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Copenhagen

    Editors Note: This is a reprint fromn our 2010 Calendar, exerpting from "The Copenhagen Diagnosis," a ground breaking report prepared for the 2009 Copenhagen Climate Conference.
    As a report to aquaint the lay-person with climate science, I cannot reccoment this report more highly. Please do take a moment to download and read the full report.

Executive Summary

   Climate change accelerating beyond expectations, urgent
    emissions reductions required, say leading scientists

    Global ice-sheets are melting at an increased rate; Arctic sea-ice is disappearing much faster than recently projected, and future sea-level rise is now expected to be much higher than previously forecast, according to a new global scientific synthesis prepared by some of the world’s top climate scientists.
    In a special report called ‘The Copenhagen Diagnosis’, the 26 researchers, most of whom are authors of published IPCC reports, conclude that several important aspects of climate change are occurring at the high end or even beyond the expectations of only a few years ago.
    The report also notes that global warming continues to track early IPCC projections based on greenhouse gas increases. Without significant mitigation, the report says global mean warming could reach as high as 7 degrees Celsius by 2100.
    The Copenhagen Diagnosis, which was a year in the making, documents the key fi ndings in climate change science since the publication of the landmark Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report in 2007.

 

    The new evidence to have emerged includes:

    Satellite and direct measurements now demonstrate that both the Greenland and Antarctic ice-sheets are losing mass and contributing to sea level rise at an increasing rate.
    Arctic sea-ice has melted far beyond the expectations of climate models. For example, the area of summer sea-ice melt during 2007-2009 was about 40% greater than the average projection from the 2007 IPCC Fourth Assessment Report.
    Sea level has risen more than 5 centimeters over the past 15 years, about 80% higher than IPCC projections from 2001. Accounting for ice-sheets and glaciers, global sea-level rise may exceed 1 meter by 2100, with a rise of up to 2 meters considered an upper limit by this time. is is much higher than previously projected by the IPCC. Furthermore, beyond 2100, sea level rise of several meters must be expected over the next few centuries.
    In 2008 carbon dioxide emissions from fossil fuels were 40% higher than those in 1990. Even if emissions do not grow beyond today’s levels, within just 20 years the world will have used up the allowable emissions to have a reasonable chance of limiting warming to less than 2 degrees Celsius.
    The report concludes that global emissions must peak then decline rapidly within the next fi ve to ten years for the world to have a reasonable chance of avoiding the very worst impacts of climate change.
    To stabilize climate, global emissions of carbon dioxide and other long-lived greenhouse gases need to reach near-zero well within this century, the report states.
    The full report is available at: www.copenhagendiagnosis.com

Statements by Authors

“Sea level is rising much faster and Arctic sea ice cover shrinking more rapidly than we previously expected. Unfortunately, the data now show us that we have underestimated the climate crisis in the past.”
— Professor Stefan Rahmstorf, Professor of Physics of the
Oceans and a Department Head at the Potsdam Institute for
Climate Impact Research in Germany.

“Carbon dioxide emissions cannot be allowed to continue to rise if humanity intends to limit the risk of unacceptable climate change. The task is urgent and the turning point must come soon. If we are to avoid more than 2 degrees Celsius warming, which many countries have already accepted as a goal, then emissions need to peak before 2020 and then decline rapidly.”
— Professor Richard Somerville, Scripps Institution of
Oceanography, University of California, San Diego, USA.

“We have already almost exceeded the safe level of emissions that would ensure a reasonably secure climate future. Within just a decade global emissions need to be declining rapidly. A binding treaty is needed urgently to ensure unilateral action among the high emitters.”
— Professor Matthew England, ARC Federation Fellow
and joint Director of the Climate Change Research Centre of
the University of NSW, Australia.

 

From the Chapter: “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, although the precise timing of this remains uncertain.
.. Satellite observations show a small increase of Antarctic sea-ice extent and changes to seasonality, although there is considerable regional variability. This is most likely due to changes in Southern Ocean winds associated with stratospheric ozone-depletion.

 

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 (see Figure 12), 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). The median September minimum sea ice extent since observations with the current generation of multi-frequency passive microwave sensors commenced in 1979 to 2000 was 6.7 million square kilometers. Compared to the median, the 2007 record involved melting 2.6 million square kilometers more ice (~40% of the median).
     The September Arctic sea ice extent over the last several decades has decreased at a rate of 11.1 ± 3.3%/decade (NSIDC 2009). This dramatic retreat has been much faster than that simulated by any of the climate models assessed in the IPCC AR4 (Figure 13). This is likely due to a combination of several model deficiencies, including: 1) incomplete representation of ice albedo physics, including the treatment of melt ponds (e.g., Pedersen et al. 2009) and the deposition of black carbon (e.g. Flanner et al. 2007; Ramanathan and Carmichael 2008); and 2) incomplete representation of the physics of vertical and horizontal mixing in the ocean (e.g. Arzel et al. 2006). Winter Arctic sea ice extent has also decreased since 1979, but at a slower rate than in summer. The February extent has decreased at a rate of 2.9 ± 0.8%/decade (NSIDC 2009).
     The thickness of Arctic sea ice has also been on a steady decline over the last several decades. For example, Lindsay et al. (2009) estimated that the September sea ice thickness has been decreasing at a rate of 57 centimeters per decade since 1987. Similar decreases in sea-ice thickness have been detected in winter. For example, within the area covered by submarine sonar measurements, Kwok and Rothrock (2009) show that the overall mean winter thickness of 3.64 meters in 1980 decreased to only 1.89 meters by 2008 — a net decrease of 1.75 meters, or 48%. By the end of February 2009, less than 10% of Arctic sea ice was more than two years old, down from the historic values of 30%.

SeaIce





    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. Nevertheless, the warming commitment associated with existing atmospheric greenhouse gas levels very likely means that a summer ice-free Arctic is inevitable. Evidence is also emerging to  suggest that the transition to an ice-free summer in the Arctic might be expected to occur
abruptly, rather than slowly (Holland et al. 2006), because of amplifying feedbacks inherent within the Arctic climate system.
    In fact, in one of the simulations of the NCAR Climate System Model version 3 (CCSM3) discussed in Holland et al (2006), the Arctic summer became nearly ice-free by 2040. As noted by Lawrence et al. (2008), an abrupt reduction in Arctic summer sea ice extent also triggers rapid warming on land and subsequent permafrost degradation.

Antarctic Sea Ice
    Unlike the Arctic, Antarctic sea-ice extent changes have been more subtle, with a net annual-mean area increase of ~1% per decade over the period 1979–2006 (Cavalieri and Parkinson 2008; Comiso and Nishio 2008). There have however been large regional changes in Antarctic sea-ice distribution: for example, the Weddell and Ross Sea areas have shown increased extent linked to changes in large-scale atmospheric circulation, while the western Antarctic Peninsula region and the coast of West Antarctica (Amundsen and Bellingshausen Seas) show a significant decline consistent with more northerly winds and surface warming observed there (Lefebvre et al. 2004; Turner et al. 2009; Steig et al. 2009). These regional changes are linked to a major change in the seasonality of the ice; that is, its duration and the timing of the annual advance and retreat (Stammerjohn et al. 2008)

GlobalTemp

   Since Antarctica is a land mass surrounded by the vast Southern Ocean, whereas the Arctic is a small ocean surrounded by vast amounts of land, 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. In addition, Turner et al. (2009) note that stratospheric ozone depletion arising from the anthropogenic
release of chlorofl uorocarbons (CFCs) has led to the strengthening of surface winds around Antarctica during December to February (summer). They argue that these strengthened winds are in fact the primary cause for the slight positive trend in Antarctic sea ice extent observed over the last three decades. However, as CFCs are regulated under the Montreal Protocol and have declining atmospheric concentrations, the ozone hole over Antarctica is
expected to recover and hence one anticipates an acceleration of sea ice melt in the Southern Hemisphere in the decades ahead.
    There are few data available on the thickness distribution of Antarctic pack ice, and no information on any changes in the thickness of Antarctic sea ice.

Isn’t Antarctica cooling and Antarctic sea ice increasing?

    Antarctica is not cooling: it has warmed overall over at least the past 50 years. 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 (Chapman and Walsh 2008; Monaghan et al. 2008; Goosse et al. 2009; Steig et al. 2009) 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 (Figure 14 shows the mean trend from 1957-2006). Furthermore, there is direct evidence from borehole measurements that warming in West  Antarctica began no later than the 1930s (Barrett et al. 2009). 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 chlorofl uorocarbons. As a consequence, much of East Antarctica has cooled in the summer and autumn seasons since the late 1970s. Ironically, human emissions of CFCs are thus helping to partly off set interior Antarctic warming, analogous to the global dimming due to sulphate aerosols. As the ozone hole gradually repairs over the coming century, the cooling off set is likely to diminish.
     The factors that determine sea ice extent around Antarctica are very different from those in the Arctic, because Antarctica is a continent sited around the pole and surrounded by water, just the opposite of the Arctic geography. The extent of sea ice around Antarctica is strongly determined by the circumpolar winds which spread the ice out from the continent, and by the position of the polar front where the ice encounters warmer ocean waters. Sea ice cover in Antarctica shows a slight upward trend, consistent with the increase in circumpolar winds mentioned above. In West Antarctica, where the temperature increases are the greatest, sea ice has declined at a statistically significant rate since at least the 1970s.

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