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Saturday, July 25, 2015
Wednesday, July 22, 2015
"Linking interannual variability in extreme Greenland blocking episodes to the recent increase in summer melting across the Greenland ice sheet," by J. T. McLeod & T. L. Mote, International Journal of Climatology, 19 July 2015: doi: 10.1002/joc.4440
International Journal of Climatology, 19 July 2015; doi: 10.1002/joc.4440
Linking interannual variability in extreme Greenland blocking episodes to the recent increase in summer melting across the Greenland ice sheet
Jordan T. McLeod and Thomas L. Mote
Abstract
Linking interannual variability in extreme Greenland blocking episodes to the recent increase in summer melting across the Greenland ice sheet
Jordan T. McLeod and Thomas L. Mote
Abstract
Atmospheric blocking commonly occurs over the high latitudes of the Northern Hemisphere, resulting from the development of persistent areas of high pressure that lead to warmer-than-average surface temperatures west of the high centre. While the variability and trends in anticyclonic circulation patterns (including blocking) over Greenland have been previously documented, an analysis of the most extreme blocking events within the observational record is lacking. In this study, a historical climatology of extreme Greenland blocking episodes (GBEs) from 1958 to 2013 is examined within the context of anomalous anticyclonic circulation patterns over the North Atlantic region during recent years. Based on a combination of the ERA-40 (1958–1978) and ERA-Interim (1979–2013) reanalysis data sets, the Greenland Blocking Index (GBI) is used to quantify 500-hPa geopotential-height anomalies for the identification of extreme GBEs. The annual rate of extreme blocking days has doubled since 1958, reaching an average of approximately 20 days per year by 2013. The frequency and, to some extent, duration of extreme GBEs were unprecedentedly high from 2007 to 2013 compared to the 56-year period of record, with a majority of the increase occurring during the spring (MAM) and summer (JJA). A multiple linear regression analysis reveals that interannual variability in extreme blocking and the Atlantic Multidecadal Oscillation (AMO) are the two predominant drivers of surface meltwater production across the entire Greenland ice sheet (GrIS), but Arctic sea ice extent and North Atlantic cyclone activity can also influence the extent of summer melting over portions of the GrIS. Thus, in addition to the larger-scale atmospheric and oceanic variability, smaller-scale features such as extratropical cyclones can play a significant role in modulating GrIS surface melting each summer.
Key words: atmospheric blocking, Greenland ice sheet, extratropical cyclones, North Atlantic, Arctic amplification, cryosphere, Arctic sea ice, climate change
http://onlinelibrary.wiley.com/doi/10.1002/joc.4440/abstract
Key words: atmospheric blocking, Greenland ice sheet, extratropical cyclones, North Atlantic, Arctic amplification, cryosphere, Arctic sea ice, climate change
http://onlinelibrary.wiley.com/doi/10.1002/joc.4440/abstract
James Hansen: ‘Emergency Cooperation Among Nations’ Is Needed to Prevent Catastrophic Sea Level Rise
If a new scientific paper is proven accurate, the international target of limiting global temperatures to a 2 °C rise this century will not be nearly enough to prevent catastrophic melting of ice sheets that would raise sea levels much higher and much faster than previously thought possible.
According to the new study—which has not yet been peer-reviewed, but was written by former NASA scientist James Hansen and 16 other prominent climate researchers—current predictions about the catastrophic impacts of global warming, the melting of vast ice sheets and sea level rise do not take into account the feedback loop implications of what will occur if large sections of Greenland and the Antarctic are consumed by the world’s oceans.
A summarized draft of the full report was released to journalists on Monday, with the shocking warning that such glacial melting will “likely” occur this century and could cause as much as a 10 foot sea-level rise in as little as 50 years. Such a prediction is much more severe than current estimates contained in reports issued by the Intergovernmental Panel on Climate Change (IPCC) — the UN-sponsored body that represents the official global consensus of the scientific community.
“If the ocean continues to accumulate heat and increase melting of marine-terminating ice shelves of Antarctica and Greenland, a point will be reached at which it is impossible to avoid large scale ice sheet disintegration with sea level rise of at least several meters,” the paper states.
Separately, the researchers conclude that “continued high emissions will make multi-meter sea level rise practically unavoidable and likely to occur this century. Social disruption and economic consequences of such large sea level rise could be devastating. It is not difficult to imagine that conflicts arising from forced migrations and economic collapse might make the planet ungovernable, threatening the fabric of civilization.”
The Daily Beast‘s Mark Hertsgaard, who attended a press call with Dr. Hansen on Monday, reports that the work presented by the researchers is
"...warning that humanity could confront “sea level rise of several meters” before the end of the century unless greenhouse gas emissions are slashed much faster than currently contemplated.
This roughly 10 feet of sea level rise—well beyond previous estimates—would render coastal cities such as New York, London, and Shanghai uninhabitable. “Parts of [our coastal cities] would still be sticking above the water,” Hansen said, “but you couldn’t live there.”
This apocalyptic scenario illustrates why the goal of limiting temperature rise to 2 degrees Celsius is not the safe “guardrail” most politicians and media coverage imply it is, argue Hansen and 16 colleagues in a blockbuster study they are publishing this week in the peer-reviewed journal Atmospheric Physics and Chemistry. On the contrary, a 2C future would be “highly dangerous.”
If Hansen is right—and he has been right, sooner, about the big issues in climate science longer than anyone—the implications are vast and profound.
In the call with reporters, Hansen explained that time is of the essence, given the upcoming climate talks in Paris this year and the grave consequences the world faces if bold, collective action is not taken immediately. “We have a global crisis that calls for international cooperation to reduce emissions as rapidly as practical,” the paper states.
Hansen said he has long believed that many of the existing models were under-estimating the potential impacts of ice sheet melting, and told the Daily Beast: “Now we have evidence to make that statement based on much more than suspicion.”
Though he acknowledged the publication of the paper was unorthodox, Hansen told reporters that the research itself is “substantially more persuasive than anything previously published.”
For his part, Eric Holthaus, a meteorologist who writes about weather and climate for Slate, said the “bombshell” findings are both credible and terrifying. Holthaus writes:
To come to their findings, the authors used a mixture of paleoclimate records, computer models, and observations of current rates of sea level rise, but “the real world is moving somewhat faster than the model,” Hansen says.
[…] The implications are mindboggling: In the study’s likely scenario, New York City—and every other coastal city on the planet—may only have a few more decades of habitability left. That dire prediction, in Hansen’s view, requires “emergency cooperation among nations.”
In response to the paper, climate scientist Michael Oppenheimer of Princeton University affirmed: “If we cook the planet long enough at about two degrees warming, there is likely to be a staggering amount of sea level rise. Key questions are when would greenhouse-gas emissions lock in this sea level rise and how fast would it happen? The latter point is critical to understanding whether and how we would be able to deal with such a threat.”
The new research, Oppenheimer added, “takes a stab at answering the ‘how soon?’ question but we remain largely in the dark. Giving the state of uncertainty and the high risk, humanity better get its collective foot off the accelerator.”
And as the Daily Beast‘s Hertsgaard notes, Hansen’s track record on making climate predictions should command respect from people around the world. The larger question, however, is whether humanity has the capacity to act.
“The climate challenge has long amounted to a race between the imperatives of science and the contingencies of politics,” Hertsgaard concludes. “With Hansen’s paper, the science has gotten harsher, even as the Nature Climate Change study affirms that humanity can still choose life, if it will. The question now is how the politics will respond—now, at Paris in December, and beyond.”
Alun Hubbard, Jason Box: Heavy summer rain in Greenland speeds up ice melt
Rapid ice melt in Greenland is driven by intense late summer rainfall, making the vast ice sheet even more vulnerable as the earth warms, new research shows
Formation of lakes on the Greenland Ice Sheet after the main melt-season due to warm wet weather and heavy rain. Later this water drains to the bottom of the ice sheet enabling it to move and melt faster. This makes the ice sheet more vulnerable to such rapid melt events as the climate warms, scientists say in a new study. (Photo: Sam Doyle)
by Catherine Jex, ScienceNordic, July 13, 2015
You may think that warmer temperatures alone were enough to drive the melting of the Greenland Ice Sheet, and you would not be alone -- many glaciologists thought roughly the same.
But a new study shows for the first time that the Greenland Ice Sheet melts rapidly, not just with warm summer temperatures, but also after intense late-summer rain.
The new study shows that these heavy rain events have occurred frequently due to the warmer, wetter weather of the last 20 years, penetrating deeper into the ice sheet, making it move and melt faster.
“We saw 10 to 15 % of the total annual surface melt occur in this unusual week of warm, wet weather in late summer 2011. When this water reached the bed, the ice sheet lifted up and moved faster,” says Sam Doyle, from Aberystwyth University, UK, lead author of the new study, which has just been published in Nature Geoscience.
According to his colleague and co-author, professor Jason Box of the Geological Survey of Denmark and Greenland, their results add more weight to scientists’ concerns of the sensitivity of the entire Greenland Ice Sheet to melting.
“We’re seeing that warm wet weather, that is increasing with climate change, is driving more melt of the Greenland Ice Sheet than we thought. And worryingly, this melt is reaching further into the ice sheet,“ says Box, speaking via satellite phone, along with Doyle, as they work together on another field trip on Greenland.
Scientist: An interesting, thorough, and novel study
Edward Hanna, professor in climate change with the University of Sheffield, UK, is impressed with the new study.
“This is an interesting, thorough and novel study based on a wealth of robust meteorological and glaciological datasets, and is based on meteorological models as well as observations,” says Hanna, who was not involved in the research.
“The study adds to an already complex story of how Greenland Ice Sheet dynamics appear to be influenced by changes in weather conditions, sometimes on a daily timescale,” he says.
Heavy rain makes the Greenland Ice Sheet move
The study began after observing exceptionally warm wet weather in late summer 2011, causing huge amounts of melt at the ice surface. At this time of the year, there was no snow on the surface of the ice to absorb and act as a buffer for all this rain and melt water, which then moved very quickly through the ice sheet.
“At first our observations of this late-August intense rainfall and melt event were met with the attitude of 'hey big deal, it’s summer, it melts,’ ” says professor Alun Hubbard from the Center for Arctic Gas Hydrate, Environment and Climate at the Arctic University of Norway, and principle investigator of the project that led to the study.
A monster moulin on the Russell Glacier, Greenland. You can see a helicopter for scale. These large gaping holes allow rain and melt water to drain quickly into the ice sheet. (Photo: Alun Hubbard)
“But this new data, shows these periods of rapid movement of the ice are in fact tied to a particular type of rainfall event, the kind of storm you would expect to see in the mid-latitudes -- UK or Scandinavia -- and not so much in Greenland where high-pressure systems are more common.”
“We can now reinterpret these big late summer melt events to get a much better understanding of what is going on -- what is affecting the ice sheet system and how it is responding,” he says.
Imagine a storm drainage system after heavy rain
Hubbard uses the analogy of a storm drainage system, to explain how the rain and melted ice moves through the ice sheet so quickly.
“Imagine a big downpour in a city. You get so much rain so quickly, on to an impermeable concrete surface that the water is immediately shunted into the city's drainage system -- which due to the huge volumes of water -- can't cope. Drains back up and it floods. Basically, the same thing happens on the bare ice sheet surface, which like the city is literally comprised of pipes, conduits and cavities,” he says.
According to him, when these pipes back up, the hydraulic pressure lifts the ice sheet up, like a gigantic iceberg. There is less friction at the bed and so it moves faster.
Melting extends far into glacier interior
But observing one event was not enough to convince some glaciologists that this process is important over longer periods of time, or that rain could cause melting over such a wide area.
Doyle and his team analysed the strange event in more detail.
They monitored the ice-movement by GPS, set up their own weather stations on the ice, and measured how much water was being discharged from the ice sheet.
According to Hubbard, the rain event covered a huge area, and melting extended deep into the interior of the ice sheet, by about 140 km. This is where you would expect it to be frozen, and for rain to fall as snow.
They also gathered rainfall data for the last 20 years and compared it with other known melt events across Greenland.
“We saw these August rainfall events were in fact quite common, a major one occurring once every couple of years, and hence could explain some of the other rapid flow acceleration events recorded by other research groups working on the ice sheet,” says Hubbard.
Timely rainfall catches the ice sheet off guard
A scientist watches melt water drain from the Russell glacier catchment in Greenland. Scientists do not yet know just how often these rapid melt events may occur in the future as the earth warms and Greenland receives more rain. (Photo: Sam Doyle)
According to Doyle, it is not only the amount of rainfall that is important, but also the precise timing of this rain.
“The late-summer timing was critical. The event occurred after the end of the melt season and the ice sheet's drainage system had started to close down,” he says.
“In this closed-down state the ice sheet's drainage system just couldn’t cope with the exceptional water inputs from melt and rain, and this is why the ice sheet reacted so dramatically, lifting off its bed and accelerating the flow of the ice towards the sea,” he says.
Scientists expect more melt due to rain
Box emphasizes how climate change will continue to drive this process in the future.
“It is rain and warmer temperatures that cause melt, and we show that this melting trend has been underway for the last two decades as the atmosphere has warmed,” he says.
According to Hubbard, a succession of these rain events would have a big impact on the ice sheet -- allowing it to melt more and move faster.
“By speeding up the ice sheet, we are effectively turning up the tap -- opening the sluice gate and hastening the speed at which the ice moves and melts and hence ends up in the ocean. But we don’t yet know for sure how frequent or how intense these late-season rainfall events will be in the future,” he says.
Hanna echoes this caution.
“The trouble is we don't really have a clear prediction of how such events will change in frequency and intensity over Greenland with ongoing climate change,” he says.
“But certainly this is another, now well-evidenced potential feedback that should be included in studies of ice sheet change during the rest of this century,” says Hanna.
Country Denmark
http://sciencenordic.com/heavy-summer-rain-greenland-speeds-ice-melt
Monday, July 20, 2015
Updated ice sheet model matches wild swings in past sea levels: Changes make Antarctica lose more ice, faster
by Scott K. Johnson, arstechnica, January 25, 2015
It has been a bit of a head scratcher. Records of sea level during the last few million years tell us that there have been some warm periods where sea level may have been as much as 20 meters higher than it is today. When fed the conditions that prevailed at the time, however, our computer models of ice sheets haven’t been able to reproduce such a swelling of the ocean.
The models can simulate that much sea level rise, but it requires temperatures much higher than were seen during those warm periods. Realistic losses of ice from Greenland and the fragile, western part of Antarctica (the West Antarctic Ice Sheet) could only provide something in the neighborhood of 3 to 10 meters of sea level rise. That leaves 10 to 17 meters for the East Antarctic Ice Sheet—the largest and most stable ice sheet—to chip in. Convincing the miserly East Antarctic Ice Sheet to be that generous with its contents isn’t easy, which is why the models required such high temperatures.
Updating the models
So what are the models missing? Penn State’s David Pollard and Richard Alley, and University of Massachussetts, Amherst’s Robert DeConto had an idea for something to try. Two things to try, really. They added a pair of physical processes to an ice sheet model that weren’t simulated previously. The first was hydrofracturing. When water reaches the ice sheet from rain or ice melt at the surface, it fills crevasses in the ice.
If they're filled to a great enough depth, the water pressure forces the crevasse to open even deeper—that's termed hydrofracturing. The other process results from the simple fact that a sheer cliff of ice can only be so tall before it collapses under its own weight—a condition not encountered in too many places today.
One place it does occur is where floating glaciers calve large icebergs. These occur on the coastal outlet glaciers at the edges of ice sheets that are the most vulnerable to warming. The glacier thins towards its outer edge, and at some point it grows thin enough that it begins to float. The point at which it floats off the bottom is called the “grounding line”—from there out to the end of the ice is called an ice shelf. Ice shelves that grind against the shore (think of it floating in a bay or fjord) act to hold back the flow of ice behind them. These shelves gradually melt from below as they float in their seawater bath. But they can also melt from above and shed large bergs of ice at their outer edge.
Both hydrofracturing and cliff failure can increase the shedding of icebergs from shelves, hastening their demise and uncorking the glacier behind them. Once these things are happening near the grounding line, though, they can really accelerate its retreat if it's in an unstable configuration where the ground surface drops as you head inland. (Significant portions of Antarctica match that description.) Once you start retreating in that situation, the glacier may have to retreat a long way to find a stable position again.
A hasty retreat
Having added representations of these two processes to the model, the researchers simulated a sudden change from modern conditions to warmer conditions like those past periods of very high sea level. Then they watched the virtual Antarctic glaciers respond.
The results were dramatic. The new processes combined to have a huge impact. Instead of about 2 meters of sea level rise, Antarctica lost enough ice to raise global sea level 17 meters over several thousand years. The fragile West Antarctic Ice Sheet collapses in a matter of decades, rather than centuries or millennia. There’s 5 meters of sea level rise in the first two centuries, after which retreat in portions of the East Antarctic Ice Sheet really get going.
Much more work will be required to make sure these new processes are being simulated accurately, but the early returns show it could put researchers in the ballpark of solving the puzzle of past high sea levels.
The relevance for our present situation is less direct, as the warming in the simulation was not realistic, but the possibility that West Antarctica could lose ice faster than we thought is a serious one. Richard Alley, whose work on this possibility we’ve covered before, explained to Ars via email, “I believe (and I suspect many people do) that it is important for us as scientists to provide not only the most-likely future outcome, but also the range of possibilities, including some sort of assessment of best-case and worst-case outcomes. Best-case is fairly easy, I believe, but worst-case is not; however, providing both is likely to be useful to many people.”
“The physical knowledge that too-tall cliffs fail is very old and familiar to every miner or quarry-worker. The physical knowledge that ice is not the strongest rock on the planet is also rather old. And, the suggestion that cliff failure could affect West Antarctic stability dates back to 1962,” Alley wrote. “We now have stronger evidence that sufficient West Antarctic retreat could lead to a higher calving front than any on Earth today, and higher than a stability limit suggested by recent papers. Putting that understanding into projections of the future, as in our new paper, has implications for the worst-case scenario. And, testing against the paleoclimatic record provides support for that understanding.”
He continued, “It is still too early to say that this is an accurate worst-case scenario. Step-application of the [warming] is too extreme, clearly… but, it is within the realm of possibility that for the time-scale of collapse, the true worst worst-case scenario could be even a bit faster than modeled here; the renewed interest in this topic is recent, and the number of scientific papers exploring the physics remains low.”
Earth and Planetary Science Letters, 2014. DOI: 10.1016/j.epsl.2014.12.035
Beneath the permafrost, fountains wait to burst forth
by Ned Rozell, Alaska Dispatch, June 27, 2015
While tight-roping on tussock heads in a bog off the Chandalar River, two companions and I heard a waterfall. Strange.
Looking through binoculars, we saw a knee-high fountain of clear water in the tundra. The flow was as thick as your leg. We squished over to investigate.
The three of us had never seen water spewing from the ground in such a way. The clear water was so cold it burned, forcing us to pull our hands back after a second or two.
A few days later, on our flight out of the Bush, pilot Dirk Nickisch said yes, he had seen tundra "hydrants" in a few Arctic valleys. When I got back, local experts watched this video.
A fountain in a tussock bog near the Middle Fork Chandalar River.
They theorized that we had seen the effect of high-pressure groundwater finding a way through permafrost.
They theorized that we had seen the effect of high-pressure groundwater finding a way through permafrost.
Permafrost pressure
Dan White is a hydrologist by trade who wears his Xtratuf boots less often now as the University of Alaska's vice president of academic affairs and research. He thinks the hydrant may be an artesian well pressurized by a permafrost barrier.
"Looks like water entering the subsurface from higher on the mountain," he wrote in an email. "That is just the place it found to get out through the frozen ground. My guess is that water is channeling though a thawed ice wedge or something."
The gusher is about 75 miles north of the Arctic Circle, on the south slope of the Brooks Range. That part of northern Alaska has remained cold enough to preserve permafrost — ground that remains frozen through the heat of at least two summers (it often has endured thousands of summers). The area featured other permafrost-related landforms, such as a house-size pingo. We ate lunch on top of the mound one day, noticing the birch trees that grew on it were rare in the surrounding spruce forest.
Permafrost researcher Kenji Yoshikawa said sometimes pingos and frost blisters generate fountains. He thinks what we saw might be related to a frost blister, a pimple caused when freezing ground in early winter blocks groundwater already restricted from beneath by permafrost. The fountain we saw might be what happened when the pimple popped.
'Uncontrolled artesian well'
Water held under pressure by permafrost can be a problem if we try to use it. In 1946, workers for the Army Corps of Engineers drilled a well near the eastern end of Farmers Loop in Fairbanks. They penetrated a permafrost layer and the non-frozen layer beneath it. At about 100 feet, they hit water. It was under so much pressure that a 4-foot gusher erupted from the drill hole.
Water flowed around the well casing in what engineers called an "uncontrolled artesian well." Corps workers pumped cement down the casing to seal the well. They topped it with a 10-foot square of concrete that was 1 foot thick.
"In August 1948, the final loss of control occurred," wrote geologist Troy Péwé in the chilling publication Geologic Hazards of the Fairbanks Area. "Water began escaping from beneath the 10-foot square, and during the summer of 1949 the slab collapsed into an enormous, water-filled thermokarst cavity. Eventually the slab sank as much as 50 feet below the surface."
Two years later, engineers injected refrigerant brine in the ground and installed freeze probes around the wellhead. That refroze the well shaft and reestablished the permafrost seal.
Thirty years later, a drilling company sunk a well in the same area. It flowed out of control all winter, covering a portion of Farmers Loop with 2 feet of ice and inspiring lawsuits from local homeowners whose houses and cars became glaciated.
Ned Rozell is a science writer at the University of Alaska Fairbanks Geophysical Institute. Used with permission.
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