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"Same as it ever was"... Ice caps melting faster than previously thought!!!

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  • "Same as it ever was"... Ice caps melting faster than previously thought!!!

    Just in time for Doha and Junk Science Palooza 2012...
    Ice sheets melting at poles faster than before

    8:53PM EST November 29. 2012 - WASHINGTON (AP) — Fueled by global warming, polar ice sheets in Greenland and Antarctica are now melting three times faster than they did in the 1990s, a new scientific study says.

    So far, that's only added about half an inch to rising sea levels, not as bad as some earlier worst case scenarios. But the melting's quicker pace, especially in Greenland, has ice scientists worried.

    One of the biggest wild cards in climate change has been figuring out how much the melting of the massive sheets of ice at the two poles would add to the seas. Until now, researchers haven't agreed on how fast the mile-thick sheets are thawing — and if Antarctica was even losing ice.

    The new research concludes that Antarctica is melting, but points to the smaller ice sheet in Greenland, which covers most of the island, as the bigger and more pressing issue. Its melt rate has grown from about 55 billion tons a year in the 1990s to almost 290 billion tons a year recently, according to the study.

    [...]

    Study lead author Andrew Shepherd of the University of Leeds in England, said their results provide a message for negotiators in Doha, Qatar, who are working on an international agreement to fight global warming: "It's very clear now that Greenland is a problem."

    [...]


    USA Today
    Note to the AP... Only one pole (the South Pole) has an ice sheet; so ice sheets aren't melting at the poles.

    By the sound of the AP report every climate scientist in the world except Richard Alley and Waleed Abdalati participated in this research. Abdalati is NASA's chief scientist and knows a lot about satellite imagery and Alley is one of the top experts on Greenland's ice sheet. You’d think they might have been invited to the party too…

    The Wall Street Journal article was a little less idiotic than the AP piece in the USA Today...
    ENVIRONMENT & SCIENCE Updated November 29, 2012

    Polar Ice Sheets Melt Faster

    Shrinkage in Greenland, Antarctica Has Sent Ocean Levels Higher, Study Says

    By GAUTAM NAIK
    Higher temperatures over the past two decades have caused the polar ice sheets to melt at an accelerating rate, contributing to an almost half-inch rise in global sea levels, according to the most comprehensive study done so far.

    Scientists long have struggled to get a fix on whether the permanent ice sheets of Greenland and Antarctica are gaining or losing ice. Past satellite-based measurements either were limited in scope or suffered from methodological inconsistencies.

    The new study, published Thursday in the journal Science, estimates that the melting of the ice sheets as a whole has raised global sea levels by 11.1 millimeters (0.43 inch) since 1992. That represents one-fifth of the total sea-level increase recorded in that period.

    In the 1990s, melting of the polar ice sheets was responsible for about 10% of the global sea-level rise, but now it represents about 30%, the data suggest.

    [...]

    "The signals suggest there is no immediate threat" from rising sea levels, Prof. Shepherd said. "But we can at least warn people that there are instabilities that need to be investigated."

    The study involved 26 laboratories and was supported by the European Space Agency and the U.S. National Aeronautics and Space Administration. The data used for the study were based on measurements from 10 separate satellite missions.

    WSJ
    The best thing about the WSJ article was the graph...



    One tiny little problem... If the accelerating ice loss is accelerating sea level rise... Why is sea level rise decelerating?

    Watts Up With That? | The world's most viewed site on global warming and climate change.

  • #2
    The abstract to the paper...
    Science 30 November 2012:
    Vol. 338 no. 6111 pp. 1183-1189
    DOI: 10.1126/science.1228102
    •Research Article
    A Reconciled Estimate of Ice-Sheet Mass Balance

    Andrew Shepherd... [Long list of co-authors]

    We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth’s polar ice sheets. We find that there is good agreement between different satellite methods—especially in Greenland and West Antarctica—and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by –142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes year−1, respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year−1 to the rate of global sea-level rise.

    LINK
    Unfortunately my level of membeship in the AAAS (the free level) doesn't provide access to the full text of this particular paper. However, there clearly is nothing alarming or remotely similar to the news reports in the abstract.

    Greenland is alleged to have lost between 93 and 191 gigatonnes of ice per year from 1992 (ten years before GRACE was launched) and 2011. If we assume 1 Gt of ice = 1 km^3 of ice and that the current volume of the Greenland ice sheet is ~5 million km^3 and that Greenland continues to melt at a rate of 142 km^3/yr over the next 90 years... The Greenalnd ice sheet will lose a bit more than 0.3% of its ice volume.

    ~142 Gt of ice per year equates to about 0.003% of ice mass loss per year. At 142 Gt/yr, Greenland will be ice-free in 35,211 years.


    GRACE (Gravity Recovery And Climate Experiment) consists of two satellites, launched in 2002, that measure subtle variations in Earth's gravitational field. GRACE is the ideal tool for measuring changes in Earth's polar ice caps.


    Figure 1. GRACE Mission (Source University of Texas).

    One of the most prolific authors on GRACE has been Dr. Isabella Velicogna, UC Irvine (one of Sheppard's co-authors). Back in 2009 Dr. Velicogna published this paper in GRL:

    Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE

    Dr. Velicogna concluded that the ice mass-loss was "accelerating with time." She found that "in Antarctica the mass loss increased from 104 Gt/yr in 2002–2006 to 246 Gt/yr in 2006–2009."

    Since the launch of GRACE, Dr. Velicogna has participated in several papers on GRACE and ice mass loss estimates for Antarctic and Greenland. Each paper has presented a more dire situation than the previous one, yet GRACE has not actually measured a significant ice mass loss in Antarctica. The actual GRACE measurements indicate a net mass gain (44 ±20 Gt/yr) from October 2003 through February 2007.


    Figure 2. Total Mass Difference: TMD = Actual GRACE measurements. TMD - IJ05 and TMD - ICE5G = GRACE measurements adjusted for GIA (Riva et al., 2007).

    Furthermore, the GIA-adjusted Total Mass Differences (TMD) from the TU Delft publication are significantly lower than those of Velicogna 2009.


    GIA is the abbreviation for "glacial isostatic adjustment," sometimes referred to as post-glacial rebound (PGR). The areas of the Earth's crust that were covered by thick ice sheets during the last glacial maximum were depressed by the ice mass. As the ice sheets have retreated over the last 15-20,000 years, the crust has rebounded (risen) in those areas. So, the GRACE measurements have to be adjusted for GIA. The problem is that no one really knows what the GIA rate actually is. This is particularly true for Antarctica.

    Riva et al., 2007 concluded that the ice mass-loss rate in Antarctica from 2002-2007 could have been anywhere from zero-point-zero Gt/yr up to 120 Gt/yr. Dr. Riva recently co-authored a paper in GRL (Thomas et al., 2011) which concluded that GPS observations suggest "that modeled or empirical GIA uplift signals are often over-estimated" and that "the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too high." (I don't have access to the full text of Thomas et al., 2011, just the abstract).

    So, there's no evidence that the Antarctic ice sheets have experienced any significant ice mass-loss since GRACE has been flying. The GIA has generally been as large or larger than the asserted ice mass-loss.

    In 2009, Velicogna asserted that Antarctic ice mass loss "increased from 104 Gt/yr in 2002–2006 to 246 Gt/yr in 2006-2009." In the current paper, supposedly showing accelerated melting, they claim that Antarctica is lost an average of 71 Gt/yr from 1992-2011. Both of those estimates add up to about 1,400 Gt from 1992-2011. This would mean that Antartica didn't lose any ice before 2002 or after 2009.

    The steepening of the trend occurred in mid-2006. So there were 5.5 years of +72 Gt/yr and 3.5 years of -70 Gt/yr measurements. Velicogna didn't repeat the mistake she made in 2006, when she actually published the pre-GIA (PGR) measurements...



    Before the PGR (GIA) adjustment, GRACE indicated a gain in ice mass. This means that from 2002-2006, GRACE was measuring a mass gain of 72 ±76 Gt/yr. Note: the error bar of the GIA is larger than the measured anomaly. From 2006 to 2009, GRACE recorded a net loss of 70 ±76 Gt/yr.


    Now, there should be some PGR or GIA. However, prior to Thomas et al., 2011 PGR/GIA had been model-derived. Now it appears that PGR/GIA is actually much smaller than the models indicated and its distribution is highly variable.

    But... Let's assume that the Velicogna GIA/PGR adjustment is correct and ice mass loss did accelerate from 2002-2009. Where did the water go?

    The rate of sea level rise has decelerated since 2002...



    Where did the meltwater go?

    More fun with numbers... Let’s assume that Antarctica is losing 190 Gt of ice mass per year. 190 Gt sounds like a really big number, doesn’t it?

    360 Gt of ice melt will yield 1 mm of sea level rise. 190 Gt is good for ~0.5 mm/yr of sea level rise.

    The volume of ice in the Antarctic ice cap is ~30,000,000 km3. 190 Gt is roughly 0.0006% of 30 million km3. GRACE is measuring no net change in the ice mass; yet a 0.0006% annual change is being calculated from the PGR adjustment.

    At 0.0006% per year, Antarctica will have lost 0.06% of its ice mass by the end of this century (99.94% of Antarctica will not have melted)! And sea level will have risen by… (drum roll)… 46 millimeters!!!… Almost 2 inches!!! Very extreme...
    Last edited by The Doctor; 30 Nov 12, 10:54.
    Watts Up With That? | The world's most viewed site on global warming and climate change.

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