January 29, 2012

Abrupt Change, Tipping Points and Past and Future Climate

Climate Change Research Centre: Climate Science 2009

Abrupt Change and Tipping Points

  • There are several elements in the climate system that could pass a tipping point this century due to human activities, leading to abrupt and/or irreversible change.
  • 1°C global warming (above 1980-1999) carries moderately significant risks of passing large-scale tipping points, and 3°C global warming would give substantial or severe risks. …

(p 40)

Lessons from the Past

  • The reconstruction of past climate reveals that the recent warming observed in the Arctic, and in the Northern Hemisphere in general, are anomalous in the context of natural climate variability over the last 2000 years.
  • New ice-core records confirm the importance of greenhouse gases for past temperatures on Earth, and show that CO2 levels are higher now than they have ever been during the last 800,000 years. …

[Recent] Arctic warming is without precedent in at least 2000 years reversing a long-term millennial-scale cooling trend caused by astronomical forcing (ie orbital cycles).

Figure 20
Blue line: estimates of Arctic air temperatures over the last 2,000 years based on proxy records from lake sediments, ice cores and tree rings.
[Green line:] best fit long-term cooling trend for the period ending 1900.
[Red line:] recent warming based on actual observations.
(Science, modified by the University Corporation for Atmospheric Research)
(pp 43-4)

The Future

  • Global mean air-temperature is projected to warm 2-7°C above pre-industrial by 2100. …
  • There is a very high probability of the warming exceeding 2°C unless global emissions peak and start to decline rapidly by 2020.
  • Warming rates will accelerate if positive carbon feedbacks significantly diminish the efficiency of the land and ocean to absorb our CO2 emissions.
  • Many indicators are currently tracking near or above the worst case projections from the IPCC AR4 set of model simulations. …

(p 49)


Abrupt Change and Tipping Points

Lessons from the Past

The Future

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.

    Abrupt Change And Tipping Points

    What is a tipping point?

    A tipping point is a critical threshold at which the future state of a system can be qualitatively altered by a small change in forcing).
    A tipping element is a part of the Earth system … that has a tipping point. …
    Abrupt climate change is the subset of tipping point change which occurs faster than its cause.
    Tipping point change also includes transitions that are slower than their cause …
    In either case the change in state may be reversible or irreversible. …

    Reversibility in principle does not mean that changes will be reversible in practice.
    A tipping element may lag anthropogenic forcing such that once a transition begins to be observed, a much larger change in state is already inevitable.

    Figure 18
    [Some] of the potential policy-relevant tipping elements …
    • Atlantic Deep Water Formation
    • Change in ENSO Amplitude or Frequency
    • Boreal Forest Dieback
    • Dieback of Amazon Rainforest
    • Sahara Greening
    • Arctic Sea-Ice Loss
    • Melt of Greenland Ice Sheet
    • Instability of the West Antarctic Ice Sheet
    • Antarctic Ozone Hole
    [Systems] whose status as tipping elements is particularly uncertain.
    • Climatic Change-Induced Ozone Hole
    • Permafrost and Tundra Loss
    • West African Monsoons Shift
    • Changes in Antarctic Bottom Water Formation
    (p 40, italics added)

    Are there tipping points in the Earth’s climate system?

    [Recent] observations show abrupt changes already underway in the Arctic.
    Recent work has identified a shortlist of nine potential policy-relevant tipping elements in the climate system that could pass a tipping point this century and undergo a transition this millennium under projected climate change.

    Which ones are of the greatest concern?
    How has this been assessed?

    The tipping points of greatest concern are
    • [Those] that are the nearest (least avoidable) …
    • [Those] that have the largest negative impacts … the more rapid and less reversible a transition is, the greater its impacts.
    • [Any] amplifying feedback … whereby tipping one element encourages tipping another.
    The proximity of some tipping points has been assessed through expert elicitation.
    Proximity, rate and reversibility have been also assessed through literature review …

    The Greenland ice sheet (GIS) may be nearing a tipping point where it is committed to shrink.
    Striking amplification of seasonal melt was observed in 2007 associated with record Arctic summer sea-ice loss.
    Once underway the transition to a smaller Greenland ice cap will have low reversibility, although it is likely to take several centuries (and is therefore not abrupt). …


    The West Antarctic ice sheet (WAIS) is currently assessed to be further from a tipping point than the GIS, but this is more uncertain.
    The WAIS has the potential for more rapid change and hence greater impacts.
    The loss of ice-shelves around the Antarctic Peninsula, such as Larsen B, followed by the acceleration of glaciers they were buttressing, highlights a mechanism that could threaten parts of the WAIS. …


    The Amazon rainforest experienced widespread drought in 2005 turning the region from a sink to a source (0.6–0.8GtC per year) of carbon.
    If anthropogenic-forced lengthening of the dry season continues and droughts increase in frequency or severity the system could reach a tipping point resulting in dieback of up to ~80% of the rainforest and its replacement by savannah.
    This could take a few decades, would have low reversibility, large regional impacts, and knock-on effects far away.
    Widespread dieback is expected in a >4°C warmer world and it could be committed to at a lower global temperature, long before it begins to be observed.

    West Africa

    The Sahel and West African Monsoon (WAM) have experienced rapid but reversible changes in the past including devastating drought from the late 1960s through the 1980s. …
    [If] the WAM circulation collapses, this could lead to wetting of parts of the Sahel as moist air is drawn in from the Atlantic to the West … greening the region in what would be a rare example of a positive tipping point.


    The Indian Summer Monsoon is probably already being disrupted by an atmospheric brown cloud haze [which] causes heating of the atmosphere rather than the land surface, weakening the seasonal establishment of a land-ocean temperature gradient which is critical in triggering monsoon onset.
    In some future projections, brown cloud haze forcing could lead to a doubling of drought frequency within a decade with large impacts, although transitions should be highly reversible.
    [A] region of permafrost known as the Yedoma, which stores up to ~500GtC … could be tipped into irreversible breakdown driven by internal, biochemical heat generation …
    However, the tipping point is estimated to be relatively distant.

    How do tipping points relate to amplifying feedbacks on climate change?

    All tipping elements must have some strong amplifying feedback … in their own internal or regional climate dynamics in order to exhibit a threshold, but they need not have an amplifying feedback to global climate change.

    Tipping elements that could have an amplifying feedback to global climate change include
    • the Amazon rainforest (dieback would make it a CO2 source, which could ultimately release up to ~100GtC),
    • the thermohaline circulation (weakening or collapse would lead to net out-gassing of CO2), and
    • the Yedoma permafrost (release of up to ~500GtC).
    (p 41)

    Tipping elements that could have a diminishing feedback on global climate change include boreal forest (dieback would release CO2 but this would be outweighed by cooling due to increased land surface albedo from unmasked snow cover), and the Sahel/Sahara (greening would take up CO2 and probably increase regional cloud cover).

    Should we be concerned about global amplifying feedbacks?

    Amplifying feedbacks from individual tipping elements are mostly fairly weak at the global scale.
    [Non tipping element] amplifying feedbacks [such as the] potential future switch in the average response of the land biosphere from a CO2 sink to a CO2 source, could significantly amplify CO2 rise and global temperature on the century timescale.
    The Earth’s climate system is already in a state of strong amplifying feedback from relatively fast physical climate responses (eg water vapor [feedback); consequently,] relatively small additional feedbacks can have a disproportionate impact on the global state … because of the non-linear way in which amplifiers work together.

    Is there a global tipping point?

    Despite much talk in the popular media about such "runaway" climate change there is as yet no strong evidence that the Earth as a whole is near such a threshold.
    Instead "amplified" climate change is a much better description of what we currently observe and project for the future.

    Which anthropogenic forcing agents are dangerous?

    The total cumulative emissions of CO2 (and other long-lived greenhouse gases) determine long-term committed climate changes and hence the fate of those tipping elements that are sensitive to global mean temperature change, are slow to respond, and/or have more distant thresholds. …
    Uneven sulfate and soot aerosol forcing are most dangerous for monsoons.
    Soot deposition on snow and ice is a key danger to Arctic tipping elements as it is particularly effective at forcing melting.
    Increasing soot aerosol, declining sulfate aerosol and increasing short-lived greenhouse gases (methane and tropospheric ozone) have also contributed to rapid Arctic warming, and together far outweigh the CO2 contribution.
    The current mitigation of SO2 emissions … may … be benefiting the Sahel region but endangering the Amazon and the Arctic sea-ice.
    Land cover change may also drive large areas of continents from being relatively robust to climate change to being highly vulnerable.

    Is there any prospect for early warning of an approaching tipping point?

    Slowing down in response to perturbation is a nearly universal property of systems approaching various types of tipping point.
    Flickering between states may also occur prior to a more permanent transition.
    Other early warning indicators [include] increasing variance, skewed responses and their spatial equivalents.
    (p 42, italics added)


    Reconstructing the last two millennia

    The first of these reconstructions has come to be known as the ‘hockey stick’ reconstruction.
    Some aspects of the hockey stick reconstruction were subsequently questioned, eg whether the 20th century was the warmest at a hemispheric average scale, and whether the reconstruction is reproducible, or verifiable, or might be sensitive to the method used to extract information from tree ring records.
    [These] criticisms have been rejected in subsequent work. …
    [The US National Research Council] report published in 2006 largely supported the original findings … and recommended a path toward continued progress in this area.
    (p 43)

    [The] “Medieval Climate Anomaly” of roughly AD 900-1100 may have rivalled modern warmth for certain regions …
    However, such regional warming appears to reflect a redistribution of warmth by changes in atmospheric circulation, and is generally offset by cooling elsewhere (eg the eastern and central tropical Pacific) to yield hemispheric and global temperatures that are lower than those of recent decades.

    Ice Core Records of Greenhouse Gases

    Changes in past atmospheric Carbon Dioxide (CO2) and Methane (CH4) concentrations can be determined by measuring the composition of air trapped in ice cores and through the analyses of leaf stomata density and geochemical analyses of marine sediment cores. …

    The newly extended [ice core] records reveal that current greenhouse gas levels (~385ppm) are at least 40% higher than at any time over the past 800,000 years.
    We must travel back at least two to three million years, and perhaps as far as fifteen million years … to find equivalent greenhouse gas levels in the atmosphere. …

    Changes in the Earth’s orbit around the Sun are the pacemaker for glacial-interglacial cycles … but these rather subtle orbital changes must be amplified by climate feedbacks in order to explain the large differences in global temperature and ice volume, and the relative abruptness of the transitions between glacial and interglacial periods …
    (p 44, italics added)

    Palaeo Constraints on Climate and Earth System Sensitivity

    … “Climate Sensitivity”, defined as the equilibrium global temperature response to a doubling of atmospheric CO2 concentration.

    [According to the] IPCC AR4
    climate sensitivity is likely to lie in the range 2°C to 4.5°C, with a most likely value of about 3°C.
    More recent studies have agreed with this assessment.

    Isn’t climate always changing, even without human interference?

    [Past climate changes] tell us that the Earth’s climate is very sensitive to changes in forcing. … Climate has always responded strongly if the radiation balance of the Earth was disturbed.
    That suggests the same will happen again, now that humans are altering the radiation balance by increasing greenhouse gas concentrations. …

    Impacts of past climate changes have been severe. …
    When the Earth last was 2-3°C warmer than now, during the Pliocene 3 million years ago, sea level was 25-35 meters higher due to the smaller ice sheets present in the warmer climate. …

    Climate reconstructions suggest that over the past two millennia, global temperature has never changed by more than 0.5°C in a century.
    (p 45)

    Are we just in a natural warming phase, recovering from the “little ice age”?

    No. …
    By far the biggest change in the radiation balance over the past 50 years, during which three quarters of global warming has occurred, is due to the human-caused increase in greenhouse gas concentrations.
    Natural factors have had a slightly cooling effect during this period.
    (p 46)

    In climate history, didn’t CO2 change in response to temperature, rather than the other way round?

    It works both ways:
    • CO2 changes affect temperature due to the greenhouse effect, while
    • temperature changes affect CO2 concentrations due to the carbon cycle response. …
    During the warming at the end of ice ages, CO2 was released from the oceans – just the opposite of what we observe today, where CO2 is increasing in both the ocean and the atmosphere. …

    A rapid carbon release, not unlike what humans are causing today, has also occurred at least once in climate history, as sediment data from 55 million years ago show.
    This “Paleocene-Eocene thermal maximum” brought a major global warming of ~ 5°C, a detrimental ocean acidification and a mass extinction event.
    (p 47)


    Climate Projections

    The wide range in the projection envelope is primarily due to uncertainty in future emissions.
    At the high end of emissions, with business as usual for several decades to come, global mean warming is estimated to reach 4-7°C by 2100, locking in climate change at a scale that would profoundly and adversely affect all of human civilization and all of the world’s major ecosystems.
    At the lower end of emissions, something that would require urgent, deep and long-lasting cuts in fossil fuel use, and active preservation of the world’s forests, global mean warming is projected to reach 2-3°C by century’s end.
    While clearly a better outcome than the high emissions route, global mean warming of even just 1.5-2.0°C still carries a significant risk of adverse impacts on ecosystems and human society.
    For example, 2°C global temperature rise could lead to sufficient warming over Greenland to eventually melt much of its ice sheet … raising sea level by over six meters and displacing hundreds of millions of people worldwide. …

    [Because of] natural variability and the 11-year solar cycle, as well as sporadic volcanic eruptions, generate short-term variations superimposed on the long term trend [we can] still expect to see the temperature record punctuated by isolated but regular ten-year periods of no trend, or even modest cooling.
    [The] peak in global temperature might not be reached until several centuries after emissions peak …

    [Dry] season rainfall reductions in several regions are expected to become irreversible …
    (p 49)

    Mitigating global warming

    While global warming can be stopped, it cannot easily be reversed due to the long lifetime of carbon dioxide in the atmosphere. Even a thousand years after reaching a zero-emission society, temperatures will remain elevated, likely cooling down by only a few tenths of a degree below their peak values. …

    The temperature at which global warming will finally stop depends primarily on the total amount of CO2 released to the atmosphere since industrialization. …
    [A] cumulative CO2 budget for the world would thus be a natural element of a climate policy agreement.
    [This] budget could then be distributed amongst countries … on the basis of equity principles. …

    The Synthesis Report of the Copenhagen climate congress [in 2009] concluded that
    Temperature rises above 2°C will be difficult for contemporary societies to cope with, and are likely to cause major societal and environmental disruptions through the rest of the century and beyond.
    [If] a total of 1000 Gigatons of CO2 is emitted for the period 2000-2050, the likelihood of exceeding the 2-degree warming limit is around 25%.
    In 2000-2009, about 350 Gigatons have already been emitted, leaving only 650 Gigatons for 2010-2050.
    (p 50)

    At current emission rates this budget would be used up within 20 years. …

    [The] required decline in emissions combined with a growing population will mean that by 2050, annual per capita CO2 emissions very likely will need to be below 1 ton.

    Successful limitation of the non-CO2 climate forcing would … create more leeway in the allowable CO2 emissions budget.
    [Options] for particularly rapid and cost-effective climate mitigation are the reduction of black carbon (soot) pollution and tropospheric low-level ozone.
    [These] are very short-lived gases [and would] respond rapidly to policy measures.
    (p 51)

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