January 23, 2012

Risks Associated with a Changing Climate

Climate Council: Climate Science, Risks and Responses

For coastal areas around Australia’s largest cities — Sydney and Melbourne — a rise of 0.5 m leads to very large increases in the incidence of extreme events, by factors of 1000 or 10,000 for some locations.
A multiplying factor of 100 means that an extreme event with a current probability of occurrence of 1-in-100 — the so-called one-in-a-hundred-year event — would occur every year.
(p 26)

[The] very low streamflow in the River Murray for the 1998-2008 period is … about a 1-in-1500 year event.
(p 33)

Sea-level rise


With much of our population and a high fraction of our infrastructure located close to the coast, Australia is vulnerable to the risks posed by sea-level rise. …
  • A plausible estimate of the amount of sea-level rise by 2100 compared to 2000 is 0.5 to 1.0 m. …
  • Much more has been learned about the dynamics of the large polar ice sheets through the past decade but critical uncertainties remain, including the rate at which mass is currently being lost, the constraints on dynamic loss of ice and the relative importance of natural variability and longer-term trends.
  • The impacts of rising sea-level are experienced through “high sea-level events” when a combination of sea-level rise, a high tide and a storm surge or excessive run-off trigger an inundation event.
    Very modest rises in sea-level, for example, 50 cm, can lead to very high multiplying factors — sometimes 100 times or more — in the frequency of occurrence of high sea-level events.
(p 23)


Ocean acidification


[Marine] organisms that form calcium carbonate shells are at risk from decreasing alkalinity of the ocean, which reduces the concentration of carbonate ions in seawater.
Corals are probably the most well-know of these organisms, but other calcifying organisms are important for the marine carbon cycle and play fundamental roles in the dynamics of marine ecosystems. …
  • The contemporary rate of increase in ocean acidity (decrease in alkalinity) is very large from a long-time perspective.
  • The effects of increasing acidity are most apparent in the high latitude oceans, where the rates of dissolution of atmospheric CO2 are the greatest.
  • Increasing acidity in tropical ocean surface waters is already affecting coral growth;
    calcification rates have dropped by about 15% over the past two decades.
  • Rising SSTs [Sea Surface Temperatures] have increased the number of bleaching events observed on the Great Barrier Reef (GBR) over the last few decades.
    There is a significant risk that with a temperature rise above 2 °C relative to pre-industrial levels and at CO2 concentrations above 500 ppm, much of the GBR will be converted to an algae-dominated ecosystem.
(p 27)


The water cycle


Australia is the driest of the six inhabited continents, and experiences a high degree of natural climatic variability …
  • Observations since 1970 show a drying trend in most of eastern Australia and in southwest Western Australia but a wetting trend for much of the western half of the continent.
  • Given the high degree of natural variability of Australia’s rainfall, attributing observed changes to climate change is difficult. …
    Evidence points to a possible climate change link to observed changes in the behaviour of the Southern Annular Mode (SAM) and the Indian Ocean Dipole (IOD).
  • Improvements in understanding of the climatic processes that influence rainfall suggest a connection to climate change in the observed drying trend in southeast Australia, especially in spring.
    In southwest Western Australia, climate change is likely to have made a significant contribution to the observed reduction in rainfall.
  • The consensus on projected changes in rainfall for the end of this century is
    1. high for southwest Western Australia, where almost all models project continuing dry conditions;
    2. moderate for southeast and eastern Australia, where a majority of models project a reduction; and
    3. low across northern Australia.
    There is a high degree of uncertainty in the projections in (2) and (3), however.
  • Rainfall is the main driver of runoff, which is the direct link to water availability.
    Hydrological modelling indicates that water availability will likely decline in southwest Western Australia, and in southeast Australia, with less confidence in projections of the latter.
    There is considerable uncertainty in the projections of amounts and seasonality of changes in runoff.
(p 32)


Extreme events

  • Modest changes in average values of climatic parameters — for example, temperature and rainfall — can lead to disproportionately large changes in the frequency and intensity of extreme events.
  • On a global scale and across Australia it is very likely that since about 1950 there has been a decrease in the number of low temperature extremes and an increase in the number of high temperature extremes. …
  • The seasonality and intensity of large bushfires in southeast Australia is likely changing, with climate change a possible contributing factor.
  • … The global frequency of tropical cyclones is projected to either stay about the same or even decrease.
    However a modest increase in intensity of the most intense systems, and in associated heavy rainfall, is projected as the climate warms.
  • On a global scale, several analyses point to an increase in heavy precipitation events in many parts of the world, including tropical Australia, consistent with physical theory and with projections of more intense rainfall events as the climate warms.

(pp 38-9)


Abrupt, non-linear and irreversible changes in the climate system


Many projections of future changes in climatic variables are simulated and presented as smooth curves from present values to an altered state at some future point in time. …
However, smooth changes are not the norm in the climate system. …
The abrupt drop in rainfall in the mid-1970s in southwest Western Australia is a well-known Australian example.
  • A number of potential abrupt changes in large sub-systems or processes in the climate system — so-called “tipping elements” — have been identified largely through palaeo-climatic research.
    Many of these, if triggered, would lead to catastrophic impacts on human societies.
  • Examples of tipping elements include abrupt changes in the North Atlantic ocean circulation, the switch of the Indian monsoon from a wet to a dry state or vice versa, and the conversion of the Amazon rainforest to a grassland or a savanna.
  • Very large uncertainties surround the likelihood, or not, of human-driven climate change triggering any of these abrupt or irreversible changes.
    [However, experts] agree that the risk of triggering them increases as temperature rises.
  • Abrupt shifts in atmospheric circulation can occur very quickly and can have large impacts on regional climates.
(p 48)


Contents


Sea-level rise

Ocean acidification

The water cycle

Extreme events

Abrupt, non-linear and irreversible changes in the climate system

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.

    SEA-LEVEL RISE


    Projections of future sea-level rise


    An estimate for the most likely magnitude of sea-level rise in 2100 relative to 2000 taking polar ice sheet dynamics into account is about 0.8 m, and an expert assessment of Greenland ice sheet dynamics suggests that it will contribute about 20 cm to global sea-level rise by 2100.


    Dynamics of large polar ice sheets


    Observations over the past 20 years, either by satellite or aircraft altimeters that measure changes in the height of the ice sheets or by satellite gravity measurements that infer changes in mass, show accelerating decreases in the mass of the Greenland ice sheet over the past 15 years … and in the mass of the Antarctic ice sheet over the past decade.
    (p 25)


    High sea-level events (inundation)


    Many of the risks due to sea-level rise are associated with inundation events, which damage human settlements and infrastructure in low-lying coastal areas, and can lead to erosion of sandy beaches and soft coastlines. …

    For coastal areas around Australia’s largest cities — Sydney and Melbourne — a rise of 0.5 m leads to very large increases in the incidence of extreme events, by factors of 1000 or 10,000 for some locations.
    A multiplying factor of 100 means that an extreme event with a current probability of occurrence of 1-in-100 — the so-called one-in-a-hundred-year event — would occur every year.
    (p 26)

    The observed sea-level rise of about 20 cm from 1880 to 2000 should already have led to an increase in the incidence of extreme sea-level events.
    Such increases have indeed been observed at places with very long records, such as Fremantle and Fort Denison, where a 3-fold increase in inundation events has occurred. …


    OCEAN ACIDIFICATION


    Ocean acidity in a long-time context


    The rate at which ocean acidity is increasing is … likely unprecedented in the 25 million years of the [observational] record, and would no doubt place severe evolutionary pressure on marine organisms.


    Marine ecosystems


    [Conditions] deleterious for the growth of calcifying plankton species could occur as early as 2030 in winter.
    Experiments in sea water with the acidity level expected in 2100 have shown a 30% reduction in calcification rates for a common pteropod (pelagic marine snail), an important component of marine food chains.
    Even larger reductions in calcification rates of around 50% have been found in experiments with a deepwater coral. …
    Previous ocean acidification events are likely to have been significant factors in mass extinction events in marine ecosystems.
    (p 28)


    Coral reefs


    There is evidence that shows a possible impact of the increase in acidity that has already occurred, based on a study of changes in … the coral Porites.
    The observational study was carried out using 328 sites on 69 reefs and showed a precipitous drop in calcification rate, linear extension and coral density, all indicators of coral growth, in the last 15-20 years of a 400-year record.
    (p 30)


    THE WATER CYCLE


    Observations of rainfall change


    [The] continent of Australia has become slightly wetter in terms of total annual rainfall over the past century …

    While the instrumental record goes back little more than a century, not long enough to clearly discern multi-decadal patterns of variability that are repeated on century timescales, palaeo studies could offer some insights into the severity of the recent drought in a longer time perspective.
    For example, a recent study states that the very low streamflow in the River Murray for the 1998-2008 period is very rare — about a 1-in-1500 year event.
    (p 33)


    The climate change-variability interaction


    Rainfall patterns across Australia are influenced in complex ways by several modes of natural variability, the most important of which are ENSO (El Niño — Southern Oscillation), SAM (Southern Annular Mode) and IOD (Indian Ocean Dipole). …

    [For] ENSO there is no clear pattern of change in behaviour that can be observed in the observational record over the past several decades and can be linked clearly to climate change, nor is there a strong consensus in climate model projections of the future behaviour of this mode of variability.

    For the IOD, the number of “positive” events, which induce a reduction in rainfall over southern Australia in winter and spring, has been increasing since 1950, reaching a record high frequency over the past decade … the number of negative IOD events has been decreasing and [the] projected pattern of the mean ocean-atmosphere circulation change in the Indian Ocean in the future is similar to that of a positive IOD phase, implying an increase in positive IOD frequency and/or intensity and thus a reduction in rainfall over southern Australia in winter and spring. …

    There is good evidence that a southward shift of the SAM (Southern Annular Mode), which brings rainbearing fronts in autumn and winter to southwest western Australia, is an important factor in the observed drop in rainfall there over the past several decades.

    [It is estimated] that about 50% of the rainfall reduction is attributable to climate change.

    (p 34)


    Understanding hydrometeorological processes


    Several aspects of the observed decrease in rainfall, especially in Victoria and southern South Australia, are now better understood.
    First, the proximate cause of the rainfall decline is an increase in the surface atmospheric pressure over much of the continent, although the cause of the rising pressure is not clear.
    In addition, the subtropical ridge, an east-west zone of high atmospheric pressure that often lies over the southern part of the continent, has strengthened considerably since 1970.
    Furthermore, this strengthening of the pressure system correlates very well with the rise in global mean temperature, and is consistent with expectations from the basic physics of the climate system.
    (p 35)


    Projecting changes in water availability


    [Rainfall] is the main driver of runoff, which is the link to river flows and water availability.
    A change in annual rainfall is typically amplified by two or three times in the corresponding change in average annual runoff. …
    (p 36)

    {[What] we can say with certainty is that rainfall patterns will change as a result of climate change, and often in unpredictable ways, creating large risks for water availability.

    This daunting uncertainty not only challenges attempts at adaptation, but also enhances, not diminishes, the imperative for rapid and vigorous global mitigation of greenhouse gas emissions.
    (p 38)


    EXTREME EVENTS


    Average-extreme relationship


    A modest shift to higher average temperatures leads to a disproportionately large increase in the number of extreme high temperature events …
    In addition, the most extreme events become much more intense …
    (p 39)


    Temperature extremes


    Changes in Melbourne temperatures provide a good example of the shifts in the frequency and intensity of extremes …
    The long-term average in the number of days per year 35°C or above is 10.
    During the decade 2000-2009, the number of such days per year rose to 13.
    Furthermore, the increased intensity of extreme events … is clearly evident in Melbourne with the record high temperature of 46.4 °C in February 2009, and the three consecutive days of 43 °C or above in late January.


    Sea surface temperature


    Coral-dominated ecosystems are sensitive to small rises in the temperature of the water in which they reside.
    This sensitivity results from the breakdown of a symbiosis between corals and tiny organisms called dinoflagellates, which are photosynthetically active and provide corals with organic carbon.
    When the sea temperature rises 1-2 °C above normal for a six to eight week period, this symbiosis breaks down, the dinoflagellates are expelled, and the corals are “bleached”. …

    The GBR has fared better than many reefs around the world, although parts of the reef have experienced bleaching events in 1980, 1982, 1983, 1987, 1992, 1994, 1998, 2002 and 2006 — with the 1998 and 2002 events the worst on record for the GBR.
    In these events over 50% of the GBR bleached in the exceptionally warm conditions, with an estimated loss of 5-10% of corals in each event.
    (p 40)


    Bushfire intensity and frequency


    Climate change affects fire regimes in at least three ways.

    1. [Changing] precipitation patterns, higher temperatures and elevated atmospheric CO2 concentrations affect the biomass and composition of vegetation, the fuel load for fires.
    2. [Higher] temperatures tend to dry the fuel load, making it more susceptible to burning;
      drought conditions can significantly exacerbate these conditions.
    3. [Climate] change increases the probability of extreme fire weather days — conditions with extreme temperature, low humidity and high winds.

    The severity of bushfires in southeast Australia is strongly pre-conditioned by low rainfall and high temperature induced by the positive phase of the Indian Ocean Dipole.
    Since 1950, the majority of large bushfires in southeast Australia, including the Ash Wednesday, Canberra, and Black Saturday bushfires, occurred following a positive IOD event in the preceding spring season, which led to warm and dry conditions.
    Since 2002, the Indian Ocean has experienced five positive IOD events (2002, 2004, 2006, 2007, 2008), with climate change a contributor to the increasing frequency of these events.


    Tropical cyclones


    Observational records show no changes beyond natural variability in either the frequency of cyclones or their storm tracks.
    [Some] studies have found a possible link between cyclone intensity and higher sea surface temperatures [however the] time period is too short to separate out decadal patterns of natural variability from the underlying trend of rising SST.


    Heavy precipitation events


    Higher temperatures, especially of the surface ocean, lead to more evaporation;
    this leads to higher water vapour content in a warmer atmosphere (which can hold more water vapour);
    and this in turn induces more precipitation.
    (p 42)

    The IPCC assessment of observations on a global scale shows an increase in atmospheric water vapour from 1988 to 2004 as well as increases in precipitation in many parts of the world, with a substantial increase in heavy precipitation events.
    A recent study comparing observed and model-simulated patterns of extreme precipitation events found that over the Northern Hemisphere land area with sufficient data coverage (about two-thirds of the total area), human-driven increases in greenhouse gas concentrations have contributed to the observed intensification of heavy precipitation events.
    However, there is no consistent evidence of an observed increase in heavy precipitation events over most parts of Australia at this time.

    At the continental scale a 100-year record from the United States shows a sharp increase in the area of the US experiencing very heavy daily precipitation events.
    (p 45)

    [Although] a conclusive link between the southeast Queensland rainfall events [in December 2010 and January 2011] and climate change cannot be made, such a link is plausible even if it is not discernible yet.
    [Nevertheless from a risk management] perspective … it would be prudent to factor in a climate change-induced increase in intense rainfall events in urban and regional planning, the design of flood mitigation works, and any reviews of emergency management procedures.
    (p 47)


    ABRUPT, NON-LINEAR AND IRREVERSIBLE CHANGES IN THE CLIMATE SYSTEM


    The science of abrupt change


    [Abrupt,] highly nonlinear changes, which … occur when an apparently small change in a forcing agent triggers an unexpected, large, complex response in the system …
    An important feature of a tipping element is that it must contain a strong positive (reinforcing) feedback process in its internal dynamics.
    [Tipping] elements can have varying degrees of irreversibility.
    (p 48)


    Examples of tipping elements in the climate system


    [It] is useful to classify them into:
    • those associated with the melting of large masses of ice,
    • those involving significant changes to unique biomes, and
    • those associated with large-scales changes in the circulation of the atmosphere and the ocean.

    The Greenland and Antarctic ice sheets may not seem like candidates for tipping elements as their rate of change is not “abrupt” from a human perspective, but they are definitely tipping elements in that beyond a rather narrow range of temperature change, they will be committed to irreversible meltdown.

    The accelerating downward trend in the loss of Arctic sea ice is indicative of threshold-abrupt change behaviour in which the threshold may already have been crossed, although the loss of summer sea ice is not irreversible and could quickly recover with a return to a colder climate.
    (p 49)

    Simulations that incorporate these ecological processes suggest that a threshold exists around a 2 °C temperature increase, beyond which the area of the Amazon forests committed to dieback rises rapidly from 20% to over 60%.
    Severe droughts in the Amazon Basin in 2005 and 2010, along with the observation that such droughts co-occur with peaks of fire activity, support this risk assessment.

    Perhaps the archetypal example of a tipping element is the Atlantic thermohaline circulation (THC), which in its current mode contributes significantly to the mild climate experienced by western Europe and Scandinavia but which has shown threshold-abrupt change behaviour in the past.
    A collapse of the THC could lead to a reduced level of warming in the north Atlantic region compared to the global average.
    Current understanding of the THC system suggests that the threshold for collapse is still rather remote, but that a weakening of the strength of the circulation is likely through this century.
    (p 50)


    Likelihood of triggering abrupt changes


    [Loss] of significant amounts of the Greenland and Antarctic ice sheets would lead to metres of sea-level rise.
    The Asian monsoon, or more precisely the Indian Summer Monsoon, is a tipping element whose behaviour is influenced by both the warming of the Indian Ocean and the presence of an “atmospheric brown cloud” over much of the subcontinent. …
    [Over] a billion people directly depend on the reliable behaviour of the Indian Summer Monsoon for their food production …

    [A risk] assessment … based on the combination of the likelihood of the tipping element being activated and the impact on human well-being of a change of state of the tipping element [suggests that] the highest risks are associating with the loss of ice from the large polar ice sheets.


    Abrupt shifts in atmospheric circulation


    Tipping elements associated with changes in atmospheric circulation, or coupled ocean-atmosphere circulation, are especially important because of the short time scales on which they can operate.
    The bi-stability of the Indian Summer Monsoon, noted above, is an example of a large shift in atmospheric circulation that can happen very quickly, even on an annual basis.

    The recent cold, snowy winters (2005-06, 2009-10, 2010-11) in parts of northern Europe and North America … comprise another good example of risks associated with this type of tipping element.
    [The] threshold for the abrupt shift in circulation lies near 40% reduction in sea ice, but another transition, flipping the circulation back to the earlier regime, is projected to exist at about 80% reduction in sea ice.
    (p 51)

No comments:

Post a Comment