Re: [geo] Re: Why Greenland's melting could be the biggest climate disaster of all
A sideshow to sea-level questions on policy-relevant time scales. (2100-ish at best).. You're talking geological scale here. Tad Pfeffer's 2008 analysis of worst-case discharge rate still a keystone to clear thinking on this. On Mon, Jan 28, 2013 at 2:38 PM, Thomas Homer-Dixon t...@homerdixon.comwrote: “Greenland . . . is a sideshow in the sea level question.” ** ** I see nothing in the Dahl-Jensen article that could possible justify such a sweeping and dismissive claim. Alley himself says: “We have high confidence that warming will shrink Greenland, by enough to matter a lot to coastal planners.” ** ** Thomas Homer-Dixon University of Waterloo ** ** ** ** ** ** On Jan 28, 2013 5:12 PM, Andrew Revkin rev...@gmail.com wrote: There's also fresh input from Richard A. (and Waleed Abdalati) on Greenland and sea level in this new dot earth post: ** ** Eyes Turn to Antarctica as Study Shows Greenland's Ice Has Endured Warmer Climates http://nyti.ms/Yq7uhA ** ** I turned to Richard Alleyhttp://dotearth.blogs.nytimes.com/2009/02/06/richard-alleys-orbital-and-climate-dance/, who’s become a vital touchstone for me on such research, for some insights. Here’s his comment, followed by my closing thoughts: I have three immediate responses: Satisfaction in the great success of the collaboration, concern that this slightly increases worries about future sea-level rise from human-caused warming, but technical questions that may leave us more-or-less where we were before on the biggest picture. Taken in turn: Having watched colleagues go to the immense effort of learning what information is desired by policymakers and other citizens, assemble the logistical and scientific abilities to supply that information, and actually do it over a lot of years, and knowing just how many of their kids’ soccer games and recitals some of the scientist-parents missed, I have to smile when the team succeeds so well. As to the big picture, there is strong evidence from the history of sea level on coasts from the Eemian that both Greenland and Antarctic ice sheets shrank notably, contributing to a globally averaged sea-level rise of very roughly 20 feet. This occurred primarily in response to a rearrangement of where sunshine reached the planet and when during the year, with more summer sunshine in the north but very little total change. And, some uncertainty has remained on the exact balance between Greenland and Antarctic contributions. The new paper suggests that the contribution from Greenland was on the low end of the prior estimates, but has little effect on the estimated total sea-level change, which points to a larger Antarctic source than the previous best estimate. In my opinion (and I believe the opinions of many colleagues), we have greater understanding of Greenland’s ice than Antarctica’s, and we have greater confidence that Greenland will be “well-behaved” — we will more easily project changes in Greenland’s ice, with greater confidence that changes begun now will take centuries or longer to be mostly completed.*** * By shifting more of the sea-level rise into the less-understood ice, and thus into the ice with greater chance of doing something rapidly, I believe the new paper at least slightly increases the concerns for coastal planners, even if the chance of a rapid change from Antarctic ice remains small. As to the technical parts, as described in many sources, we have lots of paleothermometers for the central Greenland ice cores over the last 100,000 years, providing multiple validation and high confidence that temperatures have been estimated accurately. The very changes in the ice sheet that are of greatest interest here also make the effort quite difficult. The melting of the Eemian interferes with gas-based paleothermometry, and with the total-gas technique that provides constraints on changes in surface elevation. A U.S. government CCSP report on Arctic paleoclimates a few years ago (to which I contributed) [linkhttp://dotearth.blogs.nytimes.com/2013/01/24/eyes-turn-to-antarctica-as-study-shows-greenlands-ice-has-endured-warmer-climates/%3Ehttp:/www.climatescience.gov/Library/sap/sap1-2/final-report/default.htm] estimated changes in temperature and ice volume for this interval. The new estimates overlap with the older ones. Were I working on that report now, I would recommend expanding the uncertainties a little to include the new results. However, considering that ice shrinkage on Greenland has a feedback effect (exposing rocks allows more sun to be absorbed, causing more warming), considering the evidence of Eemian warmth from marine records around Greenland, considering climate model runs for that time, considering other studies of Greenland, and recalling the notable uncertainties associated with untangling the changes in total gas and in the ice sheet
Re: [geo] Re: Why Greenland's melting could be the biggest climate disaster of all
Hi Andy‹Your agreement with the dismissive statement on Greenland seems terribly short-sighted. Over the coming decade (if not already), we¹ll be setting a course for Greenland that will lead to much higher sea level in the future (and the contributions from Greenland and Antarctica will end up being far more than from thermal expansion and melting glaciers). A key issue at present among politicians is the impacts we are imposing on future generations (national debt, etc.)--well, dealing with Greenland melting is quite the predicament we would be posing to future generations (so the children and grandchildren of today¹s politicians). Mike On 1/28/13 9:56 AM, Andy Revkin rev...@gmail.com wrote: A sideshow to sea-level questions on policy-relevant time scales. (2100-ish at best).. You're talking geological scale here. Tad Pfeffer's 2008 analysis of worst-case discharge rate still a keystone to clear thinking on this. On Mon, Jan 28, 2013 at 2:38 PM, Thomas Homer-Dixon t...@homerdixon.com wrote: ³Greenland . . . is a sideshow in the sea level question.² I see nothing in the Dahl-Jensen article that could possible justify such a sweeping and dismissive claim. Alley himself says: ³We have high confidence that warming will shrink Greenland, by enough to matter a lot to coastal planners.² Thomas Homer-Dixon University of Waterloo On Jan 28, 2013 5:12 PM, Andrew Revkin rev...@gmail.com wrote: There's also fresh input from Richard A. (and Waleed Abdalati) on Greenland and sea level in this new dot earth post: Eyes Turn to Antarctica as Study Shows Greenland's Ice Has Endured Warmer Climates http://nyti.ms/Yq7uhA I turned to Richard Alley http://dotearth.blogs.nytimes.com/2009/02/06/richard-alleys-orbital-and-clim ate-dance/ , who¹s become a vital touchstone for me on such research, for some insights. Here¹s his comment, followed by my closing thoughts: I have three immediate responses: Satisfaction in the great success of the collaboration, concern that this slightly increases worries about future sea-level rise from human-caused warming, but technical questions that may leave us more-or-less where we were before on the biggest picture. Taken in turn: Having watched colleagues go to the immense effort of learning what information is desired by policymakers and other citizens, assemble the logistical and scientific abilities to supply that information, and actually do it over a lot of years, and knowing just how many of their kids¹ soccer games and recitals some of the scientist-parents missed, I have to smile when the team succeeds so well. As to the big picture, there is strong evidence from the history of sea level on coasts from the Eemian that both Greenland and Antarctic ice sheets shrank notably, contributing to a globally averaged sea-level rise of very roughly 20 feet. This occurred primarily in response to a rearrangement of where sunshine reached the planet and when during the year, with more summer sunshine in the north but very little total change. And, some uncertainty has remained on the exact balance between Greenland and Antarctic contributions. The new paper suggests that the contribution from Greenland was on the low end of the prior estimates, but has little effect on the estimated total sea-level change, which points to a larger Antarctic source than the previous best estimate. In my opinion (and I believe the opinions of many colleagues), we have greater understanding of Greenland¹s ice than Antarctica¹s, and we have greater confidence that Greenland will be ³well-behaved² ‹ we will more easily project changes in Greenland¹s ice, with greater confidence that changes begun now will take centuries or longer to be mostly completed. By shifting more of the sea-level rise into the less-understood ice, and thus into the ice with greater chance of doing something rapidly, I believe the new paper at least slightly increases the concerns for coastal planners, even if the chance of a rapid change from Antarctic ice remains small. As to the technical parts, as described in many sources, we have lots of paleothermometers for the central Greenland ice cores over the last 100,000 years, providing multiple validation and high confidence that temperatures have been estimated accurately. The very changes in the ice sheet that are of greatest interest here also make the effort quite difficult. The melting of the Eemian interferes with gas-based paleothermometry, and with the total-gas technique that provides constraints on changes in surface elevation. A U.S. government CCSP report on Arctic paleoclimates a few years ago (to which I contributed) [link http://dotearth.blogs.nytimes.com/2013/01/24/eyes-turn-to-antarctica-as-stu dy-shows-greenlands-ice-has-endured-warmer-climates/%3Ehttp:/www.climatescie nce.gov/Library/sap/sap1-2/final-report/default.htm ] estimated changes in temperature and ice
RE: [geo] Re: Why Greenland's melting could be the biggest climate disaster of all
Three quick points. First, Andy (if I may), I was responding to the following remark you made in your NYT blog after Richard Alley's comment on the Dahl-Jensen article: I do think [the new work] closes the case that Greenland, despite all of its drama (moulins, for example) - drama that focused my attention for a few years too - is a sideshow in the sea level question. I'm having trouble squaring the above remark with the one immediately below in this thread (Don't get me wrong. I'm not endorsing that the probable Greenland contribution to sea level doesn't matter to policy). Second, you cite Pfeffer 2008, which argues kinematic constraints will keep Greenland's contribution to SLR to less than 20 cm by 2100. But Pfeffer 2008 almost certainly isn't the last word on this matter. As Hansen notes, recent data suggest Greenland ice sheet mass loss may be exponential, and he and others have challenged the assumptions in Pfeffer 2008. So a flat declaration that Greenland is a sideshow seems a tad premature. Third, I'd like to challenge the notion that the policy-relevant time scale operative in this case ends at 2100. The relevant horizon is actually both farther away from us than 2100 or nearer to us, depending on our policy interests. The horizon should probably be pushed further away if we're focusing on climate or SLR impacts on our societies. Nearly half of the children under 10 today will still be alive in 2100 (assuming current incremental increases in longevity continue). Also, 2100 is easily within the planning horizon of major infrastructural projects, including storm sewers, sea walls, coastal roads and bridges, etc. The relevant time horizon should be brought much closer to us, given the inertia in climate and ice systems, if we are interested in taking mitigative action to prevent severe climate and SLR impacts in the future. Mike MacCracken has already made a similar point. Even if we project that Greenland's melting won't raise sea level by a meter till (say) 2150, if our emission commit us to that meter by 2050, then 2050 is the relevant policy horizon for avoiding that meter through mitigation. THD From: Andrew Revkin [mailto:rev...@gmail.com] Sent: Monday, January 28, 2013 9:23 PM To: Mike MacCracken Cc: Thomas Homer-Dixon; Geoengineering Subject: Re: [geo] Re: Why Greenland's melting could be the biggest climate disaster of all Don't get me wrong. I'm not endorsing that the probable Greenland contribution to sea level doesn't matter to policy. I'm just stating a fact related to how humans - as individuals and groups - have responded to risks that require big changes in the status quo. Thomas, I'd be eager to see any data you have showing otherwise. On Mon, Jan 28, 2013 at 9:15 PM, Mike MacCracken mmacc...@comcast.net wrote: Hi Andy-Your agreement with the dismissive statement on Greenland seems terribly short-sighted. Over the coming decade (if not already), we'll be setting a course for Greenland that will lead to much higher sea level in the future (and the contributions from Greenland and Antarctica will end up being far more than from thermal expansion and melting glaciers). A key issue at present among politicians is the impacts we are imposing on future generations (national debt, etc.)--well, dealing with Greenland melting is quite the predicament we would be posing to future generations (so the children and grandchildren of today's politicians). Mike On 1/28/13 9:56 AM, Andy Revkin rev...@gmail.com wrote: A sideshow to sea-level questions on policy-relevant time scales. (2100-ish at best).. You're talking geological scale here. Tad Pfeffer's 2008 analysis of worst-case discharge rate still a keystone to clear thinking on this. On Mon, Jan 28, 2013 at 2:38 PM, Thomas Homer-Dixon t...@homerdixon.com wrote: Greenland . . . is a sideshow in the sea level question. I see nothing in the Dahl-Jensen article that could possible justify such a sweeping and dismissive claim. Alley himself says: We have high confidence that warming will shrink Greenland, by enough to matter a lot to coastal planners. Thomas Homer-Dixon University of Waterloo On Jan 28, 2013 5:12 PM, Andrew Revkin rev...@gmail.com wrote: There's also fresh input from Richard A. (and Waleed Abdalati) on Greenland and sea level in this new dot earth post: Eyes Turn to Antarctica as Study Shows Greenland's Ice Has Endured Warmer Climates http://nyti.ms/Yq7uhA I turned to Richard Alley http://dotearth.blogs.nytimes.com/2009/02/06/richard-alleys-orbital-and-cli mate-dance/ , who's become a vital touchstone for me on such research, for some insights. Here's his comment, followed by my closing thoughts: I have three immediate responses: Satisfaction in the great success of the collaboration, concern that this slightly increases worries about future sea-level rise from human-caused warming, but technical questions that may leave us more-or-less
