|Stef Lhermitte image published in Washington Post 9-25-17|
Friday, October 6, 2017
Where are the pines on the Pine Island Glacier in West Antarctica? Of course, there aren’t any. The name comes from the US Navy ship Pine Island, which surveyed near this region during Operation High Jump in 1946.
What the Pine Island Glacier (PIG) does have is cracks in its seaward floating ice shelf. These cracks have become more frequently observed and have spawned large icebergs including one spotted in late September that amounts to one hundred square miles, dubbed B-44. Now, compared to the immense size of the Antarctic ice sheets, that's puny. Don’t think that’s the issue. Rather it’s the role these ice shelves play as a throttle to the discharge of ice from the continent, and therefore how they affect sea level.
What’s the news about this? First some background. The PIG drains a significant portion of the West Antarctic Ice Sheet. A Wikipedia page gives all the details, but the amount of ice staged to flow from the PIG drainage is enough to raise global sea level an average of over one meter (4 feet). I have blogged about this region previously [May 20, 2014], reporting research that showed there are good reasons to believe the PIG system is nearly unstable or in fact unstable. What does stable/unstable mean? Simply, it means that the rate of discharge of ice from the glacial drainage equals the rate of input of ice and snow to the drainage area. Unstable means there is more rapid input than discharging output, or in the case we worry about, more rapid discharge than the input. That would mean rapid flow of ice off West Antarctica that will raise sea level.
One of the several means that control the discharge rate is the size of the floating ice shelves in front of the glacier. They serve to buttress the glacier and regulate the speed of advance, hence its discharge rate. What’s in fact happening is the PIG ice shelf front is retreating. One of the several means that control the discharge rate is the size of the floating ice shelves in front of the glacier. They serve to buttress the glacier and regulate the speed of advance, hence its discharge rate. What’s in fact happening is the PIG ice shelf front is retreating. Stef Lhermitte, at Delft University of Technology in the Netherlands, who noticed the latest calving or break-off in satellite images told the Washington Post; “It’s the fifth large calving event since 2000. This one and 2015, they were much further inland than the previous ones. So there has been a retreat of the calving front, specifically between 2011 and 2015.” With the ice shelf retreating, we can expect an increase in discharge.
But what about the Larsen ice shelf in the Antarctic Peninsula? That’s been breaking off also, and apparently at a high rate with bigger bergs. We’ve been reading about this over the past decade, most recently in July 2017 when Larsen C broke apart. Both ice shelves buttress the glaciers and ice sheets behind them but the Larsen blocks a small ice sheet on the Peninsula while the PIG throttles a significant portion, like ten percent, of the immense West Antarctic Ice Sheet.
Back in 2014, when I alerted followers to reports of the unstable condition of the PIG, it was acknowledged as a “looks like it” moment. Now, only three years later, it seems more certain that the drainage system of the PIG is in collapse. That’s bad news—a potential outcome worse than the damage we’ve seen for storm surges due to the monster hurricanes this summer (2017). Sea level rise is a permanent problem of our times. I hope some clever coastal engineers are on the job, figuring out how to face this challenge for the twenty-first century. But before they begin to build dikes and sea walls, it will be important to put a finer point on what to expect; sea level rise will differ from place to place. The California Ocean Protection Council recently published a good model for how to approach the science of sea level rise for that state. The April 2017 report for California is available from a link on their website.
Wednesday, March 22, 2017
|N. Mortimer; Geol. Soc. America|
Twenty-two years ago, I published a study of the breakup of the eastern parts of the Gondwana supercontinent 1. My goal was to understand how the New Zealand microcontinent (the term used at the time) was ripped away and drifted north to more hospitable climes. The study was related to my research in Marie Byrd Land, the region in West Antarctica from where New Zealand broke away.
My research recognized that along with New Zealand other continental crust pieces once attached to Gondwana made the trip north. These included the submerged continental fragments of the Campbell Plateau, Lord Howe Rise, and the Chatham Rise along with other smaller fragments. They all were one mass within Gondwana. Tectonic forces separated and distorted them as they drifted with the new ocean floor created during the breakup process.
I thought it made sense to name all these distributed pieces—that once were one, a single name to recognize that fact. Zealand seemed to be a necessary part of any name, and inspired by the Finnish composer Jean Sibelius, I coined the name Zealandia in parallel with the name of his composition Finlandia.
All seemed calm for about twenty years until a gorgeous book Zealandia,2 was published. Two New Zealand geologists expanded on my suggestion and put real depth into it, chronicling the natural history of Zealandia including the country of New Zealand since Gondwana breakup. But they made the startling claim that Zealandia was in fact its own continent—a notion that was implicit in my study but I did not make explicit because that was not my purpose.
Zealandia came to fame last month when GSA Today published a definitive argument by Nick Mortimer and colleagues 3 that Zealandia is a continent—the eighth one, previously ignored, mainly because 95% of it is out of sight beneath the waves. The 5% above sea level is the nation of New Zealand. Does New Zealand then "own" Zealandia? The United Nations Law of the Sea offers guidance.
Why is it a continent? The authors argue because it meets the geologic criteria, namely; (1) high elevation relative to regions floored by oceanic crust; (2) a broad range of siliceous igneous, metamorphic, and sedimentary rocks; (3) thicker crust and lower seismic velocity structure than oceanic crustal regions; and (4) well-defined limits around a large enough area to be considered a continent rather than a microcontinent or continental fragment.
How large is it? About the size of India, which is not a continent but once was before its northward journey caused it to collide and join with Asia between forty and thirty million years ago.
Publication of the GSA Today article (online) started a global media frenzy. I was contacted and spoke to reporters from around the world, and appeared on live TV news programs (see Business Insider). For Mortimer, he tells me it was overwhelming. Just imagine why this news was so shocking. A continent was found that we didn’t know about.
The main problematic issue is that most of Zealandia is below sea level. We don’t think of continents as under water—we think of continents as places where people live. The reason much of it is below sea level is because the breakup process thinned its crust. The continental crust of Zealandia is thinner that other continents in comparison. However, the fact that much of it remains below sea level for 95% of its territory is not a limiting factor. Sea level goes up and down over time. For example, sea level was 120-130 meters lower about twenty-four thousand years ago. The land area of Zealandia (now mostly the nation of New Zealand) would have been much greater then. Consider a plug could be pulled and the oceans drained. Zealandia would stand high like other continents if a bit lower.
The main insight from the “discovery” of Zealandia is that continents are not defined by coastlines, that is current sea level. Rather they are defined by geologic criteria that are independent of sea level. My recent blogs point to the fact that sea level is rising due to global warming. In a few hundred years, most of Florida will be below the surface of the Atlantic Ocean. But Florida will not be gone. It will still be Florida. It will still be part of the North American continent but beneath the waves.
1. Luyendyk, B.P. 1995. Hypothesis for Cretaceous Rifting of East Gondwana caused by Subducted Slab Capture. Geology, v. 23, pp. 373-376.
2. Mortimer, Nick, and Hamish Campbell. 2014. Zealandia: Our Continent Revealed. Auckland, NZ: Penguin Group (NZ). 271 pp.
3. Nick Mortimer and others. 2017. Zealandia: Earth’s Hidden Continent. GSA TODAY. Volume 27 Issue 3. (March/April 2017). pp. 27–35.
Sunday, January 8, 2017
On the trail in Marie Byrd Land.
Photo © Bruce Luyendyk
Sea level has been rising for decades, now at an increasing rate. Mostly we don’t notice this except in superstorms and hurricanes, or if we have visited the same beach for decades and wonder why it looks different (less sand, and cliff retreat).
If you're reading this, you aren’t someone who lives in a bubble. You know that scientists predict even more sea level rise during the next few human generations and beyond. If you live in Miami or New York or New Orleans or Orange County (and elsewhere of course) you and your offspring will for sure deal with significant sea level rise. Up to now, contributions to future sea level rise from Antarctic ice sheets this century is predicted to be small. New research shows this is not the case.
The Intergovernmental Panel on Climate Change (IPCC) is the go-to institution where we can find best-estimates of what will likely happen to sea level during the 21st century. In earlier blogs I gave some of the IPCC predictions (5/20/14; 6/9/14; 9/17/14; 9/6/15). I also repeated the statements by the IPCC in 2013 ( AR51 ) that sea level rise models for the next century have not included the more dramatic possibility of Antarctic Ice Sheet shrinkage and its contribution to sea level rise. The reason given by the IPCC is that the process of how the ice sheet might shrink was very murky – research was underway and not available to include in the predictions.
A problem identified with modeling the growth and shrinkage of Antarctic ice has been that models did accurately reproduce the growth of the ice sheet but not the shrinkage of it. The exercise is to construct a computer model that reproduces known geologic constraints on the ice sheet history—when it was larger and when it was smaller. It proved nearly impossible to model the shrinkage of the ice sheet. Specifically, shrinkage of the vast East Antarctic Ice Sheet proved the most difficult. The marine-based West Antarctic sheet proved easier—but most of Antarctica’s ice is on the East. Cleary, this is what we want to know—shrinkage means sea level goes up. What makes it shrink? How will the ice sheet shrink under the different scenarios for global warming that the IPCC has told us to expect in this century?
In the past few months, climate modelers at Penn State and University of Massachusetts, Amherst have added new physical parameters to those that affect the melting rate of Antarctic ice, and this has made the difference2. The new computer models published in Nature can reproduce both the growth and the shrinkage of the Antarctic Ice sheet that we know happened in the geologic past. Now a model can be computed for what Antarctic ice shrinkage will contribute to sea level rise this century.
The early models keyed on removal of the floating ice shelves (9/6/15) by melting from underneath from a warming ocean. The new approach added two more impacts. First, melting of ice shelves from above by a warmer atmosphere. Second, after disappearance (retreat) of ice shelves, the collapse of remaining exposed ice cliffs over 80 meters high. The broken pieces fall into the sea as icebergs. This is a runaway process. As the ice breaks off in cliffs, the ice sheet moves downhill to the continent edge. Taller cliffs get exposed and these too break off, etc.
The researchers tested this idea against two periods in the geologic past when data show the Antarctic sheets retreated. In fact, during the last time the ice sheets shrank (130,000 – 116,000 years ago; smaller, but did not disappear) sea level was 6 to 9 meter higher than today, as reported in AR5. The researchers input atmospheric warming and ice sheet cliffing and reproduced that observation and another further back in geologic time.
These new ideas proved to be the missing “dynamic processes” needed to model Antarctic Ice Sheet shrinkage.
Armed with the new model technique the researchers asked the important question: “what is the future?” They ran three predictions starting back from 1950 and up to year 2100. Each prediction assumed different warming scenarios selected by the IPPC, the RCP2.6, 4.5, and 8,5; —translation; no change to the current rate of increasing greenhouse gas emissions (8.5), abrupt reduction of the rate (2.6) and somewhere in between (4.5). The details are in the Nature publication but the important conclusion is:
“When applied to future scenarios with high greenhouse gas emissions, our palaeo-filtered model ensembles show the potential for Antarctica to contribute >1 m of GMSL [global mean sea level] rise by the end of this century, and >15 metres of GMSL rise in the next 500 years.” (R. M. DeConto, D. Pollard, Nature 531, 591 (2016).)
Sounds like a lot and it is—but wait, this is for Antarctica alone. The IPCC GMSL estimates don’t include “dynamic” contributions for the continent ice shrinking. Dynamic means processes like described above. The IPCC AR53 estimates that at the end of this century GMSL rise due to ocean warming, mountain glacier melting, Greenland ice melting, ground water runoff and more is in the range 0.26 to 0.82 m. To these estimates Antarctic ice loss contributions need to be added. Including Antarctica, GMSL is then projected to rise up to 2 meters by 2100.
The 2013 IPCC report stated that Antarctic ice would mostly maintain itself with a balance of shrinking in the West and growing in the East. Now we must think about these dynamic models that account for what is in the geologic record. If upheld, these new estimates show that Antarctica will be the main contributor to sea level rise in the lifetimes of our children and grandchildren—and the current estimates circulating in the literature and media are too low by a factor of two.
1. IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
2. R. M. DeConto, D. Pollard, Nature 531, 591 (2016)
3. IPCC: Table SPM-2, in: Summary for Policymakers. IPCC AR5 WG1 2013, p. 21