1301.0 - Year Book Australia, 2003
ARCHIVED ISSUE Released at 11:30 AM (CANBERRA TIME) 24/01/2003
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There is a lot of confusion in the world about climate change. The first purpose of this article is to explain what is meant by ‘climate’ and ‘climate change’ in order to understand why so much of the discourse on the subject seems like the dialogue of the deaf - why the proponents of alternative perspectives still appear to be talking past each other on even very basic issues of climate science and policy; and why it has proved so difficult to achieve consensus on practical strategy for reducing whatever adverse long-term impacts humans may be having on climate and helping the world to prepare for whatever future the global climate system delivers over the coming decades and coming centuries.
The second purpose is to look back over the 20th century and show how Australian climate has changed in the past; and then to summarise what can, at present, be said, and what can not be said, about how it might change over the century ahead.
The meaning of climate change
We all have an intuitive sense of what we mean by climate. It is both our synthesis of the weather we have experienced in the past and our expectation of what it will be like in the future, at a particular place and time of year. Our recollections of the past are not so much of the monthly or yearly averages of temperature, humidity, cloud, wind and rainfall, but of the impacts on significant occasions in our lives of their hour-to-hour, day-to-day and week-to-week variability; and especially of the extreme events - the severe storms, the gales, the heatwaves and the droughts and floods - from which these long-term averages derive. We remember that we have had both hot and cold summers in the past and we sense that we must expect them again in the future. Those with long memories recall the years of widespread drought in the 1960s just as they do the floods of the 1970s and 1990s. And there are few Australians over fifty who have not asserted that 'the weather these days isn’t like it was when I was young'.
The statistics of Australia’s meteorological records tend to bear out these subjective impressions and, in a very real sense, the climate has always been changing - from year-to-year, decade-to-decade, and century-to-century. We also know from proxy - mainly geological - evidence that it has been changing also on much longer timescales, from thousands to millions of years, as the Earth moved into and out of the great ice ages of the past before returning to the more benign climates of the present ten-thousand-year-long Holocene ‘interglacial’.
Contemporary Earth system science can explain most of the features of present-day climate and how it has changed over time: why the tropics are warm and the polar regions cold; how the ‘greenhouse effect’ of water vapour, carbon dioxide and other trace gases in the atmosphere keeps the Earth’s surface some 70ēC warmer than it is 10 km above, where jet aircraft fly, and some 33ēC warmer than it would be, on average, if there were no radiatively active gases in the atmosphere; how the large-scale distribution of the continents modifies the north-south overturning of the atmosphere and oceans that is driven by the solar heating of the equatorial belt; and, perhaps most significantly of all, how the instabilities in the jet streams generated by the north-south overturning provide the energy source for most of the day-to-day weather phenomena that make up our climate. Because of the differing natural timescales of the atmosphere and ocean and the strength of the coupling between them, the explanation of the mechanisms of climate involves an integrated scientific understanding of the entire Earth system, consisting of the atmosphere, the oceans, and the land surface and inland waters and of all the physical, chemical and biological processes that take place within them.
If this is the nature and origin of climate, what then do we mean by ‘climate change’? Until a few decades ago, the term ‘climate change’ was mostly taken to mean the major astronomically-induced shifts from ice-age to interglacial on timescales of tens to hundreds of thousands of years or, less usually, systematic change of the long-term (by international convention, 30 years) statistics of the climate elements (temperature, pressure, wind, rain, etc.) sustained for several decades or longer.
The situation became greatly confused in the early 1990s as a result of the emerging concern that, in addition to the natural variability of climate on all timescales, the build-up of greenhouse gases in the atmosphere from the burning of fossil fuel and other human activities may be leading to systematic long-term increase of globally-averaged surface temperature (via an enhanced greenhouse effect) and other irreversible changes in climate. With its focus on human interference with the working of the climate system, the United Nations Framework Convention on Climate Change, signed by more than 150 countries at the 1992 Rio Earth Summit, defined climate change as 'a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods'.
Thenceforth, to those who speak and listen in the language of the Convention, any statement that climate change is occurring has meant that it is attributable to human activity. The scientific community, however, have taken a different approach. The Intergovernmental Panel on Climate Change (IPCC), the assessment body set up in 1988 by the World Meteorological Organization and the United Nations Environment Programme to provide objective, expert assessment of the state of understanding of the science, impacts and response strategy for climate change, has defined climate change as 'any change in climate over time whether due to natural variability or as a result of human activity'. It is this (IPCC) usage which is adopted here with the purpose of summarising what can be said about past and future climate change in Australia as a result of both natural variability and human interference with the global climate system.
The controls on Australian climate
The broad-scale controls on Australian climate are shown schematically in figure S1.1. The two major influences are:
In El Niņo years, when the central and eastern Pacific are warm, the ascent and cloudiness over the western Pacific are suppressed and there is a lower probability of rain-bearing systems affecting northern and eastern Australia. La Niņa events, on the other hand (which are characterised by an unusually cold eastern Pacific and a warm western Pacific), usually mean a higher probability of rain-bearing systems and flooding over northern and eastern Australia.
S1.1 LARGE-SCALE CONTROLS ON AUSTRALIAN CLIMATE
Note: The solar heating of the tropics drives the north-south (Hadley) overturning of the atmosphere (shown schematically on the left), which generates the meandering westerly jet stream (wind speed cross section shown on the right with wind speeds of a few hundred km per hour in the jet core) and the migratory weather-producing lows and highs of the middle latitudes. The east-west (Walker) circulation of the tropics is driven primarily by the temperature differences between the warm western and cool eastern Pacific Ocean. Its season-to-season and year-to-year fluctuations (and occasional reversal) exert a major influence on the occurrence of cloud and rain producing systems in Australian longitudes.
Climate change over the past century
The 20th century witnessed some major fluctuations and trends in Australian temperature and rainfall as well as in a host of other characteristics of Australian climate. Graph S1.2 shows the annual mean temperature averaged across Australia on the basis of a network of high-quality observing stations and presented in terms of anomalies (departures) from the 1961-90 ‘normal’. It is evident that, with the notable exception of 2000 and 2001, most years of the past two decades have been above the 1961-90 normal and approximately half a degree warmer than the average for the first half of the century. The general warming trend over the 20th century is evident in both summer and winter temperatures as well as in daily maxima and minima, with night-time minimum temperatures generally rising faster than daytime maxima. The distinct warming trend of the past half-century, evident in graph S1.2 which is of the same general magnitude as the observed globally-averaged warming described in the recent Third Assessment Report of the IPCC (IPCC 2001a) is not, however, uniform across Australia, as can be seen in map S1.3. Whereas parts of Queensland have warmed by more than one degree over the past 50 years (with the greatest warming evident in the night time minima), parts of New South Wales and Victoria and large areas of north-west Australia have experienced only minimal warming, or have actually cooled, over the period.
S1.2 CHANGES OF ANNUAL MEAN TEMPERATURE
Note: The changes of annual mean temperature (ēC) averaged over Australia since 1910. Temperatures are shown as departures from the 1961-90 ‘normal’.
S1.3 AVERAGE TREND IN ANNUAL MEAN TEMPERATURE (ēC/10yrs - 1950-2001)
Note: Pink and red areas have experienced average warming over the period while blue areas have experienced a cooling trend.
The long-term record of area-average rainfall over Australia is shown in graph S1.4, which highlights the large year-to-year and decade-to-decade variability of rainfall with long, dry periods following Federation and again in the 1920s, 1930s, 1940s and 1960s, and above-average rain in the mid 1950s and 1970s and for most of the past decade. While there is a very slight long-term trend towards increased rainfall for Australia as a whole, the pattern is highly variable from region to region and, over the past 50 years, most of central and north-west Australia has got wetter, while south-west Western Australia, Victoria and much of New South Wales and Queensland have got drier (map S1.5).
S1.4 MEAN ANNUAL RAINFALL
Note: It is evident that, for the country as a whole, the rainfall has changed markedly from year-to-year and decade-to-decade, with the very wet years of the 1970s and over the period 1997-2001 suggesting a slight long-term trend towards a wetter Australia.
S1.5 AVERAGE TREND IN TOTAL RAINFALL (MM/10YRS) - 1950-2001
Note:The average trend (mm/decade) in annual total rainfall over Australia over the past half century, showing the strong trend towards wetter conditions over north-west Australia and a drying trend over much of eastern Australia and the southwest corner.
The cause of the observed change
Much of the Australian and international climate research effort over recent decades has been aimed at developing sufficiently reliable models of the global climate system to enable scientists to find out how much of the observed change over the past century is the result of various forms of natural variability and how much can be attributed confidently to the influence of human activities; and then to use those models to provide an indication as to how climate might evolve over the next century, both as a result of natural processes and in response to human influences through greenhouse gas emissions or in other ways.
At the global level, the IPCC, in its Third Assessment Report, has concluded, on the basis of the longer and more closely scrutinised temperature record and new model estimates of both natural variability and climate system response to forcing by natural processes (e.g. volcanoes and changes in solar output) and human influences (especially emission of greenhouse gases and aerosols), that 'there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities' (IPCC 2001a).
Although it is more difficult to demonstrate on a sound scientific basis and there may still be extraneous influences (e.g. from the so-called ‘urban heat island’ effect) in even the high quality data sets on which graph S1.2 is based, there appears to be good reason to believe that the overall warming trend over Australia over the past half century is also largely a result of enhanced greenhouse warming. It is almost impossible, however, to separate out the effect of human influence from natural factors on smaller space and time scales in, say, explaining why Queensland has warmed more than parts of New South Wales (map S1.3). In the absence of any convincing basis for attribution of the geographic pattern of warming to human influence (albeit some plausible physical hypotheses have been advanced), the presumption must be in favour of natural processes as the primary explanation of the spatial variability of the observed rate of warming over Australia.
It is impossible to determine, with any confidence, at this stage whether the spatial pattern of trend in rainfall (map S1.5) is the product of processes associated with natural large-scale and long-term fluctuations in the oceanic and atmospheric circulation of the Southern Hemisphere (the so-called Antarctic circumpolar wave, natural long-term variability of the El Niņo-Southern Oscillation mechanisms or the like), or whether the circulation changes causing these patterns of rainfall trend (and at least part of the associated pattern of temperature trends) are an early manifestation of the systematic shifts in climate patterns that some global climate models suggest should be expected from the build-up of atmospheric greenhouse gas concentrations over the past century. Some features of the pattern (drying in south-west Western Australia and Victoria) are, however, broadly consistent with the majority of presently available model projections under an enhanced greenhouse warming scenario.
Impacts of climate change
Considerable research has been carried out over the past century on the impacts of climate change on the Australian environment, economy and way of life (Gibbs 1978; Pittock et al. 1978; Maunder 1989), and in particular on Australian water resources and agriculture. One of the most important components has been the work on assessment of the probability and return periods of extreme rainfall events of various magnitudes for design of dams and other long-term water resource-related infrastructure.
The Australasian chapter of the IPCC Special Report on Regional Impacts of Climate Change (IPCC 1998) and the corresponding section of the Third Assessment Report on Impacts, Adaptation and Vulnerability (IPCC 2001b) provide an overview of present knowledge of both past and prospective impacts of climate change in such sectors as water supply, ecosystems and conservation, food and fibre, settlements and industry, and human health.
It is clear from the experience of the past century that the challenge of living with climate change in Australia has not so much been that of adapting to long-term trends resulting from human activity, but rather that of planning and managing for the large natural year-to-year and decade-to-decade variability of rainfall and other characteristics of Australian climate. The lessons learned from this experience will be critical to the 21st century challenge of living with whatever human-induced long-term change is superimposed on the continuing natural variability.
Modelling anthropogenic climate change
The major challenge faced by climate scientists, called on to advise policymakers on how increasing anthropogenic emissions of carbon dioxide and other greenhouse gases will affect future global and regional climate, focuses on the construction of sufficiently soundly-based and demonstrably reliable global climate models to simulate how the real atmosphere and ocean would respond to a range of possible future emissions of greenhouse gases and aerosols through the 21st century. The IPCC’s Third Assessment Report (IPCC 2001a, b) has indicated that some 20 to 30 models around the world have reached a sufficient level of sophistication and reliability to justify confidence in their assessment of the sensitivity of the large scale features of global climate (global mean temperature, rainfall etc.) to increasing greenhouse gas emissions, but that it is still not possible to attach much confidence to the models’ projections of the anthropogenic component of climate change at the regional level.
The use of these climate models to explore possible future anthropogenic climate change is based on the rigorous, but widely misunderstood, methodology of feeding a range of emission scenarios (not predictions) into atmospheric chemistry models to produce concentration scenarios (not predictions) which are then used, in turn, to produce corresponding projections (not predictions) of how the enhanced greenhouse effect would be expected to modify the real climate. This methodology avoids the impossible task of trying to predict a future which would itself be significantly influenced by society’s response to the prediction. It enables us to gain an understanding of the sensitivity of the global climate system to increasing (or decreasing) emissions without making any assumption about the actual likelihood of one future emission profile relative to another. A schematic summary of the global warming and sea level projections for the 21st century included in the IPCC’s Third Assessment Report (IPCC 2001a) is shown in graph S1.6.
S1.6 MODELLED SENSITIVITY OF GLOBAL MEAN TEMPERATURE AND SEA LEVEL TO GREENHOUSE GAS EMISSIONS
Note: The graphs show, for a wide range of emission scenarios (the lowest, highest and two ‘illustrative’ scenarios - A1FI (fossil intensive) and B1 (clean technology) - published in the IPCC Special Report on Emission Scenarios (SRES) (IPCC 2000), the carbon dioxide emission profiles to 2100 (bottom left), the resulting carbon dioxide concentrations (top left), the model projections of global mean temperature rise (top right) and sea level rise (bottom right). The temperature panel provides an indication of the range of uncertainty of the projections resulting from the different climate sensitivities of the individual models (pink shading) as well as the model mean projections for the A1FI and BI scenarios and the envelope of climate change projections associated with the envelope of emission scenarios included in the IPCC Report.
The current state of knowledge
The current state of the science of climate change is reported comprehensively in the Third Assessment Report (IPCC 2001a). The key conclusions, which the IPCC includes in its Summary for Policymakers, are the following:
An increasing body of observations gives a collective picture of a warming world and other changes in the climate system.
Despite the exhaustive process of peer review and the IPCC policy of explicitly identifying areas of uncertainty and disagreement in its reports, there is a substantial body of sceptical literature taking issue with its main conclusions (e.g. Lomborg 2001). While much of this appears to be based on misunderstanding of the IPCC reports (e.g. confusion between the IPCC and Convention definitions of ‘climate change’, failure to understand the significance of the difference between scenarios, projections and predictions and even failure to understand the implications of the basic physics of the greenhouse effect), other critics have focused on perceived inconsistencies in the observational record and the various well-known sources of uncertainty in physical processes and model limitations1. While it is expected that the Fourth Assessment Report, due in 2006-07, will bring both new confidence and new sources of uncertainty, the conclusions of the Third Assessment Report remain the most up-to-date and most reliable summary of the state of knowledge of the science of climate change (Zillman 2001).
Future climate change over Australia
By contrast with short-term weather prediction which has achieved increasingly high levels of skill over the past 50 years, climate prediction is still in its infancy as a science (WMO 2002). While a number of empirical systems for assessing the probability of above or below normal rainfall and temperature are employed operationally (e.g. by the National Climate Centre of the Commonwealth Bureau of Meteorology) for producing usefully skilful seasonal climate outlooks, and coupled atmosphere-ocean models are now available which can predict the broad evolution of ocean temperature and other climate patterns for six to twelve months ahead, most of the forecast skill runs out beyond a year or so and it is not possible to indicate likely climate patterns, either globally or for individual geographic regions, with much confidence on longer timescales.
Given contemporary understanding of the mechanisms of global and regional climate, the most confident statement that can be made about the next decade and the next century is that Australia must expect to continue to experience the major El Niņo- and La Niņa-associated multi-year fluctuations of temperature and rainfall which have earned it its reputation for climate extremes and its image as a land of ‘droughts and flooding rains’. There is, as yet, no sound scientific basis for predicting any specific change to this year-to-year and decade-to-decade variability, which must be expected to continue to be the dominant climatic influence on Australia’s environment, economy and way of life. But it is certainly possible that some large-scale fluctuation, outside the range of experience of the past two hundred years of instrumental records, will manifest itself in Australian climate patterns over the next century.
The next most confident thing that we can say about future climate change in Australia is that there seems likely to be a general warming trend, as a result of the inevitable continued build-up of greenhouse gases in the atmosphere, of up to perhaps a few degrees over the century, superimposed on whatever temporal and spatial change (including short-term variability) occurs as a result of natural processes. While the IPCC Third Assessment Report (IPCC 2001a) has indicated that, for the full range of greenhouse gas emission scenarios considered by the IPCC, and allowing for uncertainties in the climate models, 'the globally averaged surface temperature is projected to increase by 1.4 to 5.8ēC over the period 1990 to 2100', and that 'it is very likely that nearly all land areas will warm more rapidly than the global average', it is clearly not possible, at this stage, to know how actual emissions will increase (or decrease) over the next hundred years, and therefore how large will be the globally averaged temperature rise due to enhanced greenhouse warming. It is even more difficult, given the possibility of significant rearrangement of the large-scale circulation (e.g. through changes in the behaviour of the El Niņo-La Niņa cycle) to predict the actual temperature rise (and any associated change in rainfall) over Australia as a whole. And it is quite impossible, given all these uncertainties and the still substantial limitations of the climate models, to indicate what the enhanced greenhouse effect might mean by way of regional changes of rainfall patterns for the individual states and territories. While the IPCC Third Assessment Report (IPCC 2001a) includes some broad indications of the extent and nature of inter-model consistency in the projections of temperature and rainfall trends for northern Australia (north of 28ēS) and southern Australia separately, for two different SRES emission scenarios (A2 and B2 which fall broadly within the envelope of the A1FI and B1 scenarios of graph S1.6, with A2 producing larger concentrations and greater warming than B2) for summer and winter (table S1.7), it seems that little can be said, with any confidence, about future climate change on the scale of the individual states of Australia at this stage.
While other, higher resolution, assessments have been undertaken for a range of global emission scenarios and using a number of different models to indicate the corresponding range of projected changes in regional climate (e.g. Whetton et al. 2002), which provide a basis for sensitivity studies as an aid to planning for adaptation to future climate, these regional model projections should not be interpreted as predictions of the human-induced component of future climate change and certainly not as predictions of future climate. It may still be decades before that is likely to be done with confidence, and it is not yet possible to say whether it will ever be done with reliability.
The challenges ahead
The attribution of observed climate change to human activities and the projection of human impacts on future climate are likely to remain controversial, but progress in both areas will be essential to planning for efficient adaptation to future climate. Until, for example, we know whether the recent systematic drying of the south-west corner of Australia (Indian Ocean Climate Initiative Panel 2002; see also map S1.5) is due to some natural long-term fluctuation in (say) the southern ocean - in which case we would expect rainfall to increase again in the future; or whether it is a manifestation of large-scale geographically-anchored circulation changes forced by enhanced greenhouse warming - in which case we would expect the drying trend to continue - it will be very difficult to provide a reliable basis for planning for adaptation on the century timescale. The importance and urgency of better monitoring and modelling of Australian climate cannot be overstated.
1. The Environment section refers to even more recent concerns raised about some of the underlying assumptions associated with the scenarios outlined in the Third Assessment Report (ed.).
Gibbs WJ 1978, The Impact of Climate on Australian Society and Economy, CSIRO Division of Atmospheric Physics, 44 p.
Indian Ocean Climate Initiative Panel 2002, Climate variability and change in southwest Western Australia, Department of Environment, Water and Catchment Protection (Western Australia) (in press).
IPCC 1998, The Regional Impacts of Climate Change: An Assessment of Vulnerability, A Special Report of the Intergovernmental Panel on Climate Change (RT Watson, MC Zinyowera, RH Moss & DJ Dokken (eds)), Cambridge University Press, 516 p.
IPCC 2000, Emission Scenarios, A Special Report of Working Group III of the Intergovernmental Panel on Climate Change, (N Nakicenovic et al. (eds)), Cambridge University Press, 599 p.
IPCC 2001a, Climate Change 2001: The Scientific Basis, Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, (JT Houghton, Y Ding, DJ Griggs, M Noguer, PJ van der Linden, X Dai, K Maskell & CA Johnson (eds)), Cambridge University Press, 881 p.
IPCC 2001b, Climate Change 2001: Impacts, Adaptation and Vulnerability, Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change, (IJ McCarthy, OF Canziani, NA Leary, DJ Dokken & KS White (eds)), Cambridge University Press, 1032 p.
Lomborg B 2001, The Sceptical Environmentalist, Cambridge University Press, Cambridge, 515 p.
Maunder WJ 1989, The Human Impact of Climate Uncertainty, Routledge, London & New York, 170 p.
Pittock AB, Frakes LA, Jenssen D, Peterson JA & Zillman JW 1978, Climate Change and Variability - a Southern Perspective, Cambridge University Press, Cambridge, 455 p.
Whetton PH, Suppiah R, McInnes KL, Hennessy KJ & Jones RN 2002, Climate Change in Victoria: High Resolution Regional Assessment of Climate Impacts, CSIRO and Department of Natural Resources and Environment, 44 p.
WMO 2002, WMO Statement on the scientific basis for, and limitations of, weather and climate forecasting, Abridged Report with Resolutions, Fifty-fourth Session of the WMO Executive Council, Geneva, 2002.
Zillman JW 2001, 'The IPCC Third Assessment Report on the Scientific Basis of Climate Change', Australian Journal of Environmental Management, 8, pp. 43-59.
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