Regarding this paper¹s conclusion, our study of polar cooling [ MacCracken, M. C., H-J. Shin, K. Caldeira, and G. Ban-Weiss, 2013: Climate response to solar insolation reductions in high latitudes, Earth Systems Dynamics, 4, 301-315, 2013; www.earth-syst-dynam.net/4/301/2013/; doi:10.5194/esd-4-301-2013 and the earlier one by Caldeira and Wood suggest that what polar cooling does is enhance the likelihood of snow on land‹so the building up of land ice. By not directly cooling the low latitudes (there is some cooling by the pulling of heat from lower to high latitudes), the hydrological cycle remains quite strong and so with a cooler Arctic (or both poles), there will be greater snow buildup on land. It is not at all clear to me that ³Cooling the poles enough to halt ice loss would devastate the rest of the world, slashing rainfall, for instance.² At least, that was not the case in our model simulations.
Mike MacCracken On 9/7/15, 8:05 AM, "Andrew Lockley" <[email protected]> wrote: > Poster's note : title piece is a box extract, immediately below. Main article > posted beneath, which is well worth reading for those not up to speed with the > sea level rise issue. > > https://www.newscientist.com/article/mg22630253-300-latest-numbers-show-at-lea > st-5-metres-sea-level-rise-locked-in/#bx302533B1 > > Can geoengineering save coastal cities? > > It¹s already too late to prevent massive sea level rise (see main story). Or > is it? Can geoengineering stop low-lying cities sinking beneath the waves? > > It certainly won¹t be easy. ³Once you kick in the melting feedbacks, it¹s very > hard to shut them off,² says Alexander Robinson of the Complutense University > of Madrid. To have any chance, we have to get the planet¹s temperature back > down to pre-industrial levels in the not too distant future. ³I personally see > that as quite unlikely,² Robinson says. > > One key problem is that most geoengineering methods, such as pumping sulphates > into the atmosphere, rely on reflecting sunlight and would cool the tropics > more than the poles (Geophysical Research Letters, doi.org/453 > <http://doi.org/453> ). Cooling the poles enough to halt ice loss would > devastate the rest of the world, slashing rainfall, for instance. > > The best solution would be to suck all the excess carbon dioxide from the > atmosphere, but the immense scale of the task and the speed required make this > seem nigh on impossible. Other suggestions, such as building huge barriers > between warming waters and glaciers, don¹t look feasible either. > > Another major problem is that until cities start drowning, it is hard to see > politicians spending trillions on megaprojects. And once they begin to drown, > it will already be too late to prevent major sea level rise. > > (main article follows) > > SPECIAL REPORT 10 June 2015 > Latest numbers show at least 5 metres sea-level rise locked in > > It¹s too late to stop the seas rising at least 5 metres and only fast, drastic > action will avert a 20-metre rise, New Scientist calculates based on recent > studies > > WHATEVER we do now, the seas will rise at least 5 metres. Most of Florida and > many other low-lying areas and cities around the world are doomed to go under. > If that weren¹t bad enough, without drastic cuts in global greenhouse gas > emissions more drastic than any being discussed ahead of the critical > climate meeting in Paris later this year a rise of over 20 metres will soon > be unavoidable. > > After speaking to the researchers behind a series of recent studies, New > Scientist has made the first calculations of what their findings mean for how > much sea level rise is already unavoidable, or soon will be. > > Much uncertainty still surrounds the pace of future rises, with estimates for > a 5-metre rise ranging from a couple of centuries possibly even less to a > couple of millennia. But there is hardly any doubt that this rise is > inevitable. > > We already know that we are heading for a rise of at least 1 metre by 2100. > The sea will then continue to climb for many centuries as the planet warms. > The question is, just how high will it get? > > No return > According to the latest report by the Intergovernmental Panel on Climate > Change (IPCC), over the next 2000 years we can expect a rise of about 2.3 > metres for each sustained 1 °C increase in the global temperature. This means > a 5-metre rise could happen only if the world remains at least 2 °C warmer > than in pre-industrial times up to the year 4100. That doesn¹t sound so bad: > it suggests that if we found some way of cooling the planet, we could avoid > that calamity. > > Unfortunately, the report, published in 2013, is not the whole story. Last > year, two teams reported that two massive glaciers in West Antarctica have > already passed the point of no return. > > Ian Joughin of the University of Washington, Seattle, modelled the fate of one > of the glaciers. ³No matter what, the glacier continued to lose mass,² he > says. > > The loss of those two glaciers alone will raise sea level 1.2 metres. If they > go, Joughin says, it¹s hard to see the rest of the West Antarctic surviving. > > Others agree. ³I think these are very convincing studies,² says Anders > Levermann of the Potsdam Institute for Climate Impact Research in Germany, one > of the authors of the sea level chapter in the last IPCC report. ³The West > Antarctic ice sheet is gone.² > > The reason is that the West Antarctic ice sheet sits in a massive basin, its > base as much as 2 kilometres below sea level. At the moment, only a little ice > on the edges is exposed to the warming waters around Antarctica. As the ice > retreats, however, ever-deeper parts of the basin will be exposed to warming > waters, leading to ever more of it being lost. The process is irreversible > because once it starts, it will continue as long as warm conditions persist. > This means a 3.3-metre rise is now unavoidable. > > And that¹s not all (see chart). Even in the unlikely event we manage to limit > warming to 2 °C, we¹re in for a 0.8-metre rise as the oceans warm and expand. > Mountain glaciers around the world will contribute 0.4 metres. Adding those > figures to the 3.3 metres, we get 4.5 metres in total, or 5 metres rounded up. > That¹s conservative, given that it doesn¹t count any melting from East > Antarctica or Greenland. > > Latest numbers show at least 5 metres sea-level rise locked in > Most of the ice in East Antarctica is more stable than that in West Antarctica > as it rests on land above sea level. There are two large basins, the Aurora > and the Wilkes, whose floors are below sea level, but these are shallower than > the West Antarctic one. We had thought only massive warming would destabilise > the ice here. > > Trough threat > However, Totten, the main glacier that drains the Aurora basin, is thinning, > says Jamin Greenbaum of the University of Texas at Austin. His team reported > in March that radar sounding has revealed a trough under the ice that could > let warm water enter the basin and trigger enough melting to eventually raise > sea level by 5.1 metres (Nature Geoscience, doi.org/27w <http://doi.org/27w> > ). ³The mind-blowing thing is that there is as much ice in one glacier in East > Antarctica as in all of West Antarctica,² says Greenbaum. > > The situation is similar in the Wilkes basin. It¹s not losing ice yet, but > once a small amount on the margins is lost it will continue disintegrating > until enough ice has melted to raise sea level 3.5 metres, Levermann¹s team > reported last year (Nature Climate Change, doi.org/snz <http://doi.org/snz> ). > > What will it take to kick-start the loss of all this ice? Not much. During the > Pliocene period around 4 million years ago, for instance, when the planet was > 2 or 3 °C warmer at times, sea level was over 20 metres higher than now. > Researchers suspect that much of this came from the Aurora and Wilkes basins. > > Support for this idea comes from an improved ice sheet model that, for the > first time, includes dynamic processes such as cliff collapse resulting from > ice sheets being undercut by warming waters. In January, a team including > Richard Alley of Pennsylvania State University reported that Pliocene > conditions will lead, so the model indicates, to ice loss not only in Aurora > and Wilkes but also in several smaller East Antarctic basins. Together, they > hold enough ice to add at least 15 metres to global sea level (Earth and > Planetary Science Letters, doi.org/42m <http://doi.org/42m> ). > > We are currently on course for a world even warmer than the Pliocene, which > means we could soon trigger the loss of the Wilkes and Aurora ice if we > haven¹t already. > > Latest numbers show at least 5 metres sea-level rise locked in > This break-up will be traumatic (Image: NASA) > > Then there¹s Greenland. The ice here mostly rests on land above sea level, so > should take thousands of years to melt. You might think, then, that there is > plenty of time left to save it. Not so, says Alexander Robinson of the > Complutense University of Madrid, Spain. > > He says his team¹s studies show that we are already nearing the point of no > return for Greenland (Nature Climate Change, doi.org/kkw <http://doi.org/kkw> > ). ³Within the next 50 years, we could be committing ourselves to continuous > sea level rise from Greenland over the next thousands of years,² he says. > ³That¹s a very profound thing to think about.² > > The reason is that as warming continues, various positive feedbacks will kick > in. As the surface of the ice sheet lowers, for instance, it experiences > higher temperatures. In theory, the melting could still be stopped if > temperatures fall, but because carbon dioxide persists in the atmosphere for > many centuries, says Robinson, it is hard to see how that could happen (see > ³Can geoengineering save coastal cities?³). > > The loss of Greenland¹s ice would add at least 6 metres to global sea level. > And in this business-as-usual scenario, ocean warming would contribute 1.6 > metres or more. Adding all this up leads to the frightening conclusion that we > don¹t have much time left before we¹re on a one-way street to a world with > seas 20 metres higher. ³It¹s kind of scary,² says Robinson. > > It will take thousands of years for the seas to rise to this extent, but much > of the rise could happen early on within the first few centuries although > no one can say for sure. Joughin thinks the IPCC estimate of up to 1.2 metres > by 2100 could still be in the right ball park. ³It¹s likely to be on the high > end [of the IPCC estimate] but not far outside.² > > Yet in the improved ice model that Alley¹s team ran, Antarctica alone added 5 > metres to sea level in the first two centuries. That model was run with warm > Pliocene-like conditions from the start, not where we are at now. > > It might not take too long to reach a similar point, though. We¹re in danger > of soaring past Pliocene levels of warmth as early as the middle of the > century if we don¹t slash emissions soon. In the study, the West Antarctic ice > sheet collapsed in mere decades in response to this kind of warmth. > > What¹s more, the model might still leave out some melting processes, Alley > says. ³It is possible that this rather short timescale is not the worst > possible case.² > > This article appeared in print under the headline ³Five metres and counting² > > By Michael Le PageMagazine issue 3025 published 13 June 2015 -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/geoengineering. For more options, visit https://groups.google.com/d/optout.
