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Greenland rising faster as ice loss accelerates

Posted on 24 May 2010 by John Cook

We have several independent lines of evidence that Greenland is losing ice at an accelerating rate. Satellite altimetry find glaciers are sliding faster downhill and dumping more ice into the ocean. Altimetry data also find the ice sheet is thinning. An overall picture is obtained by satellites measuring the gravity around the ice sheet. Another line of evidence has now been added to this picture with GPS measurements finding that Greenland is losing ice so quickly, the land is now rising up at an accelerating rate.

These results are published in Accelerating uplift in the North Atlantic region as an indicator of ice loss (Jiang 2010). The study looks at high-precision global positioning system (GPS) data that measure the vertical motion of the rocky margins around Greenland, Iceland and Svalbard. The weight of ice sheets push down on the bedrock it rests on. As the ice sheets lose mass, the bedrock rises. This process, known as Glacial Isostatic Adjustment (GIA), has been happening since the planet came out of an ice age around 17,000 years ago. How do we know whether current uplift might be a delayed response to glacial retreats from thousands of years ago? To avoid the effect of past events, this study focuses on vertical acceleration rather than velocities. The results are therefore insensitive to GIA-related motions from past ice mass changes.

What they find is crustal uplift in Greenland, Iceland and Svalbard is accelerating. Extrapolating the acceleration backwards in time finds the acceleration began after 1990. The acceleration of uplift over the past decade represents an essentially instantaneous, elastic response to recent accelerated melting of ice throughout the North Atlantic region.

Vertical Uplift in Greenland, Iceland, Canada
Figure 1: GPS measurements for the North Atlantic region. The numbers (eg - 0.6 mm/yr2) show the amount of acceleration. The red (upper) time series (Greenland, Iceland) show positive acceleration and the blue (lower) time series (Fennoscandia, Canada) show no significant acceleration.

From the rates of uplift around Greenland, they estimate ice loss is accelerating at 21.2 gigatonnes/yr2. This agrees well with other estimates of ice loss accelerating at around 21 gigatonnes/yr2. The following shows estimates of the rate of Greenland ice loss measured from satellite altimetry, GRACE gravity data and net accumulation/loss measurements.

Greenland ice loss measured by net accumulation/loss, altimetry and GRACE gravity observations
Figure 2:  Rate of ice loss from Greenland. Vertical lines indicate uncertainty, horizontal lines indicate averaging time. Blue circles are from altimetry, red squares are from net accumulation/loss and green triangles are from GRACE. The black line is a straight-line (constant acceleration) fit through the mass balance data for the period 1996–2008 with a slope of 21 gigatonnes/yr2.

So combining altimetry, net accumulation/loss, GRACE gravity data and GPS measurements, we find multiple lines of evidence converging on a single answer: Greenland is losing ice mass at an accelerating rate. If this acceleration continues, Greenland could soon become the largest contributor to global sea level rise

Lastly, one interesting point. The amount of uplift in Greenland varies from location to location, from 1.4 mm per year in northwest Greenland to over 10 mm per year in other places. In some locations, this exceeds the current rate of global sea level rise which is around 3.2 mm per year. Greenland's uplift rate is predicted to double by 2025. Sadly, this doesn't mean we can now relax about sea level rise - unless you have a huge melting ice sheet in your neighbourhood, you're unlikely to see uplift rates like those seen in Greenland.

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Comments 51 to 62 out of 62:

  1. You guys realize that it is hard to get to excited about Greenland's icecap melting at 300gt annually out of 2.8 million gt, especially since that's got to be the high end of the estimate. GRACE estimates the melt at only 185gt annually. It took 33 years to go from 0 to 300gt, at that rate accelerating technological evolution will have yield innovations both social and economically beyond our wildest dreams long before even 1.0% of Greenland's ice is lost. Hard to imagine our hydrocarbon-based economy surviving the next 40 years of technological evolution. If this is the signal of AGW, then just what part of it is catastrophic and unprecedented? "This is a big change, and it is a big deal." Why? Just what does 30 years warming trend really mean to the life cycle of 3 million gt icecap left over from the last ice age?
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  2. HumanityRules at 12:18 PM on 26 May, 2010 HumanityRules at 12:45 PM on 26 May, 2010 There’s nothing particularly alarmist in the data on Greenland ice melt, but there’s no question that the prognosis is alarming. Obviously during the past 100’s and 1000’s of years sea levels have waxed and waned a little as long term temperature variations cause polar ice sheets to advance and retreat. However over the last 2000 years (where we have a reasonable handle on sea levels; [*]), the evidence indicates little net change in sea levels. So sea levels likely did rise above present levels (by of the order of 8-17 cm according to [**]) during the Medieval period, and were likely at a minimum (of perhaps 24-30 cm below current levels [**]) during the LIA. During Roman times sea levels were apparently similar to mid 20th century levels. Now it’s going to be all in one direction during the coming century (barring astonishing solar phenomena or massive volcanic activity, or a truly remarkable change in technologies for energy production or CO2 sequestration). It’s pretty well understood that the rate of sea level rise is roughly proportional to the temperature difference between some temperature at which polar ice variation is balanced, and the extant temperature [***]. So as the Earth temperature rises during the coming century the rate of sea level rise will very likely continue to increase. That’s the expectation of greatest likelihood. I would say that's pretty concerning… …but I don’t see how a straightforward consideration of the evidence is alarmist. [*] G. A. Milne et al. (2009) Identifying the causes of sea-level change Nature Geoscience 2, 471 - 478 abstract [**] A. Grinsted et al (2010) Reconstructing sea level from paleo and projected temperatures 200 to 2100 AD Clim. Dynam. 34, 461–472. abstract [***] S. Rahmstorf (2007) A Semi-Empirical Approach to Projecting Future Sea-Level Rise Science 315, 368 – 370 abstract [***] M. Vermeer and S. Rahmstorf (2009) Global sea level linked to global temperature PNAS 106, 21527-21532 abstract
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  3. "Maybe they print this "alarmist" data because that is what all the data look like." Seems logical to me. And I'd take skeptics a bit more seriously if they'd give up their use of the term "alarmist." Either you have a valid scientific objection to the data, or you don't. If you don't, accusations of "alarmism" are meaningless. If you do, they're unnecessary.
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  4. Wes, read this to get up to reasonable speed on the whole subject: Spencer Weart's The Discovery of Global Warming. In particular your remarks about technological advances superseding the amount of C02 we've added to the air suggest a lack of sufficient background in the topic.
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  5. Wes, "Hard to imagine our hydrocarbon-based economy surviving the next 40 years of technological evolution." It was hard to imagine in 1970, too. And yet, here we are.
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  6. Further to Phila's remarks on "alarmists", "warmists" and the like, this research was done and will continue to be done quite apart from any ideological considerations; folks such as Phil Jones are nice examples of somebody's curiosity leading them inadvertently into the limelight, as Pielke Sr. recently remarked. The muse researchers are following is mostly a private spirit. Public reactions to research findings are quite divorced from motivations leading to new discoveries; Charles Darwin did not set out to upset Christian orthodoxy regarding creation but inadvertently blundered into that controversy. The exact behavior of ice sheets, the response of the crust to changes in mass balance, all these matters and more will be scrutinized to a fare-thee-well not because researchers are grinding some axe but instead due to the human inclination to resolve mysteries. That may be a hard thing to believe but I think most scientists would largely agree.
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  7. wes george writes: You guys realize that it is hard to get to excited about Greenland's icecap melting at 300gt annually out of 2.8 million gt, especially since that's got to be the high end of the estimate. GRACE estimates the melt at only 185gt annually. No, actually, GRACE shows that the loss of mass from Greenland has been accelerating, and is now over 300 GT/year. You really need to look at this quantitatively. Not many years ago, Greenland was actually neutral or gaining mass because warming was slight enough that the increased ablation was more than compensated for by precipitation. But the mass balance is now on a clearly accelerating downward trend. From a discussion in another recent thread: [O]ver the past decade the rate of ice loss from Greenland accelerated by about 11% per year. This is highly unlikely to continue (there would be no ice left by ~2075 or so). If that acceleration dropped to 3% per year tomorrow and continued for the rest of the century, you'd end up with 0.3 to 0.4 m sea level rise from Greenland, 1.0 m SLR total. One meter of sea level rise would be very problematic. But that's assuming a much lower rate of acceleration than we've seen over the past decade ... and Greenland would still have 93% of its ice. That's a pretty scary thought, IMHO.
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  8. Present and future climates of the Greenland ice sheet according to the IPCC AR4 models Bruno Franco1 , Xavier Fettweis, Michel Erpicum and Samuel Nicolay Climate Dynamics DOI 10.1007/s00382-010-0779-1 Abstract "......We also show that the GrIS surface mass balance anomalies from the SRES A1B scenario amount to −300 km3/year with respect to the 1970–1999 period, leading to a global sea-level rise of 5 cm by the end of the 21st century.........." 5cm would be less problematic.
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  9. #49 Humanity Rules, where did I say that the Earth as a whole was not losing ice over the last couple hundred years? I said GREENLAND was not. Now it is. That's a fundamental change. The blue diamonds on your figure may be the red squares on Figure 2 in John's original post. I prefer that other figure because it shows all of the independent estimates rather than a single source (and it lacks the overly squiggly "linear reconstruction", which is based on heaven knows what (not the observations!). Not to say that the general trend of that reconstruction is necessarily wrong, just that without a lot more information about how it was estimated, it shouldn't be assumed to be right, and its continuity is misleading.
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  10. #58 Humanity Rules, you can take solace in the model prediction if you like, but the present rate of mass loss rate of 300 Gt/year is about 300 km3/year of ice (1 km3 of ice masses about 0.9Gt), and by my count there remain 90 years of warming in the A1B scenario to go before we hit 2100. Meaning that the model is massively underpredicting the rate of ice loss. My guess is that the model ignores glacier flow, but that's a guess based on a quick look at Fettweis' web site, which shows figures for melting and precipitation. This may also provide some context for wes george's question about why this is a big deal (#51). I also got quite a chuckle out of someone's claim (I forget who) that Science and Nature were somehow competing to publish "alarmist data". You must be joking!
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  11. HumanityRules, like the IPCC AR4 sea level rise projections, that paper doesn't attempt to include discharge of ice by marine-terminating glaciers. Its mass balance estimates are based solely on precipitation vs surface melt. This is the same problem that people have been discussing ever since IPCC AR4 came out -- their projections of sea level rise do not include ice discharge and are thus far too low. There are some further issues with the paper HR cites (they don't include several other factors that increase the rate of ice loss, and their model doesn't deal with spatial variability across Greenland -- they themselves point out in the paper's Discussion that this means their estimates of negative mass balance may be too low). So, it's not an uninteresting paper, but its projections of sea level rise are obviously incomplete. I am a bit surprised that they didn't say anything about the fact that their "-300 GT/year mass balance" prediction for 2100 has already come true by 2009-2010.
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  12. Wait a second. The annual loss of ice off Greenland is about 200-300km^3 per year, but that icecap is big – between 2.4 and 3,400,000 km^3. And old too. Yet you guys keep evincing ridiculously short melting trends then say if these trends not only continue on this very short slope but accelerate exponential then Greenland’s icecap will be gone in 65 years. Oh, then you concede that probably won't happen, it will probably be about 7% melt of total ice in 65 years. Still catastrophic. Yet, the observed data shows 7% of Greenland's ice will take about 1000 years to melt. So you're postulating an exponential rate of acceleration starting today. This should be easy to test. I understand there is a theoretical apparatus behind such claims, but to those of us who are less committed to theory and more to the observed data, it sure looks like Greenland has been melting at a rate of about 0.007% recently. Even if the trend accelerates there seems to be little chance of the Greenland icecap disappearing before the end of the current interglacial. This is because your projections assumes absolutely no interruption in a very short term trend, socially, technologically or climatically (other than accelerated AGW) over the next century. Philia’s comment illustrates that the one truly exponential rate of change that is robust and long observed seems to carry little weight here. That is technological evolution is occurring at a rapidly accelerating rate, rendering any long term forecasts for climate based on today’s level of technology simplistic. Moreover, extrapolating a very short trend forward 10 to 1000 times its length seems to ignore how complex nonlinear systems far from equilibrium evolve. Even if AGW theory is robust, climate is unlikely to respond in a mechanical, direct way to forcing in the same way a steam engine might to a governor adjustment. To those old enough to remember how “futurology” once worked back in the 1960’s and 70’s where someone put a ruler and pencil to a trend of, say, current known global petroleum reserves then drew a line straight into the future arriving at the conclusion that peak oil must occur in 1979…this smells very similar. Doubters back then were told they didn’t understand the confidence levels experts had in total amount of undiscovered reservoirs based on some now long forgotten theories of oil formation and the rate of evolution in extraction technologies.
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    Response: I posted an article on this very line of argument yesterday: Why Greenland's ice loss matters. Please continue any further discussion on this topic over there.

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