January 29, 2012

Ice and Ocean

Climate Change Research Centre: Climate Science 2009

Rapid Arctic sea-ice decline:
Summer-time melting of Arctic sea-ice has accelerated far beyond the expectations of climate models.
The area of summertime sea-ice during 2007-2009 was about 40% less than the average prediction from IPCC AR4 climate models.

Current sea-level rise underestimated:
Satellites show recent global average sea-level rise (3.4 mm/yr over the past 15 years) to be ~80% above past IPCC predictions.
This acceleration in sea-level rise is consistent with a doubling in contribution from melting of glaciers, ice caps, and the Greenland and West-Antarctic ice-sheets.

Glaciers and Ice-Caps

  • There is widespread evidence of increased melting of glaciers and ice-caps since the mid-1990s.
  • The contribution of glaciers and ice-caps to global sea-level has increased from 0.8 millimeters per year in the 1990s to be 1.2 millimeters per year today.
  • The adjustment of glaciers and ice caps to [the] present climate … is expected to raise sea level by ~18 centimeters [and under] warming conditions … as much as ~55 centimeters by 2100.

(p 23)


Ice-Sheets of Greenland and Antarctica

  • The surface area of the Greenland ice sheet which experiences summer melt has increased by 30% since 1979, consistent with warming air temperatures.
    Melt covered 50% of the ice sheet during the record season in 2007.
  • The net loss of ice from the Greenland ice sheet has accelerated since the mid-1990s and is now contributing as much as 0.7 millimeters per year to sea level rise due to both increased melting and accelerated ice flow.
  • Antarctica is also losing ice mass at an increasing rate, mostly from the West Antarctic ice sheet due to increased ice flow.

(p 24)


Ice Shelves

  • Ice-shelves connect continental ice-sheets to the ocean.
    Destabilization of ice-shelves along the Antarctic Peninsula has been widespread with 7 collapses over the past 20 years.
  • Signs of ice shelf weakening have been observed elsewhere than in the Antarctic Peninsula, eg in the Bellingshausen and Amundsen seas, indicating a more widespread influence of atmospheric and oceanic warming than previously thought.
  • There is a strong influence of ocean warming on ice sheet stability and mass balance via the melting of ice-shelves.

(p 27)


Sea-Ice

  • The observed summer-time melting of Arctic sea-ice has far exceeded the worst-case projections from climate models of IPCC AR4.
  • The warming commitment associated with existing atmospheric greenhouse gas levels means it is very likely that in the coming decades the summer Arctic Ocean will become ice-free …
  • Satellite observations show a small increase of Antarctic sea-ice extent and changes to seasonality, although there is considerable regional variability. …

(p 29)


The Oceans

  • Estimates of ocean heat uptake have converged and are found to be 50% higher than previous calculations.
  • Global ocean surface temperature reached the warmest ever recorded for each of June, July and August 2009.
  • Ocean acidification and ocean de-oxygenation have been identified as potentially devastating for large parts of the marine ecosystem.

(p 35)


Global Sea Level

  • Satellite measurements show sea-level is rising at 3.4 millimeters per year since these records began in 1993.
    This is 80% faster than the best estimate of the IPCC Third Assessment Report for the same time period.
    [The 2007 IPCC report projected essentially the same sea level rise as those of the TAR, to within 10%.]
  • Accounting for ice-sheet mass loss, sea-level rise until 2100 is likely to be at least twice as large as that presented by IPCC AR4, with an upper limit of ~2m based on new ice-sheet understanding.

(p 37)


Contents


Glaciers and Ice-Caps

Ice-Sheets of Greenland and Antarctica

Ice Shelves

Sea-Ice

The Oceans

Global Sea Level


CLIMATE CHANGE RESEARCH CENTRE

  • The Copenhagen Diagnosis, 2009: Updating the World on the Latest Climate Science, Climate Change Research Centre, The University of New South Wales, Sydney, 2009.
    Ian Allison, Nathan Bindoff, Robert Bindschadler, Peter Cox,Nathalie de Noblet-Ducoudré, Matthew England, Jane Francis, Nicolas Gruber, Alan Haywood, David Karoly, Georg Kaser, Corinne Le Quéré, Tim Lenton, Michael Mann, Ben McNeil, Andy Pitman, Stefan Rahmstorf, Eric Rignot, Hans Joachim Schellnhuber, Stephen Schneider, Steven Sherwood, Richard Somerville, Konrad Steffen, Eric Steig, Martin Visbeck and Andrew Weaver.

    ICE-SHEETS OF GREENLAND AND ANTARCTICA


    IPCC AR4 concluded that net ice loss from the Greenland and Antarctic ice sheets together contributed to sea level rise over the period 1993 to 2003 at an average rate estimated at 0.4 millimeters per year.
    [A] number of new studies … show that the rate of loss from both Greenland and Antarctica has increased recently [and that] the rate of ice discharge into the sea can occur far more rapidly than previously suspected.
    (p 24)


    Greenland


    [Estimates] of the mass budget of the Greenland Ice Sheet since 1960 [are based on:]
    • [Satellite] or aircraft altimeter measurements of height change of the ice sheet surface …
    • [Satellite] gravity measurements [and]
    • [The] balance between mass influx and discharge …
    Near-coastal surface melt and run-off have increased significantly … but total snow precipitation has also increased.
    The average Greenland surface temperature rose by more than 1.5°C over the period 2000 to 2006 …
    (p 25)


    Antarctica


    Several new studies using the GRACE satellite gravity data … show net loss from the Antarctic since 2003 with a pattern of near balance for East Antarctica, and greater mass loss from West Antarctica and the Antarctic Peninsula.
    [Like] Greenland, the rate of mass loss from the Antarctic ice sheet is accelerating, increasing from 104 Gt per year for 2002-2006 to 246 Gt per year for 2006-2009 (the equivalent of almost 0.7 millimeters per year of sea level rise).
    Gravity and altimeter observations require correction for uplift of the Earth’s crust under the ice sheets (glacial isostatic adjustment) [which] is poorly known for Antarctica.


    The Risk of Ice-Sheet Collapse


    The largest unknown in the projections of sea level rise over the next century is the potential for rapid dynamic collapse of ice sheets. …
    The major dynamic ice sheet uncertainties are largely one-sided: they can lead to a faster rate of sea-level rise, but are unlikely to significantly slow the rate of rise.
    (p 26)


    ICE SHELVES


    Ice shelves are floating sheets of ice of considerable thickness that are attached to the coast.
    (p 27)

    Warming along the [Antarctic] Peninsula has been dramatic, and on the western side has been substantially above the global average.
    [In] March 2009, more than 400 square kilometres collapsed off the Wilkins Ice Shelf on the western side of the Antarctic Peninsula.
    A number of mechanisms are [involved] in destabilizing floating Antarctic ice shelves …
    • surface warming leading to the creation of melt ponds and subsequent fracturing of existing crevasses
    • subsurface ice shelf melting from warming ocean waters; and
    • internal ice shelf stresses

    While the collapse of a floating ice shelf does not itself raise sea level, its collapse is followed by rapid acceleration of glacier outflow – which does raise sea level – due to the removal of the ice shelf buttressing effect. …
    A recent modeling study has suggested that the West Antarctic Ice Sheet would begin to collapse when ocean temperatures in the vicinity of any one of the ice shelves that surround it warm by about 5°C. …
    The widespread thinning and acceleration of glaciers along the Antarctic coast may indicate a significant impact of oceanic changes on glacier dynamics, a factor that has received little attention in past IPCC reports due to the lack of observational data …
    (p 28)


    SEA-ICE


    Arctic Sea Ice


    Perhaps the most stunning observational change since the IPCC AR4 has been the shattering of the previous Arctic summer minimum sea ice extent record – something not predicted by climate models.
    Averaged over the five-day period leading up to September 16, 2007, the total extent of sea ice in the Arctic was reduced to an area of only 4.1 million square kilometers … surpassing the previous minimum set in 2005 by 1.2 million square kilometers (about the same size as France, Spain, Portugal, Belgium and Netherlands combined). …
    Compared to the [1979 to 2000] median, the 2007 record involved melting 2.6 million square kilometres more ice (~40% of the median).
    (p 29)

    The September [summer] Arctic sea ice extent over the last several decades has decreased at a rate of 11.1 ± 3.3%/decade.
    The February [winter] extent has decreased at a rate of 2.9 ± 0.8%/decade.

    The thickness of Arctic sea ice has also been on a steady decline over the last several decades.

    When Will the Arctic Ocean be Ice-Free?


    Due to the existence of natural variability within the climate system, it is not possible to predict the precise year that the Arctic Ocean will become seasonally ice free. …
    Evidence is … emerging to suggest that the transition to an ice-free summer in the Arctic might be expected to occur abruptly, rather than slowly. …
    [An] abrupt reduction in Arctic summer sea ice extent also triggers rapid warming on land and subsequent permafrost degradation.
    (p 30)


    Antarctic Sea Ice


    Antarctic sea-ice extent [shows] a net annual-mean area increase of ~1% per decade over the period 1979–2000. …

    Since Antarctica is a land mass surrounded by the vast Southern Ocean … and as oceans respond less rapidly than land to warming because of their thermal stability, one would expect, and indeed climate models show, a delayed warming response around Antarctica.
    (p 30)

    Isn’t Antarctica cooling and Antarctic sea ice increasing?

    Although the weather station at the South Pole shows cooling over this period, this single weather station is not representative.
    For example, there is a warming trend at Vostok, the only other long-term monitoring station in the interior of the continent.
    Several independent analyses … show that on average, Antarctica has warmed by about 0.5°C since wide-scale measurements began in the 1957 International Geophysical Year, with particularly rapid warming around the Antarctic Peninsula region and over the West Antarctic Ice Sheet …

    Since the development of the Antarctic ozone hole in the late 1970s, there has been a strengthening of the circumpolar winds around Antarctica, which tends to reduce the amount of warmer air reaching the interior of the continent.
    The stronger winds are due to cooling in the upper atmosphere, which are in turn a result of ozone depletion caused by chlorofluorocarbons. …
    Ironically, human emissions of CFCs are thus helping to partly offset interior Antarctic warming …
    As the ozone hole gradually repairs over the coming century, the cooling offset is likely to diminish.
    (p 33)


    THE OCEANS


    Ocean Warming


    Satellite measurements for the surface ocean showed 2007 to be the warmest year ever recorded, despite the extremely strong El Niño of 1997/1998. …
    Increases in oceanic heat content in the upper ocean (0-700m) between 1963 and 2003 have been found to be 50% higher than previous estimates [and] are now consistent with observations of sea-level rise over the last 50 years, resolving a long standing scientific problem in understanding the contribution of thermal expansion to sea-level.


    Salinity and the Hydrological Cycle


    More comprehensive analyses of ocean salinity show a freshening of high latitudes, while regions of excess evaporation over precipitation have become saltier. …
    We now have increased evidence that the long-term trends in patterns of rainfall over the global ocean, as reflected in salinity, can be attributed to human influence.


    Climate Change and Ocean Circulation


    Currently the influence of regional climate modes [such as the North Atlantic Oscillation, El Niño, and the Southern Annular Mode] on ocean circulation is larger than the underlying trends attributable to anthropogenic climate change.

    The stability of the North Atlantic Ocean circulation is vitally important for North American and European climate.
    For example, a slowdown of these ocean currents could lead to a more rapid rise of regional sea level along the northeast US coast.
    (p 35)

    The IPCC AR4 concluded that there is greater than 90% probability of a slowdown of this ocean current system, and less than 10% risk of a “large abrupt transition” by the year 2100.


    Ocean Acidification, Carbon Uptake and Ocean De-oxygenation


    The CO2 content of the oceans increased by 118 ± 19 Gt (1 Gt = 109 tons) between the end of the pre-industrial period (about 1750) and 1994, and continues to increase by about 2 Gt each year.
    [This] has caused a direct decrease in surface ocean pH by an average of 0.1 units since 1750 and an increase in acidity by more than 30%.
    Calcifying organisms and reefs have been shown to be particularly vulnerable to high CO2, low pH waters …
    [The] polar regions of the Arctic and Southern Oceans are expected to start dissolving certain shells once the atmospheric levels reach 450ppm (~2030 under business-as-usual).

    There is … a continuing decrease in dissolved oxygen concentrations in the global oceans, and … significant evidence that the large equatorial oxygen minimum zones are already expanding in a warmer ocean.
    Declining oxygen is a stress multiplier that causes respiratory issues for large predators and significantly compromises the ability of marine organisms to cope with acidification.
    Increasing areas of marine anoxia have profound impacts on the marine nitrogen cycle, with yet unknown global consequences.
    (p 36)


    GLOBAL SEA LEVEL


    Currently 160 million people live less than 1 meter above sea level.
    This allows even small sea level rise to have significant societal and economic impacts through coastal erosion, increased susceptibility to storm surges and resulting flooding, ground-water contamination by salt intrusion, loss of coastal wetlands, and other issues. …
    Since 1870, global sea level has risen by about 20 centimeters.
    Since 1993, sea level has been accurately measured globally from satellites.
    Satellite and tide-gauge measurements show that the rate of sea level rise has accelerated.
    Statistical analysis reveals that … the warmer it gets, the faster sea level rises. …

    For the period 1961-2003, thermal expansion contributed ~40% to the observed sea level rise, while shrinking mountain glaciers and ice sheets have contributed ~60%.
    (p 37)

    [The] IPCC AR4, the coupled models used in developing the 21st century sea level projections did not include representations of dynamic ice sheets. …
    [The] 18-59 centimeters projected sea level rise only included simple mass balance estimates …
    [The] assumed positive mass balance over the Antarctic ice sheet … contributed to [a projected] global sea level decline during the 21st century in that report.
    However, the Antarctic Ice Sheet is currently losing mass as a consequence of dynamical processes. …
    [Accordingly,] the 2009 Copenhagen Climate Congress concluded that
    updated estimates of the future global mean sea level rise are about double the IPCC projections from 2007.
    Sea level will continue to rise for many centuries after global temperature is stabilized …
    [Unchecked] global warming is likely to raise sea level by several meters in coming centuries, leading to the loss of many major coastal cities and entire island states.
    (p 38)

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