertise of the participants/readers of this list to gain a better understanding of the potential ramifications of geoengineering as a response to climate change emissions. In this regard, Mr. Tobis’s commentary is welcome and appreciated.
I, nonetheless, believe that issues raised by geoengineering should be subjected to scientific treatment and rigor. It is my estimation that this is the next logical step of climate science with respect to climate change.
Comment by George A. Gonzalez — 28 Jun 2006 @ 5:33 PM
Concerning the possibility of meltwater percolating to the base of the Greenland icesheet, it is instructive to look at Greenland with Google Earth. Check the area, for example, on the west side at about 68 deg N, 49 deg W, from an eye altitude of 50-100 mi. You will see an incredible number of meltwater lakes. You can track them for miles inland from the western edge of the ice, and northward to some 75 deg N. Zooming in, they are accompanied by structures that appear to include sinkholes, crevasses and even streams between the lakes. With these structures in a context of shifting, fracturing and moving ice, it seems inevitable that water would be reaching the base.
The correlation between solar activity and the apparent heating and cooling of the Earth seems obvious. And, there seems to be a recent uptrend in solar activity that readily explains the apparent 20th warming.
Unless this data can be explained and modeled, I don’t see how any climate model can be taken seriously. Seems like certain scientists are cherry picking their model parameters to fit thier politics.
Emboldened by the reponse to my previous post, thank you Gavin, I thought I might grace you with my later thoughts.
They are dominated by by reading Jim Hansen’s review, and his apology for not helping Al Gore earlier. I agree with nearly everything Jim wrote, but I do feel that it is too little too late :-( Jim seems to think that we still have ten years in which to act. But the Greenland glaciers have begun to melt and that melting is accelerating. They have not collapsed, but now it is inevitable that they will.
Mauri argues in his first post that rapid collapse of the Greenland ice sheets is not posssible, but that is based on the premise that they are mountain glaciers which have melted slowly in the past. In his opinion, it is only low lying ice sheets, such as those that covered Canada that collapse suddenly. However, it seems to me to be more likely that sudden collapse is caused by undermining by the sea. That would have applied to the Canadian sheet, where much of the land was isostatically depressed and the rest, such as Hudson Bay, is still below sea level. Greenland may be three islands, and so its ice sheet culd also be undermined by the sea when melting begins. In fact the rise in level of 2 meters caused by melting must be due to the ice sheet floating.
Mauri concludes his second post with the sentence “But the bottom of the glaciers are truly a land of unknowns.” My point is that whether we have ten years before the Greenland ice collapses is therefore also an unknown.
… almost everywhere they looked, glaciers started to retreat about 17,500 years ago, …
The only place that does not fit the warming pattern they observed is Greenland, which lagged about 2,500 years behind the rest of the planet in starting to thaw certain glaciers. …
If you are going to rely on paleo to sort this problem, you have an unlimited choice of models to choose from ranging from creationist to balance theory. WHICH DO YOU PICK?
[Response: Simple. You pick the ones that are based on sound physics which has been tested in field observations and laboratory experiments. That knocks out the "creationist" models, whatever that may mean. Then you look at how well the models reproduce global mean temperatures, seasonal cycles, patterns of rainfall, etc., and how well they reproduce the 20th century climate trend. Maybe you also see how well the models reproduce the last glacial maximum, for which the climate forcing is pretty well known. Once you've done all that, you're still left with a spread of forecast temperature increase, all of which should be considered in the realm of possibility when assessing risk. The situation for ice sheet models is more problematic, since they're less easy to test. However, the same principles apply. --raypierre]
Jim seems to think that we still have ten years in which to act. But the Greenland glaciers have begun to melt and that melting is accelerating. They have not collapsed, but now it is inevitable that they will.
After Greenland, there is the WAIS. After the WAIS, there is the EAIS. Preventing the melt of the WAIS though GHG emissions cuts is still feasible. The need to plan for the melting of Greenland’s Ice Sheet(*) does not mean we should throw up our hands and abandon GHG emissions cuts. I am not convinced that any of the geo-engineering schemes mentioned in this thread (assuming they work) could allow us to abandon GHG emissions cuts. So I’m opposed to the constant repetition of the notion that it is already too late to act, or that it will someday be too late act. There is no point at which more GHG emissions will not make matters worse.
(*note: I believe the need to plan for the melting of Greenland is real, even though I think avoiding the melting of Greenland is still possible, if only barely.)
Someone told me that Kadhafi had at some point had a grand plan to build a canal and detour water from the sea into the low-lying areas of the Sahara desert. Would anyone like to comment on this as a possible response to significant sea-level rise?
Boreholedate from Greenland show, that the temperature has been higher there, for most of the holocene, as far as I understand these graphics.
How was the sealevel behaving 1000, 2000 and 8000 Years ago.
The glaciers in Europe (Alps) have retreated 2000 Years ago 300 Meters more than today, for a long period, so that trees have grown there and farmers have raised there cattle on the “alms”.
What do we know about the sealevels of these times?
Thanks for your hard work here!!!
[Response: What Greenland graphic are you looking at? Do you mean oxygen isotopes or thermometry based on the downcore temperature profile? Neither one gives support for your statement that Greenland temperature has been higher for most of the Holocene, so the premise of using the Holocene to test ideas about warming and sea level isn't very promising. That's why Hansen uses previous interglacials -- some of which were warmer than the present time -- to give an idea of how much Greenland and Antarctica could melt in warm climates. We're probably not yet warmer than the warmest interglacials of the Pleistocene (roughly the past 2 million years, but unless sensitivity comes in at the very low end of the range, if we double CO2 we're almost certain to exceed the temperature of the warmest interglacials. That's what has Hansen worried about sea level. --raypierre]
The author in Sciam made a hypothesis, that men had induced the global warming 8000 Years ago. I’ve seen that graphic already years before.
As far as I know, the glaciers of Europe had nearly totally disappeared some 6000 years ago.
What I mean, is, that if we consider Greenland alone, or the Northern Hemisphere, the ice must have melted in the same way than today.
In Sciam, I also read about the most probabel models predicting a rise in temperatures of 3° C (from 2 to 4) for a doubling of Co2. I think you have been asked as well as Mr. Rahmstorf, closer to my country. Is that still your opinion today?
But, does a rise of 3° C over a rather short? period have the same effects as a whole warm period.
Is it true that the Eemian warm period was perhaps 5° C above todays temperatures?
I hope you excuse my very bad english writing ;-)
And thank you for the very quick answer!!! :-)
[Response: Estimates of Eemian temperatures look to be about a degree or so warmer on average. Because of the orbital configuration, you might have expected NH summers to warmer still (possibly up to 5 deg locally, but you'd have to look at the primary literature for examples if they exist). The response to 2xCo2 is just a metric used to compare models (and 3 degrees is about right), estimates of what actually may occur range from 1.5 to 5 deg C by 2100. -gavin]
“… a massive climate shift to a cooler regime that occurred just over 5,000 years ago, and a more recent reversal to a much warmer world within the last 50 years….
“… in most of the world, glaciers and ice caps are rapidly retreating, even in areas where precipitation increases are documented. This implicates increasing temperatures and not decreasing precipitation as the most likely culprit.
“… Ohio State University’s Byrd Polar Research Center and three other universities combined the chronological climate records retrieved from seven remote locations north and south of the equator.”
Re #66: I had never seen Greenland ice core temperatures presented for this time period, and I am rather surprised. I spot checked the graphs against the NOAA reference data, and they look correct.
I think it is misleading to claim temperatures 3,000 years ago were three degress warmer on the basis of one or two hundred year peak. The key unknown is the time for the ice cap to reach equilibrium for a given temperature. Lets take that to be one thousand years. Hansen thinks it is less. Mentally running a thousand year smoothing filter over the Greenland data, I get a one degree difference. From what I have been able to find from paleoclimate data, he equilibrium temperature rise per degree C (global average) is about 6 meters. But we are now comparing Greenland local temperatures with the global average. Using another approximate relationship, which is 2.5 degrees of Arctic warming per degree global average, I get 6/2.5 = 2.4 meters that sea level should have been higher 3,000 years ago.
This is all very approximate, and I would love to see this simplistic calculation corrected, but it looks to me there should have been a significant sea level difference that does not seem apparant in the paleo data.
I’ve read a few things about the glaciers of Europe (I like skiing, and I would cry if the snow would not come in wintertime), that tell me that our glaciers behave like they did 1000 years ago and 2000 years ago. So this must not be so exceptional. But the temperature is rising quickly …!!!
But on the map shown in your last link I see a glacier close to Punta Arenas. This link
Remember, while you have ice, the temperature measured stays at 32 degrees the whole time the ice is melting — the energy’s going into separating the molecules of water (melting) although the thermometer won’t change til the ice is gone. I don’t know where the GISS temps are measured but you say you picked sites around glaciers, so look at the averages. Look also at the Ohio State paper linked above that documents tropical ice cap melting.
FWIW I just did a Q&D numerical version of this smoothing and came up with -31.94 C for 0-1kybp and -30.30 for 3-4kybp, which is more like a difference of 1.6Â°C. The plot was average of the temperatures aggregated against floor(kypb). I can post the plot if anyone is interested.
I have tried to pick out important temperature series from places, where global warming can be felt or seen, like the Alps or Skandinavia or for example Greenland. We also have a station in Luxemburg, that shows a warming according to the global temperatures.
I was born in 1961 and I love the snow. The warm winters I’ve lived when I was in highschool made me depressed and I was happy baout our cold winter 2005-2006, making me hope to live some more happy years … ;-)
I have looked at all longtime temperatureseries in the Nasa-records and the only ones I found really extremely rising were the Japanseries. I got an E-mail-friend there, who told me about that.
The temperatures around Antarctica seemed to be the ones with the least rise for the last 100 Years, like for example Punta Arenas about which I read a book (canoe).
Environment protection is very important for me, but I hope that the predictions of the models are wrong ;-) because I like my Alps and Switzerland like they are ….
I am not a scientist, but I “believe” in Science. Let’s hope that this time they are wrong ;-) (like R2D2 sometimes)
If any of you have links to free papers like for example the glacier-report I would be very grateful.
But thanks again for sharing your precious time with me!!!!! :-)
Re #71: Richard, I would like to see your graph. The thousand year figure for ice caps reaching equilibrium with temperature is rather arbitrary, so other values such as 500 years or 2500 years might also be interesting.
This data leaves me with the impression that equilibrium takes longer than a thousand years, contrary to what James Hansen is saying.
Eddie, see this thread, it may explain why you need to do the analysis rather than look at one station’s few data points to determine whether there’s a trend. Esp. response 11. http://www.realclimate.org/index.php?p=231
I’ve read the whole thread and again looked at many series in the Northern Hemisphere, where most of the data comes from. What striked me again is that, the longer the time period of records, the less the warming of today appears to be soo exceptional.
We go back to 1880, that was a rather cool period here in Europe, as far as I know. I try not to be cherrypicking, but only considering longtime records. I think that’s the only reliable way to compare.
There are many stations that go from 1960 to 1990 for example. How can they be part of it? The same happens with stations from 1990 to 2006. If there is a huge warming, how was it 100 years ago?
I’ve read about Russia showing a real warming trend:
That’s why I ask myself, how actual temperature series are put together with old data.
This is the only site I can expect a serious answer to the question…
Thank you all again and have a nice weeekend!!!!
[Response: All of the series you show have a positive trend so I'm a little unclear about you are asking. If you want to know how these are put together to get the composites then I suggest you look at the papers describing the procedures themselves. They are quite accessible and will provide more information than we can get into here. In particular, read:Hansen et al (1999). - gavin]
This post is somewhat off-topic, but I can’t find an open thread that’s more appropriate. Please forgive the intrusion.
I was reading LeGrande et al’s recent paper (LeGrande et al, Consistent simulations of multiple proxy responses to an abrupt climate change event (2006), PNAS, Vol. 103, No. 4, 837-842), to which you’ve referred a couple of times and for which you are a co-author. I have a few questions which I hope you can clarify.
This paper states that:
“In 12 coupled GCM simulations, we introduced fresh, isotopically depleted water (0°C, -30 permil d18Oseawater) into the Hudson Bay (adding to the total volume of water in the climate system). Additionally, we added a passive tracer to the melt water to track its movement through the simulation. The results discussed here use a range of volumes from 2.5 to 5 Sv yr, applied from 0.5 to 1 yr. Further experiments with volumes from 1.25 to 10 Sv and applied from 0.25 to 2 yr are qualitatively similar. The range of 2.5 Sv yr (0.788 x 10^14 m3 or ~23 cm rise of sea level) to 5 Sv yr (1.576 x 10^14 m3, 45 cm rise in sea level) is the most consistent with recent hydrologic model estimates for the volume and duration of the 8.2-kyr drainage of glacial Lakes Aggasiz and Ojibway (2).”
In examining the referenced paper (GKC Clarke et al (2004) Q. Sci. Rev. 23, p 389-407), I was looking for the model which corresponded to volumes described above (which, if I’ve done my sums correctly, are 78,800 and 157,600 km3 respectively).
Clarke et al contains 10 different models for possible flood volumes from Lake Agassiz-Ojibway (summarized in Table 4, each corresponding to different flood routes). The C(230) model contains the largest flood volume of 70,800 km3. The smallest is their case A1(230) which represents 27,900 km3. Their “preferred” model for an initial flood is B(230), containing 53,900 km3 of flood water, which corresponds to a 15 cm sea level rise. Clarke makes the distinction between an initial flood (Kinojevis event) and a subsequent one (Fidler event), the combined magnitude of which would fall within the ranges that LeGrande considers. However, Clarke also makes clear that the interval between such floods would range between 10 and 50 years, which doesn’t correspond to the method described in LeGrande where the fresh water pulse is introduced in no more than one year.
In other words, the largest single flood volume model in Clarke is less than the minimum volume model whose results are discussed in LeGrande. The smallest pulse discussed in LeGrande is 46% larger than Clarke’s “preferred” model for a single flood event, while the largest is nearly three times the volume.
These differences in this input parameter appear quite substantial, and lead to a couple of questions. First, can you clarify how LeGrande et al arrived at the flood volume range of 78,800 to 157,600 km3 which forms the basis of the discussed results? Second, assuming for the moment that Clarke et al’s flood volumes are correct, would using these figures significantly impact the results of the GCM runs in LeGrande and (potentially) lessen the match between the model runs and the paleoclimate proxies? If not, why not?
Thanks and best regards;
[Response: We made no attempt to exactly map the Clarke et al cases to the model experiments. Instead we took ballpark estimates of flood volume and duration and investigated the response. The context for these choices can be seen by examining other estimates of the flood volume (up to 15 Sv yr in von Grafenstein et al, 1999) and previous modelling work (which also used much larger estimates). Our simulations used a range from 1.25 Sv yr up to 10 Sv yr which encompasses Clarke's estimates, but also those of other workers. One of the key results was that the ocean response is not linearly tied to the volume - thus the model cannot be easily used to support any one value (or range) for the outflow. However, the range of results did allow us to calibrate the influence of appropriately-sized NADW slowdowns on the rest of climate, and in particular, on the proxies used to detect the 8.2 event itself. Thus in the results, we show the pooled response, not any particular run, and use that pooled response to come up with a likely change in the NADW (i.e. around 50% decrease). In doing this we are trying to avoid some of the obvious problems, such as the uncertainty in the initial floods and the uncertainty in the sensitivity of the model's ocean. As long as the inputs and the responses are in the ballpark, this calibration is possible. We are not claiming that any one of these model results was close to the 'truth', merely that the ensemble of model responses to reasonable perturbations are consistent with observations. This doesn't for instance rule out a smaller actual flood but a more sensitive ocean. -gavin]
Thank you for the response Gavin. Reading through LeGrande, I was under the impression that the range of volumes had been selected only from Clarke as that was the only paper referenced in the particular paragraph, thus my confusion. Thanks also for cleaning up the formatting.
Re #78: Thanks, Richard. I would have preferred to cut the data off at 10 ky to vertically stretch the data of interest around 3 ky ago, but I get the idea. As to which window (500y, 1ky, 2.5ky) most reflects reality, that depends entirely on sea level data, which I don’t have. The searching I did yielded rather inconsistent results.
Here are my assumptions:
1) Sea level is a function of temperature.
2) The intertia of the ice cap delays the response.
3) The sea level response to a temperature increase follows a bell curve. For example, if the equilibrium period is 1000 years, half the rise in sea level will happen by 500 years, and the rate of increase will be the greatest at this time.
4) Paleoclimate data suggests the sea level response to temperature rise is 6 meters per degree C, global average. We are measuring Greenland specific temperature, which will be much higher (about 3 times) than global average, suggesting 2 meters per Greenland degree.
5) I am assuming that Antarctica has little response in this temperature range.
All this suggests that the averaging you did around the center point is not quite right. You need to shift the curve to the right by one half of the averaging period.
I don’t know if you want to persue the any further. I appreciate what you did.
The problem of modeling ice sheets is real, and it is largely the fault of the water underneath, just as pointed out. In this article I addressed the issue in relation to a subglacial lake:
Erlingsson, Ulf (2006) Lake Vostok Behaves Like A ‘Captured Lake’ and May Be Near To Creating An Antarctic JÃ¶kulhlaup. Geografiska Annaler: Series A, Physical Geography 88 (1), 1-7. doi: 10.1111/j.0435-3676.2006.00278.x
This complication may also affect the Pleistocene ice sheets, which makes understanding past relations between ice sheets and temperature problematic. The final word is probably not said yet, just as pointed out above.
[...] Why is ice flow rate important? An ice sheet affects sea level when it melts faster, or when some of its ice flows into the sea. There is evidence that portions of both the Greenland and Antarctic ice sheets are doing the latter at an increasing rate. The causes are unclear and the changes may not continue in the future, but parts of these ice sheets seem to be far less stable than previously thought. Changes such as these are very difficult to model. [...]
[...] movement has often been a confusing one, but it is an important distinction. As Michael Oppenheimer explains, The issue still seems to perplex many journalists and others because there are two entirely [...]
[...] movement has often been a confusing one, but it is an important distinction. As Michael Oppenheimer explains, The issue still seems to perplex many journalists and others because there are two entirely [...]