January 23, 2012

Developments in the Science of Climate Change

Climate Council: Climate Science, Risks and Responses


Observations of changes in the climate system

  • The average air temperature at the Earth’s surface continues on an upward trajectory at a rate of 0.17 °C per decade over the past three decades.
  • The temperature of the upper 700 m of the ocean continues to increase, with most of the excess heat generated by the growing energy imbalance at the Earth’s surface stored in this compartment of the system.
  • Recent observations confirm net loss of ice from the Greenland and West Antarctic ice sheets;
    the extent of Arctic sea ice cover continues on a long-term downward trend [and most] land-based glaciers and ice caps are in retreat.
  • The alkalinity of the ocean is decreasing steadily as a result of acidification by anthropogenic CO2 emissions.
  • Sea-level has risen at a higher rate over the past two decades, consistent with ocean warming and an increasing contribution from the large polar ice sheets.
  • The biosphere is responding in a consistent way to a warming Earth, with observed changes in gene pools, species ranges, timing of biological patterns and ecosystem dynamics.

(p 6)


Why is the climate system changing now?

  • There is no credible evidence that changes in incoming solar radiation can be the cause of the current warming trend.
  • Neither multi-decadal or century-scale patterns of natural variability, such as the Medieval Warm Period, nor shorter term patterns of variability, such as ENSO (El NiƱo-Southern Oscillation) or the North Atlantic Oscillation, can explain the globally coherent warming trend observed since the middle of the 20th century.
  • There is a very large body of internally consistent observations, experiments, analyses, and physical theory that points to the increasing atmospheric concentration of greenhouse gases, with carbon dioxide (CO2) the most important, as the ultimate cause for the observed warming.
  • Improved understanding of the sensitivity of the climate system to the increasing atmospheric CO2 concentration has provided further evidence of its role in the current warming trend, and provided more confidence in projections of the level of future warming.

(p 13)


How is the carbon cycle changing?

  • Despite the dip in human emissions of greenhouse gases in 2009 due to the Global Financial Crisis, emissions continue on a strong upward trend, on average tracking near the top of the family of IPCC emission scenarios.
  • Ocean and land carbon sinks, which together take up more than half of the human emissions of CO2, appear to be holding their proportional strengths compared to emissions, although some recent evidence questions this conclusion and suggests a loss of efficiency in these natural sinks over the past 60 years.
  • If global average temperature rises significantly above 2 °C (relative to pre-industrial), there is an increasing risk of large emissions from the terrestrial biosphere, the most likely source being methane stored in permafrost in the northern high latitudes.

(p 17)


How certain is our knowledge of climate change?

  • The IPCC’s Fourth Assessment Report has been intensively and exhaustively scrutinised and is virtually error-free.
  • The Earth is warming on a multi-decadal to century timescale, and at a very fast rate by geological standards. …
  • Human emissions of greenhouse gases — and CO2 is the most important of these gases — is the primary factor triggering observed climate change since at least the mid 20th century. …
  • Many uncertainties surround projections of the particular risks that climate change poses for human societies and natural and managed ecosystems, especially at smaller spatial scales.
    However, our current level of understanding provides some useful insights:
    1. some social, economic and environmental impacts are already observable from the current level of climate change;
    2. the number and magnitude of climate risks will rise as the climate warms further.

(p 19)


Contents


Observations of changes in the climate system

Why is the climate system changing now?

How is the carbon cycle changing?

How certain is our knowledge of climate change?

Would you like to know more?

CLIMATE COUNCIL

  • The Critical Decade: Climate science, risks and responses, Climate Commission Secretariat, Department of Climate Change and Energy Efficiency, Commonwealth of Australia, June 2011.
    Will Steffen.

    OBSERVATIONS OF CHANGES IN THE CLIMATE SYSTEM


    Surface air temperature


    For the most recent 10-year period (2001-2010), global average temperature was 0.46°C above the 1961-1990 average, the warmest decade on record. …

    [The] month of November 2010 was exceptionally warm, with extremely high temperatures around the northern high latitudes more than compensating for the cold, snowy weather in western Europe and parts of North America …
    (p 6)


    Ocean temperature


    Since the 1960s measurements of the heat content of the upper 700 m of the ocean have been available, and since 2004, measurements to lower depths (up to 2 km) have become widely available with the deployment of Argo floats …

    [The] record of ocean thermal expansion from 1950 … indicates that multi-decadal warming has continued to the end of the record in December 2008 …

    This record is quantitatively consistent with the observed rate of sea-level rise over the past half-century.
    Although most of the additional heat stored in the ocean is found in the upper 700 m, recent observations show that warming of the deeper ocean waters in both the Southern and Atlantic Oceans is now occurring …


    Ocean acidification


    Observations of the acidity of the ocean’s surface waters show the expected decrease of about 0.1 pH unit since the pre-industrial era …


    Sea ice and polar ice sheets


    [Sea] ice covering the Arctic Ocean has decreased significantly over the last several decades …
    Changes to the sea ice surrounding Antarctica are more complex, with no appreciable change in overall extent over the past several decades.

    The large polar ice sheets on Greenland and West Antarctica, which are important factors influencing sea-level rise, are currently losing mass to the ocean through both melting and dynamical ice loss [—] that is, by break-up and calving of blocks of ice. …
    [It] is not entirely clear whether these are long-term trends that will be maintained into the future or are at least partly the result of natural decadal-scale variability …

    [Recent] re-analysis of the Greenland gravity change data suggests that the rate of ice loss has been overestimated by a factor of two …

    Nevertheless, a synthesis of all observations shows that there is a net loss of mass from the Greenland (and West Antarctic) ice sheets [—] the uncertainty refers to the rate at which this ice loss is occurring, with some evidence that this rate of loss may be accelerating …
    (p 8)


    Land-based glaciers and ice caps


    Most glaciers and mountain ice-caps around the world have been in retreat the past century …
    [The climate] will almost surely continue to warm through this century, with … an estimated mass loss equivalent to about 55 cm of sea-level rise by the end of the century.


    Sea-level rise


    Global sea level has risen by about 20 cm since the 1880s,when the first global estimates could be made.
    The rate of increase has risen to about 3.2 mm yr^-1 for the 1993-2009 period, based on satellite altimeter data, compared to a rate of 1.7 mm yr^-1 for the 1900-2009 period.
    (p 11)

    [About 40% of the rise between 1961-2003] can be attributed to the thermal expansion of the ocean as it warms, about 35% to the melting of continental glaciers and ice caps (eg, the Andean and Himalayan mountain glaciers) and about 25% from the large polar ice sheets on Greenland and Antarctica. …

    Global average values of sea-level rise mask large regional differences.
    [Around Australia] recent sea level rise … has been below the global average along the east coast, near the global average along the much of the southern coast, but at least double the global average along much of the northern coastline.


    Terrestrial and marine biosphere


    [There] have been eight mass bleaching events on the [Great Barrier Reef] since 1979 with no known such events prior to that date.
    (p 12)


    WHY IS THE CLIMATE SYSTEM CHANGING NOW?


    The longer-term context


    [The] last 12,000 years — the Holocene … provides a useful, human-relevant baseline against which to test possible explanations for contemporary warming.


    Changes in solar radiation


    Variation in the amount of solar radiation … has been implicated … in the Medieval Climate Anomaly …
    Variations in solar radiation … could have contributed at most 10%to the observed warming trend in the 20th century.
    [However,] there has been no significant change in solar radiation over the past 30 years, when global average temperature has risen at about 0.17 °C per decade.
    [Furthermore,] stratospheric cooling has been observed, inconsistent with solar forcing but consistent with CO2-dominated forcing.
    (p 13)


    Modes of natural variability


    The Medieval Climate Anomaly (MCA), a somewhat warmer period from about 1000 to about 1250 or 1300 AD …
    [A] spatially explicit synthesis of all available temperature reconstructions around the globe suggests that the MCA was highly heterogeneous … with globally averaged warming much below that observed over the last century …

    Shorter-term modes of natural variability, such as ENSO and the NAO (North Atlantic Oscillation), are very important influences on the weather that people experience from year to year, but they cannot explain recent multi-decadal, globally synchronous trends in temperature.


    Greenhouse gas forcing


    The physics by which greenhouse gases influence the climate at the Earth’s surface … was first proposed in 1824 by Joseph Fourier, experimentally verified in 1859 by John Tyndall and quantified near the end of the 19th century by Svante Arrhenius. …
    [The] very large differences in surface temperature among Earth, Venus and Mars can only be explained by the very different amounts of CO2 in their atmospheres.
    [The] difference in globally averaged temperature between an ice age and a warm period, about 5-6 °C, can only be explained by changes in greenhouse gas concentrations and in the reflectivity (albedo) of the Earth’s surface that amplify the original modest changes in temperature due to variations in incoming solar radiation caused by cyclical changes in the Earth’s orbit around the sun [Milankovitch cycles].
    (p 14)

    [In addition to stratospheric cooling, other] “fingerprints of greenhouse gas forcing” include … the observation that winters are warming more rapidly than summers and that overnight minimum temperatures have risen more rapidly than daytime maximum temperatures.
    An apparent inconsistency between observations with greenhouse theory was the alleged failure to find a so-called “tropical hot spot”, a warming in the tropical atmosphere about 10-15 km above the Earth’s surface.
    In reality, there was no inconsistency between observed and modelled changes in tropical upper tropospheric temperatures, allowing for uncertainties in observations … large internal variability in temperature in the region [and] recent thermal wind calculations [which have] shown greater warming in the region …


    Climate sensitivity


    [Climate] sensitivity usually refers to the equilibrium temperature rise resulting from a doubling of CO2 concentration (from 280 to 560 ppm; ppm = parts per million).
    With no other responses of the climate system (eg changes in water vapour, albedo or clouds), a doubling of CO2 alone would result in around 1 °C warming …
    Theory, modelling studies and observations all strongly support there being a strong positive (reinforcing) water vapour feedback, which roughly doubles the initial warming from CO2.
    Other feedbacks [include relate to changes in] surface albedo (positive), temperature ‘lapse rate’ (negative) and clouds …

    Multiple simulations by climate models … have generated a probability density function with most of the values for sensitivity falling between 2 and 4.5°C and a peak near 3°C.
    An analysis of the transition of the Earth from the last ice age to the Holocene … also estimates a value of about 3°C.
    Much of the uncertainty on the magnitude of climate sensitivity is associated with the direction and strength of cloud feedbacks.
    Recent observational evidence from short-term variations in clouds suggests that short-term cloud feedbacks are positive, reinforcing the warming …

    A recent model study comparing the relative importance of various greenhouse gases for the climate estimates a sensitivity of approximately 4 °C for a doubling of CO2 …
    Although CO2 accounts for only about 20% of Earth’s greenhouse effect … it is the one that effectively controls climate because of its very long lifetime in the atmosphere. …
    [Indeed, without] long-lived greenhouse gases, the Earth’s temperature would drop rapidly and drive the planet into an ice-bound state.
    (p 16)


    HOW IS THE CARBON CYCLE CHANGING?


    Human emissions of CO2


    [There has been an] average annual rise in fossil fuel CO2 emissions of 3.2% for the 2000-2008 period. …
    [Current] emissions are about 37% larger than those in 1990 [and lie] near the top of the envelope of IPCC projections.
    [Coal] has overtaken oil as the largest source of CO2 from fossil fuel combustion. Despite the drop in the absolute amount of emissions in 2009 [due to the Global Financial Crisis], the atmospheric concentration of CO2 still rose by 1.6 ppm during the year, compared to an average growth rate of 1.9 ppm per year for the 2000-2008 period.
    (p 17)


    Ocean and land carbon sinks


    Natural sinks of carbon on land and in oceans (eg, uptake of CO2 by growing vegetation; dissolution of CO2 in seawater) have historically removed over half of the human emissions from the atmosphere …
    The carbon is taken up in approximately equal proportions by land and ocean …
    Over the past half-century the capability of these natural sinks has generally kept pace with the increasing human emissions of CO2.
    However, there is evidence that the efficiency of these sinks is declining …
    There are uncertainties with some of these results, and some scientific controversy over the declining trend …

    The ongoing strength of these natural sinks is crucially important for the level of effort that will be required to limit climate change to no more than a 2°C rise above pre-industrial …
    There is considerable scientific evidence that values of temperature rise above 2°C are “dangerous” by most definitions, but this evidence also shows that there are significant risks of serious impacts in various sectors and locations at temperature increases of less than 2°C.
    (p 18)


    Vulnerable new sources of carbon


    [There] is the potential for activating new natural sources of carbon emissions from pools that are currently inactive.
    Examples include methane hydrates stored under the sea floor, organic material stored in tropical peat bogs and organic material stored in permafrost in the northern high latitudes and the Tibetan plateau. …

    [There] are over 1,700 billion tonnes of carbon stored in permafrost , which is about twice the amount stored in the atmosphere at present.
    There is uncertainty about the vulnerability of this potential new source of carbon, but there is already evidence of some loss of methane from the northern high latitudes …


    HOW CERTAIN IS OUR KNOWLEDGE OF CLIMATE CHANGE?


    Figure 13
    The process by which the IPCC carries out an assessment …

    The IPCC


    The Fourth Assessment Report (AR4), published in 2007, involved about 1250 expert authors and 2,500 reviewers, who produced about 90,000 comments on drafts, each one of which was addressed explicitly by the authors.

    [The InterAcademy Council (2010) found] two peripheral errors, both of them in the WG2 report on impacts and adaptation, … in a publication containing approximately 2.5 million words …
    No errors have been found in any of the main conclusions, nor have any errors been found in the 996-page WG 1 report [on fundamental climate science].
    (p 20)


    Certainty of warming


    [There is unequivocal] evidence that the Earth is warming on a multi-decadal timescale …


    Human causation


    [The] IPCC in 2007 stated that:
    [There is a greater than 90% probability that most] of the observed increase in global average temperatures since the mid-20th century is … due to the observed increase in anthropogenic greenhouse gas concentrations.

    Large uncertainties


    [The] ways in which the large polar ice sheets on Greenland and Antarctica are responding and will respond in future to warming …
    [The] influence of climate change on spatial patterns of precipitation across the Earth’s surface and on the temporal patterns of precipitation — droughts and intense rainfall events. …
    [The] risks that climate change poses for human societies and natural and managed ecosystems …
    [Specifically:]

    1. uncertainties in the projections of potential impacts from future climate change;
    2. uncertainties associated with the dynamics of systems being impacted by climate, such as agricultural systems, natural ecosystems, or urban systems; and
    3. uncertainties in the ways in which humans will respond to the threats of climate change by reducing their vulnerability or increasing their adaptive capacity.

    (p 21)

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