Arguments
Andy Lacis responds to Steve Koonin
Posted on 13 April 2015 by Guest Author
This is a re-post from And Then There's Physics
I know Eli’s already posted Andy Lacis’s response to Steve Koonin on Judith Curry’s blog, but I thought it worth repeating. It’s a pretty impressive comment in terms of what it covers, so it’s worth reading in it’s own right. I do find myself amazed at what Steve Koonin has been willing to say. Ignoring that much of what he says suggests a woeful lack of understanding of the topic itself, that anyone of his supposed intellectual calibre would construct an argument that essentially goes “look, this number is small, nothing to worry about” is remarkable, and not in a good way. It’s one thing to suffer from hubris, but it’s hard to see why if one’s argument is so obviously silly. Maybe Eli’s right that the best description is beyond contempt.
credit : xkcd
Anyway, Andy Lacis’s comment is below (bolds mine).
Physicists should take the time to understand their physics better (Comment: some of us are trying :-) )
Only 1% to 2% . . . that may sound small and insignificant . . . but it isn’t.
It is well known that the normal human body temperature is about 310 K. Furthermore, it is also well known that a seemingly small change (up or down) in absolute body temperature by only 1% (3.1 K, or 5.6 F) would make one sicker than a dog, and, that a 2% change in body temperature (up or down by 6.2 K, or 11.2 F) will virtually guarantee a dead body. From this, it should be sufficiently clear that, when viewed in absolute energy terms, the viable margin between life and death in the Earth’s biosphere is remarkably narrow – so much so that a seemingly insignificant 1% to 2% change in the total energy of the global environment will invariably result in serious disruption of the established infrastructure of life in the biosphere.
There is no substitute for appealing directly to basic physics for physical insight and better understanding of the ongoing global warming problem. And I do recall one particular case in the 1970s (in which you might have participated) when the JASON group of physicists was tasked to weigh in on the then open question of radiative forcing due to doubled CO2. At that point in time, the JASONs had available the computational resources to calculate one of the earliest line-by-line radiative forcing determinations for doubled CO2. They found the downward flux change at the ground surface to be less than 1 W/m2, from which they erroneously concluded that the radiative forcing caused by the doubling of atmospheric CO2 was “not all that significant”.
While the JASON group’s radiative calculations were numerically on target, the JASONs were clearly mistaken in their interpretation of the calculated results. Radiative forcing takes place over the entire atmosphere, and not just at the ground surface. If they had to select a single point on the vertical profile that best describes the radiative forcing by CO2, they should have selected the tropopause point, where the instantaneous flux change due to doubled CO2 is nearly 5 W/m2 for a clear-sky atmosphere. Moreover, the JASONs did not take into account the additional radiative magnification that is invariably contributed by the longwave opacity from water vapor and cloud feedbacks, which are several times larger than the radiative forcing due to CO2 alone, and therefore should have been included in their analysis.
In simple terms, the basic essence of the global warming problem is best understood as a straightforward problem in global energy conservation (Comment : I like this, because this is precisely how I normally think of this issue), as was first noted by Joseph Fourier in 1824. Specifically, the global-mean surface temperature of the Earth is about 288 K, which implies that the Planck emission from the ground surface must be about 390 W/m2. Furthermore, the global-mean solar energy absorbed by the Earth is observed to be about 240 W/m2 (with about 100 W/m2 reflected directly back to space).
Given that the Earth should be in near-global energy balance, this implies that the Earth must radiate about 240 W/m2 of longwave energy out to space (as has also been verified by satellite measurements). Absent the greenhouse effect, the 240 W/m2 of absorbed solar energy can only support a surface temperature of 255 K. This “missing energy” circumstance led Joseph Fourier to conclude that there must be thermal heat energy radiated downward from the atmosphere to supply the additional heating of the ground surface.
The flux difference of 150 W/m2 between the 390 W/m2 emitted by the ground surface and the 240 W/m2 of LW flux going out to space at the top of the atmosphere is a direct measure of the strength of the terrestrial greenhouse effect. Greenhouse action builds up the surface-emitted flux to 390 W/m2 and creates the ensuing reduction by 150 W/m2 of the outgoing longwave flux to space – all accomplished by radiative energy transfer means (via sequential emission, absorption, and re-emission interactions).
Physicists should also appreciate the nature of the Clausius-Clapeyron relation, and the fact that it is exponential in temperature. Undisturbed, with a source of liquid water, the atmosphere is always striving to reach an equilibrium 100% relative humidity. In simple terms this means that the holding capacity of the atmosphere for water vapor doubles for every 10 K increase in atmospheric temperature. And, there is no doubt that water vapor is a very potent greenhouse gas.
Detailed radiative attribution calculations show explicitly that water vapor accounts for about 50% of the 150 W/m2 of greenhouse effect, and that longwave cloud opacity accounts for 25%. Both of these radiative effects are due to the climate system’s fast feedback processes. The remaining 25% of the greenhouse effect comes from the radiative forcings by the non-condensing greenhouse gases (which incidentally also act to sustain the terrestrial greenhouse effect at its present strength).Of the non-condensing greenhouse gas contributions, CO2 is by far the strongest contributor accounting for about 20% of the 150 W/m2 greenhouse effect, with the remaining 5% due to minor GHGs like CH4, N2O, O3, and CFCs.
A key point to keep in mind is that it is these non-condensing greenhouse gases that act as the principal radiative forcing agents of the climate system. Because of their thermodynamic, chemical, and radiative properties, CO2 and the minor GHGs are chemically slow-reacting with atmospheric lifetimes ranging from decades to many centuries. Once they are injected into the atmosphere these gases effectively remain there indefinitely by not condensing or precipitating at prevailing atmospheric temperatures as they continue to exert their radiative forcing.
Since CO2 is the strongest and most effective of these non-condensing radiative forcing gases, it then follows that CO2 can be identified as the principal LW control knob that governs the global climate of Earth. The fact is that the other forms of radiative climate forcing (e.g., changes in solar irradiance, surface albedo, and aerosol forcing) are small by comparison. This makes the case for recognizing CO2 as the principal climate control knob that much more compelling.
Atmospheric water vapor, on the other hand, has the role of principal fast feedback process in the climate system by condensing and precipitating from the atmosphere in response to changes in local meteorological conditions (constrained by the exponential temperature dependence of the Clausius-Clapeyron relation), meaning that the atmospheric distribution of water vapor (and clouds) can change rapidly on a time scale of hours and days in response to changing weather conditions.
Applied radiative forcings that heat (or cool) the atmosphere cause more (or less) water vapor to evaporate, which generates more (or less) longwave opacity, which then contributes more (or less) radiative greenhouse effect. Such changes in water vapor cause big changes in radiative heating or cooling, but the changes are limited in magnitude by how much change the water vapor undergoes in reaching its new equilibrium distribution.
Because of this, water vapor and clouds act to magnify the initial radiative perturbation, but cannot on their own initiative manufacture or impose a warming or cooling trend on global climate, even though they contribute more strongly to the atmospheric radiative structure than the radiative forcing gases that actually drive and control the global temperature trend.
The physics cause-and-effect nature of the global warming problem is not all that complicated. The basic “cause” component of global warming has been clearly identified and understood for many decades, and has been accurately quantified by precise measurements of atmospheric CO2 (e.g., the Keeling curve).
This is fully corroborated by the latest annual data report of fossil fuel extraction that now approaches 10 gigatons of carbon/yr (roughly equivalent to 10 cubic km of coal/yr, which when burned, adds about 5 ppm CO2 to the atmosphere, half of which remains there for many centuries). The radiative effects of CO2 are fully known from well-established understanding of greenhouse gas radiative properties and radiative transfer modeling of the atmospheric structure.
How can a physicist not comprehend that it is atmospheric CO2 that is the principal radiative forcing agent for the ongoing global warming? . . . and not be concerned that water vapor, as the climate system’s principal feedback agent, has an exponential dependence on temperature?
To be sure, there are other factors that contribute to climate change. Butdecades of measurements and analysis have shown that variations in solar irradiance, land use, aerosols, ozone, and other minor greenhouse gases, while making a contribution, are small by comparison to CO2.
Of greater interest is the “unforced” variability of the climate system on decadal time scales that arises from changes in ocean circulation patterns that are effectively un-influenced by changes in atmospheric radiative forcing. The deep ocean is a very large cold storage reservoir. An upwelling blob of cold ocean water can put a “pause” in the ongoing global warming, temporarily diverting the greenhouse “heat” to warming the ocean. But once that cold blob of ocean water has been warmed up to its equilibrium temperature, it is back to the business of continued global warming. And also note that the ocean cannot cause a decadal warming spurt – the deep ocean is colder than the surface biosphere, so it cannot be a source of heat.
Significantly, the key climate system components (water vapor, clouds, ocean) are not configured to respond to radiative and/or temperature perturbations on a sufficiently small enough incremental scale that would permit a monotonic approach to global energy balance equilibrium. Instead, there is always over-reaction such as when water vapor condenses en mass to produce storms, coupled with the similarly over-reactive responses by atmospheric and ocean dynamics to pressure-temperature and salinity differences, to produce the quasi-chaotic weather and the longer-term climate noise that characterizes the climate system.
Physicists should not be confused by these random-looking quasi-chaotic fluctuations about the local climate equilibrium point, and should instead focus more on the changing energy balance equilibrium point of the climate system. They should also pay attention to the geological record that points to an atmospheric CO2 level of 450 ppm as being incompatible with polar ice caps, a level that is expected to be reached by the end of this century. While it may take a thousand years for the polar ice to melt, the future course is being prepared for a 70 meter rise in sea level.
"They should also pay attention to the geological record that points to an atmospheric CO2 level of 450 ppm as being incompatible with polar ice caps..."
That figure seems much too low. 450 ppm is the most common 'target limit' to avoid the worst impacts of AGW, but I've never seen a claim that we'd eventually lose the ice caps and have corresponding 70 meter sea level rise at 450 ppm.
Some housekeeping may be required, as this article is breaking the formatting of other posts on the main page (no doubt due to the preview ending in a block quote).
@1, The truth may be more along the lines of " a sustained level of 450 ppm CO2"...
Note that Dr David Mills was on youtube years ago saying the 440ppm barrier is impossible not to break... he says it is(/was) still being worked out whether or not it was possible to go over the limit and then duck back under it but the point I'm making is ---> 450 ppm is written in stone as reality!
Has anyone put the basic Arrhenius/Hulburt calculation in a spreadsheet? Something where at the top you input your preferred CO2 level and at bottom it tells you how much warmer, or cooler that will be, in equilibrium, compared to pre-industrial? I understand it means breaking up the atmosphere into 200-500 nodes, each of which absorbs sunlight, radiates/convects/etc. It means assumptions are made about feedbacks (clouds, vegetation, ocean response, ice cap extent).
My point is: equilibrium sensitivity, at the level of Arrhenius or Hulburt, is just a calculation, like 2+2 =4 (indeed, for them, it was a calculation done without benefit of electronic calculators). If you can put those assumptions/calculations in a spreadsheet for anyone to see and manipulate, it reinforces the notion that this is, at base, just Math (i.e., once the Physics and Chemistry have been codified into Math). It's Math anyone sufficiently trained can do/see/appreciate.
Hence, when someone says "Doubling CO2 is no big deal" you can send them the spreadsheet and say "identify the specific location where 'no big deal' " comes out of this Math". Without the spreadsheet you're left saying "It IS a big deal. My expert says so." Leading to the response "My expert says it isn't."
I agree that 450ppm by the end of the century is an understatement. Everything I've seen suggests we'll reach that mark within the next 20 years.
I have a couple of questions about the Fourier calculations. First, the global mean temperature of 288K implying 390 W/m2 of radition, some into space and some reflected back to earth by greenhouse gasses. I'd like to know more about how that's calculated.
I also don't fully understand the near-global energy balance used as the reason earth absorbs 240 W/m2 of radiation from the sun and radiates the same amount out of the upper atmosphere. The earth isn't in equilibrium now is it? We'd still warm for quite awhile even if CO2 levels remained constant, right? Doesn't that imply we're absorbing more radiation from the sun than we're radiating back into space? I feel like I'm missing something obvious here.
gregcharles - The Earth isn't in equilibrium right now, increasing ocean heat content (OHC) measures show that over the last 50 years we've averaged about a 0.6 W/m2 mbalance over that period, meaning the earth has been receiving about 240 W/m2 and radiating about 239.4 W/m2. The difference points to the (currently) unrealized warming due to thermal lag, primarily in the oceans.
The difference between the near-blackbody IR radiation at the Earths surface (~396 W/m2, IR emissivity about 0.95-0.98) and what's radiated to space (~240 W/m2) is entirely due to the radiative greenhouse effect - in essence energy isn't emitted to space until much higher altitudes (and colder) due to greenhouse gases, making the effective emissivity of the Earth in IR around 0.61. The Earth's just not as effective a radiator as a bare rock would be, and with increasing GHGs it becomes even less effective - hence a higher and higher surface temperature required to radiate the same energy to space.
CBDunkerson @1, Bozza @2, the most recent credible data that I know of is encapsulated in this figure:
(Source)
Total melting of the ice caps is associated with a sea level rise greater than 70 meters. Ergo, from paleo data, we cannot expect that unless we have sustained CO2 concentrations of 800-1500 ppmv. From that paper (Foster and Rohling 2013), we lean that "our results imply that acceptance of a longterm 2 °C warming [CO2 between 400 and 450 ppm (46)] would mean acceptance of likely (68% confidence) long-term sea-level rise by more than 9 m above the present." That probably represents the melting of the West Antarctic Ice Sheet (WAIS), and the partial melting of the Greenland Ice Sheet (GIS), with only limited melting of the East Antarctic Ice Sheet (EAIS).
Looking at the figure, Andy Lacis may be basing his claim on van der Wal et al, 2011, except that when I actually look at van der Wal et al, the modelling shows a total loss of polar ice does not occur until a sustained temperature anomaly of plus 20 C is reached, ie, around 1600 ppmv with an Earth System Climate Sensitivity of 8 C. It appears, therefore, that Foster and Rohling have incorrectly represented van der Wal et al's results.
Aslak Grinstead gives a more detailed discussion.
gregcharles @5, the CO2 in the atmosphere does not reflect IR radiation back towards the Earth. Rather, it absorbs it, and then reradiates it. The difference is important, because if reflected the energy returned would be the difference between that which the surface emits (398 W/m^2) and that which escapes the atmosphere (239 W/m^2), ie 159 W/m^2. In fact, downwelling IR radiation (or back radiation) averages around 342 W/m^2 because the IR radiation from the atmosphere (figures from IPCC AR5):
The much higher back radiation is due to the air mass immediately above the Earth's surface (from which most of the back radiation originates) having a temperature very close to that at the surface. In constrast, most of the IR radiation to space comes from high in the troposphere, where the temperatures are much lower.
Further, the actual back radiation is not important to the greenhouse effect (although may be important for local weather events). That is because if the back radiation were to increase, with no change in the greenhouse effect, evaporation and sensible heat transfers would also increase to maintain a balance, and if it were to decrease, evaporation and sensible heat would also decrease. The greenhouse effect is determined by the top of atmosphere energy balance.
I think it would be accurate however to say the 450ppm is incompatible with the ice-age cycle. (Didnt have ice ages when atmosphere was last at 450ppm, though there were still polar ice caps.)
Tom Curtis @7: In looking closely at the graphs, it appears that a 9 meter rise corresponds to 1 std. dev. below the mean. This would imply that the chance of less than 9 meters is 16%, and the chance of more than 9 meters is 84% rather than 68%. No? Clearly not a better situation. (I do note that you are quoting the paper.)
@ 9, I think this is surely the relevant question at the moment!
@ 7, why things flatline and/or double dip I don't know but the links should prove interesting- cheers.
mrkt @10, looking carefully at the graph in the upper right panel, it becomes clear that they show a "likely range" (68%) at 9 to 59 meters at 450 ppmv. That makes their claim of a "likely (68% confidence) long-term sea-level rise by more than 9 m above the present" true, but obscure. Ie, they are saying that the likely confidence interval, using IPCC methods of expressing confidence, has a lower limit of 9 m. From that it follows that their results show an 84% probability of at least 9 meters of sea level rise for a long term CO2 concentration of 450 ppmv.
That is not how I initially read it, so thank you for drawing attention to my error.
Looking at their likely range, over that interval, it appears evident that the upper value is so large due entirely to the limitted observations. On the assumption that increased temperatures will not cause water to refreeze, I think the lowest upper limit of the likely range at higher temperatures can also be used as a more realistic upper limit for a long term 450 ppmv concentration. That produces an adjusted likely range between 9 and 30 meters, with 30 meters representing the almost complete melt of the WAIS and GIS, with any surviving ice from those ice sheets being more than compensated for by melt of the EAIS.
bozza @11, the flat line is because while the WAIS and GIS are on the verge of melting, the EAIS is much more stable , and will mostly remain intact once the WAID and GIS have melted away with little melting other than at the fringes. Only after considerable further rise in temperature will the EAIS melt away.
Thanks for a very clear explanation.
As Tom has pointed out, when one is considering eustatic sea level rise (i.e. the rise due to more water being added to the oceans) the behaviour of the East Antarctic Ice Sheet will not simply mimic that of the Greenland Ice Sheet, the West Antarctic Ice Sheet or the Antarctic Peninsula.
The EAIS is at a much higher elevation than its smaller counterparts, and would need considerably more (and longer) planetary warming before it would even come close to melting out. Those wishing to learn a bit more might care to look at the Antarctic Glaciers website, or at the British Antarctic Survey site.
There is little meaningful argument that the Mass Balance for each of the GIS, the WAIS and the Peninsula is in negative territory. However, the EAIS may actually be accumulating ice at present, as enhanced precipitation in its central regions (thanks to the good old Clausius-Claperyon relationship) could be more than compensating for increased peripheral loss.
"Earth system were to reach equilibrium with modern or future
CO2 forcing. Given the present-day (AD 2011) atmospheric CO2
concentration of 392 ppm, we estimate that the long-term sea level
will reach +24 +7/−15 m (at 68% confidence) relative to the present."
That was at 392ppm.
http://www.pnas.org/content/110/4/1209.full.pdf
As the early Pliocene was ~350-400ppm, with most of the later papers with more accurate calculations are pointing more to 350ppm, and sea levels were 20-25m higher.
This is were stats lets us down really, for everyone will grasp at the seeming chance of 16% that sea level rise will be below 9m, for this is resultant from taking a wide range of sea levels and CO2 levels uncertainities and arriving this larger uncertainity, yet no expert in the paleoclimatology would agree that the Pliocene wasn't warmer and that sea levels weren't 20m higher, when CO2 was ~350-400ppm.
Also interesting that 5cm a year rates of sea levels rise have occured in deglaciations when the general global warming was occuring an order or two magnitude slower than current rates.
Seems reasonable to suggest that the fasterthe ehat accumulates in the system the faster the ice sheets will melt, and it is well known that melting an ice sheet is an ever accelerating event due to icesheet lowering and dynamic melting processes (e.g.the heat from surface melt ponds being transfered to the heart of the ice sheet).
That is why Hansen wonders at 5m in a century and why most feel the IPCC 0.8m by 2100 is generally regarded as very conservative.
[PS] Fixed link. It would be appreciated if you did this yourself with the Link button in the comment editor.
ranyl @15, although we have a stated likely range of 9 to 31 meters sea level rise at 392 ppmv, the lower bound does not start increasing again until about 550 ppmv, and the upper bound, though it increases, then decreases to about 29 meters from about 550-800 ppmv. On the postulate that increasing temperatures will not result in decreasing sea levels, that rise in the upper bound must be treated as simply a loss of certainty due to limited data over that interval and we have a prediction of essentially no further increase in sea level other than by thermal expansion from 390-800 ppmv.
To reach that state of relative stability, the WAIS and GIS must be essentially completely melted. That in turn indicates a sea level rise of about 14 meters from WAIS and GIS plus a little more from glacial ice and thermal expansion. So, assuming we will only have 9 meters of sea level rise is indeed folly based on that data. Sustained CO2 concentrations at current levels will give us 14 plus meters of sea level rise, but the same is true at 550 ppmv, and not till you get above 1000 ppmv is their any credible risk of the complete melt of the polar ice caps (the original point queried).
So far I have broadly agreed with you, though perhaps disagreed on detail. Where I truly disagree with you is on rates. While it is reasonable that a greater TOA energy imbalance (not forcing) will result in a faster ice melt, there are a host of other factors involved. One of those is surface area subject to ice melt which is far less with the surviving ice sheets than was the case at the end of the last glacial. Another is regional energy imbalance in the melt season, which was far greater at high northern latitudes (due to milankovitch "forcing") during the end of the last glacial than during the current anthropogenic temperature increase.
Further, even if I agreed with you about the possibility of 5 meters per century sea level rise in the current warming (which I do not), it would not be possible in this century. The current rate of sea level rise is about 0.32 meters per century. Even if that were to linearly increase to 5 meters per century by 2100, that would still only give us a sea level rise over the century of slightly less than 2.5 meters. (Less than because 14% of the century has already occured at low rates.) An exponential increase to that rate gives us a lower rise, not a greater rise - although it would predict a greater sea level rise in from 2100-2200. That, however, is itself a problem. Hansen's predictions suggest that most of the WAIS and GIS will melt out in less than fifty years early next century, a prediction which is not credible.
There is a reason why virtually nobody (I know of no actual cases) who is an expert is ice sheet dynamics or sea level rise accepts Hansen's predictions. There is a reason also why those who think the IPCC is too conservative expect sea level rises of the order of 2 meters or less for this century, not 5 meters.
If you want a more realistic assessment for this century, you should turn to Jevrejeva et al (2014):
Aslak Grinsted (one of the authors) writes on his blog:
Further on the expert elicitation by Bamber and Aspinal, their 95% bound on elicited ice sheet contribution to sea level rise in 2100 was 17.61 mm per annum. Note that that is the sea level rise in that year. It means the expected average contribution over the century is approximately half that (or about 0.9 meters total contribution). The median is 5.4 mm per annum and mean of 6.9 mm per annum. The maximum elicited expert contribution was 38 mm per year (for an approximate expected contribution over the century of 1.9 meters). Hansen's views are so far outside the bounds of actual expert opinion on this topic as to be absurd.
Well Tom,
I never said I agreed with Hansen merely pointed out why he felt 5m was possible, to add to that he bases his estimates on thepotential from previous melt rates and the doubling time of the acceleration of the melt already being seen in both icesheets.
Further can you name one ice sheet expert who opinion isn't in some way curtailed by alarmist branding!
There are many factors involved, although the 6-9m from the LIG wasn't all from the NH therefore the same forcings that helped loss of the Southern Greenland Ice sheet also would have been making the SH colder yet, atleast half of the melt came from there, implying that the higher CO2 level of the LIG at ~300ppm certainly helped some.
Then there all the new findings, such as the ehat transfer into the centreand base of the ice sheet due to surface melt heat advection, which those experts wouldn't have been taking into account,
Phillips T et al, (2013) Evaluation of cryo-hydrologic warming as an explanation for increased ice velocities in the wet snow zone, Sermeq Avannarleq, West Greenland , Journal of Geophysical Research
“The sun melts ice into water at the surface, and that water then flows into the ice sheet carrying a tremendous amount of latent energy,” said William Colgan, a coauthor and CIRES adjunct research associate. “The latent energy then heats the ice.”
“It could imply that ice sheets can discharge ice into the ocean far more rapidly than currently estimated,” Phillips said. “It also means that the glaciers are not finished accelerating and may continue to accelerate for a while. As the area experiencing melt expands inland, the acceleration may be observed farther inland.”
“Previous studies estimated that it would take centuries to millennia for new climates to increase the temperature deep within ice sheets. But when the influence of meltwater is considered, warming can occur within decades and, thus, produce rapid accelerations.”
“Traditionally, latent energy has been considered a relatively unimportant factor, but most glaciers are now receiving far more meltwater than they used to and are increasing in temperature faster than previously imagined,” Colgan said. “The chunk of butter known as the Greenland Ice Sheet may be softening a lot faster than we previously thought possible.”
LINK
Then there is also the new evidence of the Pine Island glacier having gone into irreversible melt and it also looks like the Thwaites and a few others will follow suit.
And from the Jevrejeva paper, “The AR5 addresses this issue by suggesting that 'only the collapse of the marine-based sectors of the Antarctic ice sheet, if initiated, could cause sea level to rise substantially above the likely range during the 21st century”
So the question definately is rate, and I agree 5m by Hansen is a definitive high end, but previously observed (with more icesheets but land based ones), and the by 2100 somewhere between 1-2m seems more likely, although the accelerating rates being observed for icesheet melting is concerning for it can only accelerate further, and that acceleration as it lowers icesheet height and more seabed icesheets get undermined beyond critical melt back, can only accelrate again due to dynamic mechanisms fedding back on themselves.
And then waht sea level rise by 2150 as rates keep accelerating and then by 2200?? Rome been arround several 1000's por years as has London.
And as CO2 isn't going anywhere for a very long time it is essentail to realise that at 350ppm -400ppm we are committed to an early Pliocene climate, whatever efforts are made to stop emissions, only taking CO2 out of the atmosphere can change that, and to take 50ppm out is a collossal task indeed, and we still progress at top rates, and forest fires, permafrost melt and soil respirations are all going to add to the burden.
Finally what does 1-2m mean?
Well moving New York for a starter for ten!
If we get 4-5m by 2200, then that is most of Florida, Bangladesh, Holland and Miami already floods at high tides!
[DB] Shortened and hyperlinked URL that was breaking page formatting.
Ranyl
I tend to agree with Tom that Hansen's view is at the extremes on a single century scale - on multi-century all bets are off. When the high rates of melt occurred we had major ice sheets in Canada/US, Scandinavia, Northern Europe and western Russia. Also southern South America and the Australian highlands. Possibly also areas of the Arctic sea ice had become like West Antarctica, grounded on the sea floor and projecting significantly above sea level. So a much larger area from which melt could occur.
To produce similar melt rates today we would need much higher temperatures to trigger much higher melt rates per km2 since there are so much less km2 available.
That said, I recall a paper published a couple of years ago - can't find it now - looking at fossil beaches in Western Australia from the last Interglacial - the Eemian. Sea levels were reported as being around 5 meters higher than now for much of the interglacial then near the end they spiked higher to more like 9-10 meters higher.
This suggests a sudden major ice collapse with the WAIS the likely culprit.
It is interesting to look at the ice core data for temperature history during the Eemian. They spiked higher than today but only for a short period. Perhaps some major disruption in southern ocean currents around the WAIS triggered an ice collapse and the Albedo change triggered a spike in warming. Then the currents reverted and it was all reversed.
Hi Glenn,
However despite all that NH ice weren't the meltwater pulses from the last deglaciation primarily from the SH?
And this quote is interesting,
"Our new RSL chronology permits the first robust calculation of rates of relative sea-level change throughout the past 150,000 years (Fig. 3c). This reveals that rates of sea-level rise reached at least 1.2mper century during all major phases of ice-volume reduction, and were typically up to 0.7m per century (possibly higher, given the smoothing in our method) when sea-level exceeded 0m during the LIG (Fig. 3c); the latter is consistent with independent estimates21,22."
Grant K.M. et al (2012) "Rapid coupling between ice volume and polar temperature over the past 150,000 years"
And the top of WAIS is melting in a non linear fashion
"The nonlinearity of melt observed in the JRI ice-core record also highlights the particular vulnerability of areas in the polar regions where daily maximum temperatures in summer are close to 0 C and/or where summer isotherms are widely spaced, such as along the east and west coasts of the Antarctic Peninsula (Supplementary Fig. S3). In these places even modest future increases in mean atmospheric temperature could translate into rapid increases in the intensity of summer melt and in the poleward extension of areas where glaciers and ice shelves are undergoing decay caused by atmospheric-driven melting."
Abram N.J et al (2013) Acceleration of snow melt in an Antarctic Peninsula ice core during the twentieth century, Nature Geoscience
“The need to improve upon the uncertainty in the LIG ESL estimates is best seen in terms of its consequences on melting from both Greenland and Antarctica during the LIG. Current modelling and data-based estimates converge on a 2- to 4-m contribution to ESL from Greenland and on a maximum contribution of +3.3 m from West Antarctica (32). Thus, the lower limit estimate of the peak LIG ESL (+5.5 m) is consistent with such contributions from both Greenland and West
Antarctica, but the upper limit (+9 m) implies additional melt-water contribution from adjacent sectors in East Antarctica.”
A. Dutton and K. Lambeck (2012) Ice Volume and Sea Level During the Last Interglacial, Science
“However, the retreat of these southernmost terrestrial ice margins within centuries of an increase in boreal summer insolation of only 1–2 W m–2 (Fig. 5a) suggests that terrestrial ice margins near their climatic limit are responsive to small changes in radiative forcing.”
“However, the final collapse of the marine portion of the Laurentide Ice Sheet at ~8.2 kyr ago occurred in less than 130 years and raised eustatic sea level 0.8–2.2 m75, which is a timescale of more importance to global society”
Anders E. Carlson and Kelsey Winsor (2012), Northern Hemisphere ice-sheet responses to past climate warming, Nat. Geoscience
So I agree Hansen is hopefully being too bold, as I said already, however with the Weddell Icesheet under threat as sea levelsrise and Southern Ocean winds progress Sotuhwards, and the below seabed portions of EAIS, mean that 1-2m by 2100 is lookingmore liley especially when considering the amount of extra heat into the whole system, far more than in the LIG,whose overall global additional wattage input was due to GHG mainly.
We passed 300ppm a long time ago, so it is clear that to melt ice takes time indeed, however that isn't that reassuring as since 1990 ice melt has accelerated markedly in all areas and the heatinput into the oceans in the last 10 years alone is remarkable compared to previous melt periods and for melting seabed icesheets that must count, especialy as West PAC waters find their way to the Antartica quite quickly and that is warming quite rapidly.
Further sea level isn't even, and the East Coast of America gets more than its fair share, 1m globally about 2m EUSA.
Put it this way, real estate investment in New York isn't a long term investment option on solid ground I'd venture.
It si weird how the early Pliocene was 3-5C warmer at 350-400ppm (0.25 to 0.42 of a doubling), considering the CS of only 3C for a doubling or 560ppm, hhhmm, oh yeah that is right the CS for full long term equilibrium is double the CS reported by the IPCC forgot about that, it makes more sense now, but still if 350ppm (and that looks more likely now) then still need long term CS of at least 12C if the 3C lower estimate is true, even if it was 2C (some studies say 2C, mainly Hansen as by the by, just make melting more susceptible to temperature rise as sea elvels still 20-25m higher), you still need a long term equilibrium CS of 8C. And the sun was cooler a little bit only a very small bit, all the orbitial parameters actually add zero to the global overall heat input and the continents were close enough not to matter that much, so not much comfort in loooking for other heat sources.
The NH did have a totally different temperature profile though, interesting, I ownder if the Hadley cell system was different back then due to polar equatorial temperature differential being so much lower, as that would definately affect ice melt in Greenland, keeping in mind there was no GIS then.
This discussion is way above my level, but, having tried to follow it, I cannot say that I have seen the influence of gravity in anyone's comments - perhaps I missed it. Specifically, what occurs to me is that as ice sheets melt, so their gravity diminishes leading to a fall in sea-level around them. Could this be important with the WAIS? As it melts, so the sea-level falls and thus places the fracture point under more bending stress, which in turn might complete the fracture, leaving it free to float off (though grind off is probably more correct) to deeper waters?
Probably unimportant, but felt the need to raise it just in case. ('Out of the mouths of babes and innocents' and all that!)
@20. Gravity has nothing to do with it. Gravity is an expression of potential energy (water above sea level) No mass is going away by melting ice, nothing will change in the gravity. Water will get some kinetic energy when going down, converted into some heat and extra flow of seawater when it smacks into the fast inertia of the sea.
[TD] Actually, gravitational attraction from ice does pull water toward it. Amazing but true. For example, see this post about Jerry Mitrovica's work.
Ger @21... You should read this article on the relationship between gravity and glacial melt.