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  1. 1. The Science ofClimate Change Questions and Answers August 2010 The Science of Climate Change
  2. 2. Prof. Kurt Lambeck political models each with their own inherent they are authoritative within the current state of President, Australian assumptions and difficulties with data and knowledge. This Committee consists of eminent Academy of Science observations. In the presence of uncertain Fellows of the Academy and other experts with May 2006 – May 2010 scientific uncertainty, it should not be surprising both extensive research experience in related that, when it comes to recommendations about fields and in the leadership of climate-relatedForeword Summary how to respond to a threat of climate change, programs and organisations. the spectrum of opinions is broad indeed. While it is important to emphasise that it is The Australian Academy of Science is not possible to provide definitive answers to strongly committed to enhancing public many of the questions that are being askedT he science of climate is at the intersection understanding of scientific issues and how about climate change, it is also important to of a number of science disciplines and these may impact on society and the planet. stress that considerable progress has been T sub-disciplines. At its heart are physics, Through its members and through its National made in understanding climate change and he Earth’s climate has changed. Thechemistry, biology and mathematics – each with Committees for Science it is able to draw on why it occurs. The role of greenhouse gases in global average surface temperaturetheir sub-disciplines of atmospheric physics and expertise from across a broad sector of the the atmosphere is qualitatively well understood.chemistry, oceanography, hydrology, geology Australian science community to report on It is known that increasing the atmospheric has increased over the last centuryetc – and each of which can be considered important scientific issues. concentration of the principal anthropogenic and many other associated changes have beenas mature within the framework required to This includes climate science. The Academy greenhouse gas, CO2, leads to higher mean observed. The available evidence impliesdiscuss climate. It is at this intersection of the recognises that decisions on how to respond global surface temperatures. It is known that that greenhouse gas emissions from humandisciplines where uncertainty can and will arise, to climate change will have to be made by our CO2 has increased very substantially during activities are the main cause. It is expectedboth because of the yet poorly understood society as a whole. These decisions need to the last century, to the highest levels seen in that, if greenhouse gas emissions continue atfeedbacks between the different components consider the findings of climate change together the past 800,000 years, and that this increase business-as-usual rates, global temperaturesof the climate system and because of the with many considerations that go beyond the is primarily of anthropogenic origin. It is also will further increase significantly over thedifficulty of bringing these components together science and must include, amongst others, beyond serious question that some CO2 frominto a single descriptive and predictive model. ethics and equity, economics, risk management human activities remains in the atmosphere for coming century and beyond.This would include, for example, the biological and politics. The purpose of this document a very long time, as is the message that unless The science behind these statements isconsequences of how increasing carbon dioxide is to contribute to the public understanding greenhouse gas emissions are reduced, an supported by extensive studies based on four(CO2) feeds back into climate and into the of the state of the science and to attempt to upward trend in global temperature will continue. main lines of evidence:climate model, or how the consequences of tread a path through the often contradictory The uncertainties in the science do not affectatmospheric warming on water vapour, cloud public commentary on the science. It is not a such major conclusions but they will affect the Physical principles established more than acover, ocean warming and circulation feedback formulation of a policy response but an attempt precise timescales or magnitudes of the change century ago tell us that greenhouse gases, suchcan be described and quantified in a coherent to improve the public understanding of the and they will affect the global distribution of itsand integrated theory. It is these feedbacks and science upon which any policy response should impact. It is important therefore that extensive as carbon dioxide (CO2), trap heat and keepinteractions that make it difficult to realistically be constructed. research and rigorous scientific debate continue the planet warmer than it would otherwisequantify the uncertainty in the outputs of climate To this effect the Academy’s Council within the expert scientific community and be. Increasing greenhouse gas levels raise themodels at levels that the experimental scientist established two committees to address some that the communication of that research to the temperature of the Earth’s usually accustomed to. In a process as of the major questions that are frequently broader community be effective. The Academyintrinsically complex as climate it should not be asked about climate change science. First, therefore hopes that this report will provide a The record of the distant past (millionssurprising that the path to understanding is long an expert Working Group carefully formulated firmer basis for understanding the science of of years) tells us that we cannot take a stableand arduous. the questions and answers about the science climate change and its implications. climate for granted. Climate has varied In many other areas of experimental science of climate change. This group consists of The Academy is very appreciative ofthe paths to full understanding are equally internationally recognised scientists who have the contributions made to this report by the greatly through the Earth’s history. It has, forcomplex. What makes climate change different contributed extensively to the underpinning members of the Working Group and Oversight example, gone through 10 major ice age cyclesis that the consequences are not only potentially science, including contribution to the successive Committee to provide authoritative answers to over approximately the past million years. Theglobal and serious but also that they occur IPCC assessments. Seven ‘big’ questions were these important questions on the science of past few thousand years have been unusuallyover long time scales (decades to centuries) identified within each of which ‘lower-level’ climate change. The Academy also thanks the stable. Together with our understanding ofso that actions need to be contemplated before questions have also been addressed. Second, Department of Climate Change and Energyfull understanding is achieved. These actions an Oversight Committee comprehensively Efficiency for providing financial support tothemselves are built on economic, social and reviewed the answers provided to ensure that prepare this document.Published by the Australian Academy of Science; ISBN 085847 286 4. Please cite “The Science of Climate Change: Questions and Answers”, Australian Academyof Science, Canberra. The Science of Climate Change
  3. 3. T 4physical principles, evidence from the past spells, changes to rainfall patterns and a his document aims to summarise Are human activities causingshows that climate can be sensitive to small higher global average rainfall, higher plant and clarify the current climate change?external influences. productivity in some places but decreases understanding of the science Human activities are increasing greenhouse in others, disturbances to marine and of climate change for non-specialist gas levels in the atmosphere. It is veryMeasurements from the recent past terrestrial ecosystems and biodiversity, readers. The document is structured likely that most of the recent observed(the last 100 years) tell us that the Earth’s disruption to food production in some around seven questions. global warming is caused by this increasesurface is warming along with rising regions, rising sea levels, and decreases in in greenhouse gases.levels of greenhouse gases from humanactivities, and that this warming is leadingto other environmental changes. Although Arctic ice cover. While aspects of these changes may be beneficial in some regions, the overall impacts are likely to be negative 1 What is climate change? Climate is a statistical description of weather conditions and their variations, 5 How do we expect climate to evolve in the future?climate varies from year to year and decade under the present structure of global society. including both averages and extremes. Climate models and studies of past climatesto decade, the overall upward trend of A warming of 7°C would greatly Climate change is a change in the average indicate that global warming and associatedaverage global temperature over the last transform the world from the one we pattern of weather over a long period of changes will continue if greenhouse gas levelscentury is clear. now inhabit, with all of the above impacts time. Greenhouse gases play an important keep rising as they are now. It is very likely being very much larger. Such a large and role in determining climate and causing there will be significant warming through theClimate models, together with physical rapid change in climate would likely be climate change. 21st century and beyond. Reduction ofprinciples and knowledge of past beyond the adaptive capacity of many greenhouse gas emissions could significantlyvariations, tell us that, unless greenhousegas emissions are reduced and greenhouse societies and species. There are uncertainties in climate science. 2 How has Earth’s climate changed in the distant past? reduce long-term warming.gas concentrations in the atmosphere arestabilised, global warming will continue. For example, a precise value cannot be given for the likely range of warming because of uncertainties in climate sensitivity Global climate has varied enormously through Earth’s history. Evidence from the past shows that global climate can be sensitive 6 What are the consequences of climate change? Climate change will have significant impactsClimate models estimate that, by 2100, to small disturbances, although climate to small influences. Past records also on our society and environment, both directlythe average global temperature will be models and evidence from past climate change show that climates can shift abruptly. and by altering the impacts of other stresses.between 2°C and 7°C higher than pre- provide a plausible range of values. Climateindustrial temperatures, depending on futuregreenhouse gas emissions and on the waysthat models represent the sensitivity of climate changes over small regions and changes in rainfall patterns are very hard to estimate. Tipping points or rapid climate transitions 3 How has climate changed during the recent past? Global average temperature has increased 7 How do we deal with the uncertainty in the science? Although climate forecasts are uncertain andto small disturbances. Models also estimate associated with overall global warming are over the past century. Evidence for this comes will remain so, the broad conclusions of climatethat this climate change will continue well possible but cannot yet be predicted with from instrumental temperature records in the change science as outlined above are based onafter 2100. confidence. These uncertainties work in both air and the ocean. Temperature observations many lines of evidence which together give a A 2°C global warming would lead to directions: there is a chance that climate are not the only evidence of recent climate high degree of confidence. Partly because ofa significantly different world from the change will be less severe than the current change: other sources include trends in sea scientific uncertainty but also because manyone we now inhabit. Likely consequences estimates of climate science, but there is also a levels, glaciers, ice caps and atmospheric aspects of human life are involved, decisionswould include more heat waves, fewer cold chance that it will be more severe. water vapour that are consistent with global about action on climate change will need warming. Australia’s climate has changed to involve extensive consideration of issues along with global climate. beyond science, including ethics, economics and risk management. The Science of Climate Change
  4. 4. BOx 2 If water vapour is the most important greenhouse gas, why all the fuss about CO2?1 What is climate change? Water vapour accounts for about half the present-day greenhouse effect. Its global average concentration in the troposphere (where most water vapour is found) is controlled mainly by the atmosphericClimate change is a change in the changes in weather from day to day, between Sustained and truly global changes in temperature and winds, with warmeraverage pattern of weather over a seasons, and from one year to the next, do average temperature require some global temperatures causing higher water vapourlong period of time not represent climate changes. The period for heating or cooling influence such as variations concentrations. This is in contrast with otherClimate is a statistical description of weather estimating climate is usually 30 years or more, in heat output by the Sun, changes to the greenhouse gases, for which concentrationsconditions and their variations, including long enough to sample a full range of weather. Earth’s orbit around the Sun, changes in are strongly influenced by human-inducedboth averages and extremes. Climate change Climate can be defined for a particular cloudiness, changes to the extent of ice on inputs to the atmosphere.refers to a change in these conditions that place or region, usually on the basis of local Earth’s surface, or changes in greenhouse gas If other factors warm the atmosphere,persists for an extended period, typically rainfall patterns or seasonal temperature concentrations in the atmosphere. then water vapour concentrations aredecades or longer. variations. Climate can also be defined for Identifying climate change that is truly expected to increase and, because water Weather variables such as temperature and the entire Earth. For global climate, a key global in extent requires simultaneous vapour is a greenhouse gas, the increasedrainfall fluctuate naturally (see Box 1). These variable is the average surface temperature. observations from a network of locations concentrations would amplify the initial around the world (see Question 3). Such a warming (see Figure 1.1). This is known as BOx 1 network of instrumental observations has only a positive feedback. Could the 20th century warming be just a part of the natural been available since the second half of the The water vapour feedback is supported variability of climate? 19th century. Climate changes that occurred by most evidence and analyses so far 5-11, before this time can be identified by although some views are different 12. Climate varies naturally on many timescales. In principle, a natural fluctuation could last reconstructing records from climate-sensitive Much of this variation arises from the exchange for a century. However, evidence going back indicators like ocean sediments, ice-cores, of heat and water between the deep oceans up to 20 centuries does not show changes in tree rings and coral reefs. of all of these gases are being directly and upper ocean layers (typically the top 50 to global temperature resembling those that have influenced by human activities (see Question 100 metres), which in turn has an impact on taken place in the last 100 years 1-3. Moreover, Greenhouse gases play an important 4). Once released into the atmosphere, many the atmosphere. A well-known example is the there is compelling independent evidence role in determining climate and causing of these gases remain there for a long time: El Niño oscillation in the tropical Pacific Ocean, (see Question 4) that this warming is being climate change in particular, a significant fraction of CO2 which influences temperatures and rainfall caused largely by the enhanced greenhouse Greenhouse gases include water vapour, emissions remains in the climate system for patterns throughout the tropical Pacific region effect due to human activities. The response carbon dioxide (CO2), methane, nitrous hundreds to thousands of years. and far beyond. Other ocean basins have of the climate system to human causation oxide and some industrial gases such as Water vapour is an important greenhouse similar oscillations. Such phenomena typically was foreseen by scientists more than a chlorofluorocarbons (CFCs). These gases act gas but it is not like the greenhouse gases change the global average temperature by no century ago 4. If this warming continues as like an insulating blanket, keeping the Earth’s affected directly by human activities. Its more than a few tenths of a degree, and only now projected, it will soon dwarf any change in surface warmer than it would be if they were concentration in the atmosphere is controlled for up to a year or two. the last 10,000 years. not present in the atmosphere. Except for by the climate itself, rather than by human water vapour, the atmospheric concentrations activities. Water vapour therefore reacts to, The Science of Climate Change
  5. 5. Figure 1.1 Feedbacks in the climate system There are close connections between global temperature, atmospheric water vapour, the extent of polar ice caps and levels of greenhouse gases (GHGs) in the atmosphere. When one of these is disturbed, the others react through processes that amplify the original disturbance until a new, different climate equilibrium is reached. In the glacial cycles overNASA the past million years, the disturbance came from fluctuations in the Earth’s orbit around the Sun (grey box in upper diagram). Thisand amplifies, climate change caused by caused temperatures toother factors (see Box 2 and Figure 1.1). change (green box), in turn The effects of changing greenhouse gas inducing rapid changeslevels on climate can be distinguished in water vapour (left blue box), and much slowerfrom the effects of other factors such as changes in ice caps (rightchanges to the Sun’s radiation. These blue box) and greenhousedifferent causes lead to different patterns gas levels (orange box),or “fingerprints” in the resulting climate which together amplifiedchanges, which assist in identifying the the temperature change.cause of observed changes. For example, In modern climate change,increases in solar radiation would be the disturbance comes fromexpected to warm both the upper and human-induced changes in atmospheric CO2lower parts of the atmosphere and result in and other greenhouse gasdays warming more than nights. On the levels (grey box in lowerother hand, increases in greenhouse gases diagram). In both cases,would be expected to result in a cooling, the disturbance is amplifiednot a warming, in the stratosphere (the by similar reinforcinglayer of the atmosphere above 15 km processes.elevation), and cause nights to warm morethan days. The observed patterns of changemore nearly match those expected fromincreasing greenhouse gases. The Science of Climate Change
  6. 6. 2 How has Earth’s climate changed in the distant past? Climate has varied enormously through Past temperature changes affected the the last ice age, of 5°C or more over as little as Earth’s history world dramatically. For example, in a few decades, were probably mostly regional Since the Earth was formed 4.5 billion the coldest period of the last ice age and due to sudden collapses of ice sheets or years ago, the global climate has changed (approximately 20,000 years ago) sea changes in ocean currents 14, 29, 32-34. dramatically many times due to the changing level was at least 120 metres lower 25. The configuration of continents and oceans, atmosphere was also very dusty, probably Although the millennium before the natural variations in the levels of greenhouse because of dramatic regional reductions in industrial revolution was relatively gases in the atmosphere, the Sun’s intensity, vegetation cover associated with the colder stable, there were variations in and the Earth’s orbit around the Sun 13-20. climate and reduced CO2 26-27. In even climate over that period earlier times, several million years ago, The Medieval Warm Period (AD 800-1300) Evidence from the past shows that global global temperature was several degrees and Little Ice Age (AD 1500-1800) are climate is sensitive to small influences higher than today and warm, tropical two well-known climate episodes during During the past million years, the average oceans may have reached much farther from the past thousand years. The Northern temperature of the Earth’s surface has risen the equator, causing significant changes to Hemisphere may have been up to 1°C warmer and fallen by about 5°C, through 10 major atmospheric flow patterns 28. on average during the former period than ice age cycles. The last 8,000 years have during the latter. However, several assessments been relatively stable at the warmer end Past records also show that climates indicate that Northern Hemisphere average of this temperature range 21. These cycles can shift abruptly temperatures over the last fifty years have were initiated by subtle variations in the The largest global temperature changes been warmer than during the Medieval Warm Earth’s orbit that altered the pattern of evident in the geologic record have typically Period, and temperatures over the last decade absorbed sunlight. Measurements from occurred fairly slowly over tens of thousands are warmer still. Records are sparse in the ice cores and other sources strongly or millions of years, much more gradually Southern Hemisphere, but those available suggest that as temperatures changed, than the warming over the past century 14. indicate little or no correlation with warming other changes were triggered that had However, some rapid changes have been in the Northern Hemisphere during the an amplifying effect: during warm periods, documented both in very warm past climates Medieval Warm Period, unlike the more CO2 and methane were released into the and in more recent ice ages. globally coherent cooling in the Little Ice Age atmosphere, and ice sheets receded and so One of these rapid changes took place and warming over the past century 1, 14, 35-40. reflected less sunlight to space 14, 22-24. This 56 million years ago, when the global There have also been regional variations meant that small influences were amplified temperature increased by about 5°C, in climate, particularly rainfall, that are not into larger changes (see Figure 1.1). accompanied by an unexplained release of associated with global changes. For example,PHOTOLIBRARY An important implication of this greenhouse gases into the atmosphere 29. regional droughts appear to have contributed finding from past climate changes is This release may have been so rapid as to be to the collapse of the ancient Akkadian that similar processes are likely to amplify comparable to the current human release of empire in the Middle East and the Mayans current human influences on climate. fossil fuels 14, 30, 31. Other rapid changes during in Mexico 41, 42. The Science of Climate Change
  7. 7. 3 How has climate changed Figure 3.1 Global surface temperature anomalies relative to 1951–1980, from surface air measurements at meteorological stations and ship and satellite sea surface temperature measurements. Differences between during the recent past? the series arise from different ways of deriving a global average surface temperature from measurements at numerous points. Data from: Climate Research Unit, University of East Anglia (www.cru.uea.; Goddard Institute for Space Studies (; National Climate Data Center, NOAA ( average temperatures have twice the rate of that for the past 100 years.increased over the past century The last decade has been the warmest yetMeasurements from many hundreds of recorded 43-46 (see Box 4).thermometers around the globe, on land The overall warming has led to an increaseand over the ocean, show that the average in the number of record high temperatures,near-surface air temperature increased over and decrease in frost frequency and thethe 100 years to 2009 by more than 0.7°C 43-46. number of record low temperatures over Many of these instrumental records, the past century 46, 47 (see Figure 3.3).which began in the second half of the 19th Over the past three decades, satellitecentury, were not initially designed to be used observations of temperature at the Earth’sfor climate monitoring. This means they have surface and in the lower atmosphere have alsoto be carefully analysed to deal with changes shown warming 46, 48, 49 (see Box 5). In contrastin instruments, observational practice, to the surface warming, the atmosphere abovelocation, and the growth of cities (see Box 3). about 15 km elevation (the stratosphere) hasAfter accounting for these issues, temperature cooled over the past four decades 46, 50-52. Thisincreases are largest in the continental provides one clue that the observed warminginteriors of Asia and north Africa, regions is due to human activities (see Question 4).which are distant from major population The upper 700 m of the ocean is storinggrowth areas (see Figure 3.1 and Figure 3.2) 43-46. about 90% of the additional heat absorbed The rates of observed near-surface warming by the Earth’s whole climate system sinceincreased in the mid-1970s. Since then, 1961 53. The surface ocean has warmedthe global land surface has warmed at about by 0.5°C from the 1970s to the earlydouble the rate of the ocean surface. Measured 2000s. Averaged over the upper 700 m ofwarming over the past 50 years was nearly the ocean, the average warming is much BOx 3 Does warming in cities affect global Climate researchers have made extensive efforts temperature records? Temperature anomaly (degree_ Celsius) to avoid or correct such problems, and several The temperatures recorded by some weather tests show that this has minimised any effects –3.0 –1.8 –0.6 0.6 1.8 3.0 stations in cities have been affected by non- on long-term trends, particularly when averaged climate related changes, including warming over large regions 59-61. Nonetheless regional and Figure 3.2 Distribution of global surface temperature anomalies for the period 2005-2009, relative to 1951-1980 as a baseline, from surface air measurements at meteorological stations and ship and due to their proximity to buildings and other year-to-year variability is not known precisely, satellite sea surface temperature measurements. Note that warming is greatest over continental interiors structures that emit, absorb and radiate heat. especially earlier in the record. and where there is no urban heat island effect (e.g. Antarctic Peninsula, Siberia). Data: The Science of Climate Change
  8. 8. smaller, only about 0.1°C, but very Has there been a global BOx 4 important because of the large amount of stored heat this represents 53-58. cooling trend since 1998? No, 1998 was an extremely warm year but Temperature observations are not the the overall warming trend has continued over only evidence of recent climate change the past decade. The temperature trend in Many other changes have been observed any given 10-year interval (such as 1 January that are consistent with the recorded 1990 to 31 December 1999, or 1 January 1998 increase in global average temperature, and to 31 December 2007) can be determined indicate some of its consequences: by a standard statistical process called n There has been widespread melting linear regression. Since the 1970s, decadal of mountain glaciers and ice caps. While global temperature trends have consistently many of these have been shrinking since demonstrated warming in almost all such about 1850, there has been a significant 10-year intervals, although the magnitude increase in the rate of average glacier melt of the trend varies because of natural since the 1990s 63-66. climate variability (see Box 1) 62. The decadal n Satellite instruments show that the temperature trends over recent 10-year intervals remain positive. Greenland ice sheet is losing more ice than it gains by snowfall, due to increased surface melting and increased flow of ice Submarine observations suggest that the into the ocean. The rate of loss of ice from Arctic Ocean sea ice thickness has decreased Greenland has risen since the mid-1990s. since 1958, and satellite measurements There are strong indications that West indicate a thickness decrease of about Antarctica has also been recently losing 0.6 m between 2003 and 2008 71, 72. However, ice due to increased ice flow. Most recent in the Southern Ocean, total sea ice extent estimates show Antarctica as a whole is has increased slightly 70, 73. losing ice 67, 68. n The average water vapour content in the n Sea level rise is an inevitable consequence atmosphere, both at the Earth’s surface and of global warming because ocean water higher in the atmosphere, has been increasing expands as it warms, and because melted ice at a rate of 1–2% per decade since reliable from the land adds more water to the oceans. measurements began in the 1980s 9, 46. The The rate of rise increased from the 19th to greater intensity of heavy rains expected the 20th centuries, with the result that ocean from this increasing humidity has been levels are now more than 20 cm higher than in observed in some regions 74. Observed 1870 69, 70. Satellite and coastal measurements changes in ocean salinity are consistent show that the rate of sea level rise since the with intensification of the water cycle early 1990s has been substantially larger than over the oceans 75. the average rate for the 20th century, and n There is evidence of a shift in weather larger than for any similar length period in systems toward the Earth’s polar regions, the historical record 69 (see Figure 3.4). The and an apparent strengthening in the winds observed rise is consistent with increased rates over the Southern Ocean over the last 40Figure 3.3 Changes in the number of record hot day maxima and record cold day maxima at Australia’s climate of ice melt and ocean warming 55. years. This is believed to have contributedreference stations. The number of days with record hot temperatures has increased each decade over the past n Arctic sea ice extent has decreased to observed warming over the Antarctic50 years; there have been fewer record cold days each decade; and 2000 to 2009 was Australia’s warmest significantly in all seasons, but particularly in Peninsula and Patagonia, to cooling overdecade on record. Source: CSIRO, Bureau of Meteorology (2010) “State of the Climate”. summer, since satellite records began in 1979. eastern Antarctica and the Antarctic plateau 76, The Science of Climate Change
  9. 9. to associated changes in sea ice and the increases over north-western Australia, Sea level has risen around Australia BOx 5ocean 73, 77, and probably to the decreases in and decreases over south-western and at a rate of about 1.2 mm per year Is there a disagreement betweenrainfall over south-western Australia 78, 79. south-eastern Australia since 1960 since 1920, resulting in coastal satellite and surface temperature records?n There are indications of recent changes in (see Figure 3.6) 81. The warming and inundation events becoming morethe temperatures and salinities of deep ocean decreased rainfall over south-east Australia frequent 88. Since the establishment Not any more. While a disagreement did exist in thecurrents such as those which carry North have exacerbated the background conditions of the Australian Baseline Sea-level 1990s, it has largely been resolved by correction ofAtlantic water southward at depth conducive to fire 83. In southwest Western Monitoring Project in the early biases in the satellite data, for example to account forand Antarctic bottom water north 80. Australia and the southeast coast, there is 1990s, sea level measured relative to drift in satellite orbits over time 48, 49. Given the remaining evidence for a systematic decline in rainfall the land has risen at about 2 mm per uncertainties in satellite-derived trends, there isAustralia’s climate has changed along in recent decades 79, and for declining trends year in the south east, and over 8 mm now acceptable agreement between satellite andwith the global climate in storminess 84. It is likely that these trends per year in the north west 89. ground-based measurements of surface temperature.In Australia, the average surface temperature are related to shifts in pressure patternshas increased by about 0.7°C since 1960, with over southern Australia, particularly thesome areas having warmed faster and some intensification of the subtropical highshowing relatively little warming (see Figure pressure belt 85.3.5) 81. The warming has caused an Australia- Regional ocean currents have also changed.wide average increase in the frequency of For example there has been a southward shiftextremely hot days and a decrease in the of the Antarctic Circumpolar Current 86 andfrequency of cold days 81, 82 (see Figure 3.3). an increasing southward penetration of the While the longer term trends in rainfall East Australian Current, associated withare less marked, there have been significant wind changes in the South Pacific 87. Figure 3.4 Global average sea level since 1850. The solid blue line is estimated from coastal and island tide gauges and the red line is sea level measured by satellite altimeters. From 1900 to 2000 the average rate of rise was about 1.7 mm/yr, increasing through this period. Since 1993 the rate of rise measured by satellite Figure 3.5 Trends in Australian annual temperature (°C/decade) over the periods 1910–2009 (left) and 1960–2009 altimeters has been about 3.2 mm/yr and from tide gauges about 3.0 mm/yr 54, 69, 70. (right). Source: Australian Bureau of Meteorology ( Figure 3.6 Trends in Australian annual rainfall (in mm per decade) over the periods 1910–2009 (left) and 1960–2009 (right). Source as for Figure 3.5. The Science of Climate Change
  10. 10. 4 Are human activities causing climate change?Human activities are increasing human population began growing rapidly It is very likely that most of the recent additional greenhouse gas 111. Moreover,greenhouse gas levels in the atmosphere and farming also increased. The growth observed global warming is caused by trends over the last 40 years, superimposedThe concentrations of greenhouse gases in in greenhouse gases has accelerated through increasing greenhouse gas levels on natural year-to-year variations, havethe atmosphere are well known, both from the 20th century to the present 90-96 (see Figure It was predicted more than a century ago been observed which show that the uppermodern measurements 90-94 and by analysis 4.1). Studies of the stores and sources of that increases in CO2 would act like added atmosphere has cooled and the surface ofof the air from past eras, trapped as bubbles these gases, both natural and human- insulation in the Earth’s atmosphere, the Earth and the lower atmosphere havein ice from Antarctica and Greenland 95, 96 induced, show that the main causes of trapping more heat near the surface 4. This warmed significantly (see Question 3 and(see Figure 4.1). These observations tell us the increasing concentrations are emissions extra CO2 was also predicted to make the Boxes 1, 4 and 5). These are the predictedthat atmospheric concentrations of CO2, from human activities 97-101 (see Figure 4.2). stratosphere colder 110 (see Question 1). consequences of the additional levels ofmethane and nitrous oxide began to rise Human activities cause CO2 input to Satellite measurements over recent greenhouse gases 46, 48, 50. In contrast, both thetwo to three hundred years ago, after the atmosphere from fossil fuel burning, decades have confirmed the extra insulating lower and upper atmosphere might have beenchanging relatively little since the end of other industrial sources such as cement effect not only of CO2, but also of each expected to have warmed if the amount of thethe last Ice Age thousands of years earlier. production, and deforestation. This increase in greenhouse gas Measurements over the past 50 years Figure 4.1 Atmospheric CO2 over the last 2000 years, based on direct measurements in the atmosphere at Cape Grim,concentration happened around the same show that only about 45% of the Tasmania, older air extracted from Antarctic snow (firn) and from air bubbles trapped in various ice corestime as industrialisation, when the global combined CO2 emissions from these (various symbols). The inset shows the air bubbles in Antarctic ice. Image: Australian Antarctic Division. Data: CSIRO sources remain in the air to cause BOx 6 atmospheric CO2 to rise 102, 103. About 25% Why are CO2 emissions of the total CO2 input is being absorbed from human activities regarded by the oceans, making sea water more as so significant? acidic 104, 105, and the remaining 30% is being Large amounts of CO2 are continually taken up on land, with the largest probable transferred to and from the atmosphere, cause being increased growth of plants 99 which exchanges carbon with the oceans (see Box 6). This is shown by a wide range of and vegetation on land. Until around 200 measurements and models 99, 101-103, 106, 107. years ago, these natural exchanges were in There has been a recent acceleration in rough balance, shown by the nearly constant the growth rate of CO2 emissions from concentrations of atmospheric CO2 for most of fossil fuels and industrial sources. From the last two thousand years. The importance 2000 to 2007 these emissions grew by of human-caused CO2 emissions is that they 3.5% per year, exceeding almost all are disturbing this balance, adding carbon to assumed scenarios generated in the late the atmosphere faster than it can be removed 1990s 99, 108, 109. This pulse of CO2 emissions by uptake by vegetation, the slow mixing of growth coincided with a period of rapid CO2 into the deep oceans, or the even slower global economic growth. There will be weathering processes that control the carbon a small, temporary downturn in CO2 balance on geological timescales. growth, associated with the 2008–09 global financial crisis 99, 109.0 The Science of Climate Change
  11. 11. BOx 7 Could changes in the Sun be causing global warming? Not much of it. Most estimates show that over the past 20 years, all the trends in are also likely to have contributed significantly solar output has not significantly increased the Sun that could have had an influence to climate trends that have been observed over since 1979, when satellites began measuring on the Earth’s climate have been in the the Southern Ocean in the past few decades, it accurately 121, 122. Indeed, some estimates opposite direction to that required to including stronger westerly winds and the indicate that the Sun has grown slightly explain the observed rise in global average southward shift of weather systems 76, 129, 130. cooler since 1960, a period during which temperatures 123, 124. Indirect estimates for The human contribution to the recent global temperatures have risen. While there earlier times suggest that the Sun has observed rainfall increases in northwest Australia have been some suggestions of a significant contributed only about 10% of the global and decreases in southern Australia cannot as solar contribution to the observed warming warming since 1750 125, 126. yet be clearly separated from natural climate variations 79, 82. However, the decreases in rainfall in southern Australia have been linkedSun’s energy being received by the Earth had Some recent Australian climate to stronger high pressure weather systems 82.increased (see Question 1 and Box 7). changes have been linked to rising The overall pattern of increasing pressure in As well as emitting greenhouse gases, greenhouse gases mid-latitudes and decreasing pressure at highhuman activities affect climate through Modelling studies indicate that rising latitudes over time in the Southern Hemisphere PHOTOLIBRARYthe release of small particles in industrial greenhouse gases have made a clear is consistently seen in climate model projectionshaze, which reflect sunlight. The amount contribution to the recent observed warming and is therefore likely to be due to human-of cooling by this pollution is not known across Australia 117, 127, 128. induced climate change through a combinationprecisely, but is likely to be offsetting some Decreases in atmospheric ozone over of increases in greenhouse gases and decreases inof the warming from the increases in Antarctica and increases in greenhouse gases stratospheric ozone 130-133.greenhouse gases 112-116. Another way humans change local Figure 4.2: Observed past BOx 8climates is by changing land use, building emissions of CO2 from fossil fuelscities, or introducing irrigation. These and other industrial processes Do volcanoeschanges can affect the amount of sunlight (black points) with economics-reflected from the surface, local wind flow based projections to 2014 (open emit more CO2and evaporation. The impact of these circles). Coloured lines represent than humaneffects in recent decades has been small emissions futures from a range of activities?on a global scale 112, 117. IPCC scenario families representing No. The combined combinations of economic (A) Natural factors that have changed climate annual emissions versus environmental (B) andin the distant past, such as the brightness of globalised (1) versus localised (2) from volcanoesthe Sun or volcanic activity, have made only orientations for world development on land and undera small contribution to recent climate change in the 21st century. Solid and the sea 117, 127, 128,(see Boxes 7 and 8). dashed coloured lines respectively averaged over Putting all these factors together, the represent averages of scenarios several decades, areobserved global warming during the past within families and particular less than 1% of CO2 “marker” scenarios used in climatecentury has been consistent with that emissions in 2009 projections. Data: Carbon Dioxideexpected 26, 118, 119 from the combination Information and Analysis Center from fossil fuels,of increasing greenhouse gases and ( industrial processesincreasing particulates 112, 117, 120, with small emis/em_cont.htm). Figure adapted and deforestation 99.contributions from other factors. from published sources 99, 108, 109. The Science of Climate Change
  12. 12. 5 How do we expect climate to evolve in the future? Climate models and studies of past methodologies, they both yield broadly doubling of pre-industrial CO2 levels climates indicate that global warming similar indications of where global climate is by about 2050, and possibly a tripling by and associated changes will continue headed. For example, both methods project a about 2100 138. This emission pathway for if greenhouse gas levels keep rising as long-term warming of global air temperature CO2, coupled with rises in the other they are now of around 3°C (within an uncertainty range of greenhouse gases, would be expected to Basic physical principles tell us that rising 2°C to 4.5°C) in response to a doubling of the produce a warming of around 4.5°C by levels of greenhouse gases will warm the concentration of CO2 in the atmosphere 134. 2100, but possibly as low as 3°C or as Earth’s surface. To answer more complex Evidence from Earth’s past (see Question 2) high as 7°C 139, 140. questions, computer simulations, or models, indicates that changes of this magnitude can If society were to shift rapidly away of the Earth’s climate are used. These models have major long-term ramifications, such as sea from using fossil fuels, there would be incorporate the many factors that affect our level rise of many metres 135-137. little reduction in the rate of global climate, using mathematical equations based warming in the first couple of decades, on fundamental laws of nature, together with Continued increases in greenhouse but warming later this century and beyond approximations of some physical processes gas levels are expected to lead to would be significantly reduced (see Figure 5.1). that cannot be represented exactly (see Box 9). significant warming through the Climate models and basic physical Models simulate reasonably well the 21st century and beyond principles indicate that global warming will broad features of the present climate and Continued “business as usual” reliance generally be accompanied by increases in the 20th century warming. This, however, on fossil fuels is expected to lead to a global-average humidity; more extreme hot does not guarantee accurate predictions into the future; changes could be more rapid or BOx 9 more gradual than projected. Overall, there If we can’t forecast the weather 10 days in advance, is good agreement between models and why should we believe long-term climate forecasts? observations at global and continental scales, but simulations are less reliable at the local Weather and climate are not the same: weather path becomes impossible. This is analogous scale 133. Some properties of climate are better is chaotic and unpredictable over times longer to the predictive limit for individual weather captured by models than others; for example, than a week or two (see Box 1), whereas systems in the atmosphere, which is around temperature is generally more accurately climate is the average of weather over time. 10 days. On the other hand, predicting climate simulated than rainfall. Therefore, the challenges of predicting weather is akin to predicting the flow of the whole river, Independent of climate models, another and climate are very different. Predicting the which requires a consideration of the major important way to estimate the implications weather is akin to predicting how a particular forces controlling the river, such as valleys of greenhouse gas increases is to examine how eddy will move in a turbulent river: it is possible and dams. Projections of climate change over climate has responded to such increases in the over short timescales by extrapolating the decades to centuries are possible because of past, both over geological time (see Question 2) previous path of the eddy, but eventually the our progressively improving understanding of and in recent centuries (see Question 3) 14. eddy is influenced by neighbouring eddies and the forces affecting climate, including global While these two approaches – modelling and currents to the extent that predicting its exact warming caused by greenhouse gases.CORBIS studying the past – rely on markedly different The Science of Climate Change
  13. 13. events such as heat waves but fewer cold future, atmospheric temperatures would notextremes; further decreases in the extent and be expected to fall significantly for athickness of Arctic sea-ice; shifts in rainfall thousand years, as CO2 and heat(generally an increase in the tropics and are only gradually absorbed by the deephigh latitude regions and a decrease in the oceans 143. Sea level rise is also expected tosubtropics); further ocean warming; melting continue for many centuries due to theof mountain glaciers and polar ice sheets; ongoing melting of ice sheets and theand rising sea levels 118. Most of these impacts gradual thermal expansion of the oceans inhave already been observed (see Question 3). response to atmospheric warming 143. Warming rates and other climate changes Global warming above some threshold,are not expected to be the same everywhere, believed to lie between about 2°C and 4.5°C,due to changes in atmospheric circulation would lead to an ongoing melting of theor other regional influences. Projections Greenland ice sheet. If sustained forof future climate for individual regions thousands of years, this would virtuallyremain much less certain than global-scale eliminate the ice sheet, raising sea levelprojections. Different models often disagree, by about seven metres 144. Most of theso definitive localised projections are not yet Antarctic ice sheet, by contrast, is expectedpossible 118. This is particularly the case for to remain too cold for widespread melting.regional rainfall projections. It is possible that increased snowfall over Some models also project substantial Antarctica may partially offset otherchanges to phenomena such as El Niño or contributions to sea level rise 145.dramatic changes to vegetation 141. Many In addition, accelerated outflow of ice hasaspects of climate change will likely remain been observed from Greenland and Westdifficult to foresee despite continuing Antarctica. This is poorly understood, butmodelling advances, leaving open the could make these ice sheets more vulnerablepossibility of climate change “surprises” 142. to future warming 135.Some climate change will continue Reduction of greenhouse gasfor centuries, and some change will emissions could significantly reducebe essentially irreversible on a long-term warming1,000-year timescale To have a better than even chance ofStabilisation of climate requires stabilisation preventing the global average temperatureof greenhouse gas concentrations. from eventually rising more than 2°C aboveHowever, the inertia of the climate system, pre-industrial temperatures, the worldparticularly the oceans and the ice sheets, would need to be emitting less than half Figure 5.1 Top panel: Fossil-fuel CO2 emissions for two scenarios: one “business as usual” [red] andmeans that climate change will continue the amount of CO2 by 2050 than it did in the other with net emissions peaking before 2020 and then reducing rapidly to near zero emissions byfor centuries after greenhouse gas 2000 138, 146. To do this on a smooth pathway, 2100, with the cumulative emission between 2000 and 2050 capped at 1000 billion tonnes of CO2 [blue].concentrations have stabilised. global emissions (which are still rising) would Bottom panel: Median projections and uncertainties of global-mean surface air temperature based on these two emissions scenarios out to 2100. The darkest shaded range for each scenario indicates the Even if human societies completely ceased need to peak within the next 10 years and most likely temperature rise (50% of simulations fall within this range). Adapted from Meinshausen et al.greenhouse gas emissions at some time in the then decline rapidly 147. (2009) 138. The Science of Climate Change
  14. 14. 6 What are the consequences of climate change? iSTOCKPHOTO; NEWSPIX; OvE HOEgH-guLdBERg / THE uNIvERSITY Of QuEENSLANd Climate change will have significant By around 2030, Australian temperatures frosts and changed rainfall patterns may means that biodiversity is likely to decline impacts on our society and environment are likely to be a half degree or more higher be beneficial to agriculture in some parts overall 167, in line with observed global Historically, the Australian climate has than 1990 and the frequency of hot days of Australia, but decreases in rainfall in trends 168. Higher temperatures on the been highly variable. This variability makes and nights will have increased 148, 149. Sea other Australian regions are likely to have a forested mountaintops of north-east it challenging to predict the future level is expected to be about 15 cm higher detrimental effect on agriculture. Queensland, for example, may exceed the heat consequences of human-induced climate and there is some evidence to suggest that Warmer ocean temperatures will lead to tolerance of some endemic species in the wet change. However, climate models and past tropical cyclones will become more severe, further changes in the distribution of marine tropics, resulting in their extinction 169.CSIRO experience provide some guidance. but less frequent 150. animals and plants, with some tropical fish It is likely that future rainfall patterns across moving progressively southward 158. As a result Climate change will exacerbate the Australia will be different from today. Changes of increased CO2 in the atmosphere, oceans impacts of other stresses in rainfall patterns are hard to predict: regional will become more acidic and, in combination The world’s population is approaching seven rainfall projections from different climate with the higher temperatures, coral bleaching billion people, and is expected to increase to models (or between different runs of the same events are likely to become more frequent and around nine billion by mid-century, with two model with different starting conditions) are severe around northern Australia 159, 160. thirds of the world’s population living in the frequently quite different from one another Sea level will increase, inundating parts of Asia-Pacific region 170. This population growth (see Question 7). Nevertheless, some future the Kakadu freshwater wetlands 160, 161 and will place additional stress on the planet and trends are projected fairly consistently, causing increased coastal flooding 88, 162, with its people. For example, half of all readily including increases in rainfall in northern consequent change to sandy coastlines. As available fresh water is already appropriated Australia and decreases in Victorian and sea levels rise, coastal infrastructure around for human use 171, 172. southwest WA coastal regions 148, 151, 152. The Australia will become more susceptible to Without major changes to population projections for rainfall trends across the entire damage 163-165. Tourism may be adversely growth policies, land use, city development, Murray-Darling basin remain uncertain 153. affected, in part due to the sector’s and economic and social systems, the It is likely that higher temperatures and dependence on natural assets and the built additional potential burdens of climate change changing patterns of wind and rainfall will environment, both of which are vulnerable to impacts could lead to social unrest across large change the patterns and frequency of extreme the physical impacts of climate change 166. parts of the world. Further pressures arise fire weather 149, 154, and also lead to more heat- The impact of climate change on plants because there is now little room for many related deaths and fewer cold-related deaths 155, 156. and animals will be variable. Habitat will populations to relocate in response to climate Farming in Australia is vulnerable to expand for some species, while for others it change 173, 174. These factors are likely to affect climate change but skilful management is will contract 160. However, the inability of developed as well as developing nations 173, 175. expected to be able to alleviate some of this many species to migrate as a result of both The recent global financial crisis has vulnerability 157. Higher CO2 levels, fewer land use change and habitat fragmentation demonstrated how interconnected the world The Science of Climate Change
  15. 15. Figure 6.1 Climate change may have severehas become. It is also dependent on a the world would be hotter than at any impacts across Australia. Days of extreme firefinite resource base. All of these factors time in the last few million years. Sea danger are likely to increase (top right).demonstrate the need for an integrated level would continue to rise for manyapproach to understanding how a centuries. The impacts of such changes Rainfall patterns are likely to change, leading to changes in MIKE TRENERRY river environments: the image on the far left shows strandedsustainable planet can be attained in the are difficult to predict, but are likely to reeds and saline mud flats in September 2007, caused by thepresence of population pressures, risks be severe for human populations and rapidly retreating waters of Lake Bywater (near Walkers Flat,from climate change, and other stresses 176. for the natural world. The further climate SA). This lake is fed by the River Murray, which has seen major is pushed beyond the envelope of relative falls in level since 2000, particularly below Lock 1. The imageFuture impacts are expected to be stability that has characterised the last on the top left shows heavy rain in the Northern Territory.more severe several millennia, the greater becomes The centre left image shows healthy coral and the centreIf emissions continue unabated, current the risk of passing tipping points that right image shows bleached coral near Keppel Island.mid-range estimates are for 4.5°C higher will result in profound changes inglobal average temperatures by 2100 climate, vegetation, ocean circulation Biodiversity is likely to decrease: the image to the right shows(see Question 5), which would mean that or ice sheet stability. the endangered lemuroid possum from North Queensland. The Science of Climate Change
  16. 16. 7 How do we deal with the uncertainty in the science?No scientific conclusion can ever be Some aspects of climate science are Uncertainty about future climate changeabsolutely certain still quite uncertain works in both directions: there is a chanceHowever, a balanced assessment of the The exact amount of warming that will that climate change will be less severe thanavailable evidence and prior knowledge result from any particular trajectory for current best estimates, but there is also aallows us to attach levels of confidence to future greenhouse gas emissions cannot roughly equal chance that it will be worse.the findings of climate science. be projected precisely, because it depends on details of processes that reinforce or Despite the uncertainties, climateThere is a high degree of confidence in dampen disturbances to the climate system. science has an important role tothe broad conclusions of climate science Important processes involve clouds, water play in informing public policy onWe are very confident of several fundamental vapour, ocean circulations and natural climate changeconclusions about climate change: that influences on greenhouse gas levels in the Decisions on when and how to respond tohuman activities since the industrial atmosphere. However, future warming climate change involve many factors thatrevolution have sharply increased greenhouse can be specified within plausible bounds, lie outside the realm of science, includinggas concentrations; that these added gases not only from climate models but also ethical and economic considerations. Anhave a warming effect; and that the Earth’s from interpretations of climate changes appropriate response will depend on valuesurface has indeed warmed since the in the past. judgements and an assessment of the risksIndustrial Revolution. Therefore, we are How climate change will affect individual of various courses of action. Just as in anyvery confident that human-induced global regions is very hard to project in detail, other sphere of human activity, decisionswarming is a real phenomenon. particularly future changes in rainfall will need to be made before we have absolute Another important conclusion is patterns, and such projections are highly certainty about the future. The role ofsupported unambiguously by all the evidence uncertain. Neither can “tipping points” or climate science is to inform these decisionsso far: “business as usual” emissions, with rapid climate transitions be projected with by providing the best possible knowledge ofcontinuing high reliance on fossil fuels, will any confidence, although they involve high climate outcomes and the consequences oflead to a significantly warmer world. risks should they occur. alternative courses of action. The Science of Climate Change