The Science of Climate Change Questions and Answers

1. The Science of
Climate Change
Questions and
Answers
August 2010
The Science of Climate Change

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-related
Foreword 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 asked
T
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. The
chemistry, 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 temperature
their 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 century
etc – and each of which can be considered important scientific issues. concentration of the principal anthropogenic and many other associated changes have been
as mature within the framework required to This includes climate science. The Academy greenhouse gas, CO2, leads to higher mean observed. The available evidence implies
discuss 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 human
disciplines 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 expected
both 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 at
feedbacks 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 temperatures
of 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 the
difficulty of bringing these components together science and must include, amongst others, beyond serious question that some CO2 from
into 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 is
consequences 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 affect
atmospheric 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 a
cover, 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, such
can be described and quantified in a coherent to improve the public understanding of the and they will affect the global distribution of its
and 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 keep
interactions that make it difficult to realistically be constructed. research and rigorous scientific debate continue the planet warmer than it would otherwise
quantify 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 the
models 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 surface.
is usually accustomed to. In a process as of the major questions that are frequently broader community be effective. The Academy
intrinsically 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 (millions
surprising 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 stable
and 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 of
the 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, for
complex. What makes climate change different contributed extensively to the underpinning members of the Working Group and Oversight example, gone through 10 major ice age cycles
is 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. The
global 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 unusually
over 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 of
so that actions need to be contemplated before questions have also been addressed. Second, Department of Climate Change and Energy
full understanding is achieved. These actions an Oversight Committee comprehensively Efficiency for providing financial support to
themselves 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 Academy
of Science, Canberra.
The Science of Climate Change

3. T 4
physical principles, evidence from the past spells, changes to rainfall patterns and a his document aims to summarise Are human activities causing
shows 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 very
Measurements 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 increase
surface is warming along with rising regions, rising sea levels, and decreases in in greenhouse gases.
levels of greenhouse gases from human
activities, and that this warming is leading
to 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 climates
to 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 associated
average 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 levels
century 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 the
Climate models, together with physical rapid change in climate would likely be climate change. 21st century and beyond. Reduction of
principles and knowledge of past beyond the adaptive capacity of many greenhouse gas emissions could significantly
variations, tell us that, unless greenhouse
gas 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 are
stabilised, 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 impacts
Climate models estimate that, by 2100, to small disturbances, although climate to small influences. Past records also on our society and environment, both directly
the 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. Climate
industrial temperatures, depending on future
greenhouse gas emissions and on the ways
that 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 and
to 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 climate
that 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 on
after 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 of
a 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 many
one 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, decisions
would 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. 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 atmospheric
Climate change is a change in the changes in weather from day to day, between Sustained and truly global changes in temperature and winds, with warmer
average pattern of weather over a seasons, and from one year to the next, do average temperature require some global temperatures causing higher water vapour
long period of time not represent climate changes. The period for heating or cooling influence such as variations concentrations. This is in contrast with other
Climate 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 concentrations
conditions 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-induced
both 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 are
decades 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 increased
rainfall 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. 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 over
NASA
the past million years, the
disturbance came from
fluctuations in the Earth’s
orbit around the Sun (grey
box in upper diagram). This
and amplifies, climate change caused by caused temperatures to
other factors (see Box 2 and Figure 1.1). change (green box), in turn
The effects of changing greenhouse gas inducing rapid changes
levels on climate can be distinguished in water vapour (left blue
box), and much slower
from the effects of other factors such as
changes in ice caps (right
changes to the Sun’s radiation. These blue box) and greenhouse
different causes lead to different patterns gas levels (orange box),
or “fingerprints” in the resulting climate which together amplified
changes, 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 from
expected to warm both the upper and human-induced changes in
atmospheric CO2
lower parts of the atmosphere and result in and other greenhouse gas
days warming more than nights. On the levels (grey box in lower
other hand, increases in greenhouse gases diagram). In both cases,
would be expected to result in a cooling, the disturbance is amplified
not a warming, in the stratosphere (the by similar reinforcing
layer of the atmosphere above 15 km processes.
elevation), and cause nights to warm more
than days. The observed patterns of change
more nearly match those expected from
increasing greenhouse gases.
The Science of Climate Change

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. 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.
ac.uk/cru/data/temperature/); Goddard Institute for Space Studies
(http://data.giss.nasa.gov/gistemp/); National Climate Data Center, NOAA
(www.ncdc.noaa.gov/cmb-faq/anomalies.html)
Global 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 yet
Measurements from many hundreds of recorded 43-46 (see Box 4).
thermometers around the globe, on land The overall warming has led to an increase
and 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 the
the 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, satellite
century, were not initially designed to be used observations of temperature at the Earth’s
for climate monitoring. This means they have surface and in the lower atmosphere have also
to be carefully analysed to deal with changes shown warming 46, 48, 49 (see Box 5). In contrast
in instruments, observational practice, to the surface warming, the atmosphere above
location, and the growth of cities (see Box 3). about 15 km elevation (the stratosphere) has
After accounting for these issues, temperature cooled over the past four decades 46, 50-52. This
increases are largest in the continental provides one clue that the observed warming
interiors 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 storing
growth 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 since
increased in the mid-1970s. Since then, 1961 53. The surface ocean has warmed
the global land surface has warmed at about by 0.5°C from the 1970s to the early
double the rate of the ocean surface. Measured 2000s. Averaged over the upper 700 m of
warming 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: http://data.giss.nasa.gov/gistemp/maps/
The Science of Climate Change

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 40
Figure 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 contributed
reference 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 Antarctic
50 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 over
decade 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. to associated changes in sea ice and the increases over north-western Australia, Sea level has risen around Australia BOx 5
ocean 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 between
rainfall 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 more
the 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 the
currents 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 of
Atlantic 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 for
and 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 is
Australia’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 and
with 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 patterns
has increased by about 0.7°C since 1960, with over southern Australia, particularly the
some areas having warmed faster and some intensification of the subtropical high
showing 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 shift
extremely hot days and a decrease in the of the Antarctic Circumpolar Current 86 and
frequency 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 with
are 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 (http://www.bom.gov.au/cgi-bin/climate/change/trendmaps.cgi)
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. 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, superimposed
The concentrations of greenhouse gases in in greenhouse gases has accelerated through increasing greenhouse gas levels on natural year-to-year variations, have
the 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 upper
modern 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 of
of 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 have
in 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 predicted
that atmospheric concentrations of CO2, from human activities 97-101 (see Figure 4.2). stratosphere colder 110 (see Question 1). consequences of the additional levels of
methane and nitrous oxide began to rise Human activities cause CO2 input to Satellite measurements over recent greenhouse gases 46, 48, 50. In contrast, both the
two to three hundred years ago, after the atmosphere from fossil fuel burning, decades have confirmed the extra insulating lower and upper atmosphere might have been
changing 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 the
the 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 cores
time 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. 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 linked
Sun’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 high
human activities affect climate through Modelling studies indicate that rising latitudes over time in the Southern Hemisphere
PHOTOLIBRARY
the release of small particles in industrial greenhouse gases have made a clear is consistently seen in climate model projections
haze, 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 combination
precisely, but is likely to be offsetting some Decreases in atmospheric ozone over of increases in greenhouse gases and decreases in
of 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 8
climates is by changing land use, building emissions of CO2 from fossil fuels
cities, or introducing irrigation. These and other industrial processes Do volcanoes
changes can affect the amount of sunlight (black points) with economics-
reflected from the surface, local wind flow based projections to 2014 (open
emit more CO2
and evaporation. The impact of these circles). Coloured lines represent than human
effects 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) and
in the distant past, such as the brightness of globalised (1) versus localised (2)
from volcanoes
the Sun or volcanic activity, have made only orientations for world development on land and under
a 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, are
observed global warming during the past within families and particular less than 1% of CO2
“marker” scenarios used in climate
century has been consistent with that emissions in 2009
projections. Data: Carbon Dioxide
expected 26, 118, 119 from the combination Information and Analysis Center
from fossil fuels,
of increasing greenhouse gases and (http://cdiac.esd.ornl.gov/trends/ industrial processes
increasing 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. 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. events such as heat waves but fewer cold future, atmospheric temperatures would not
extremes; further decreases in the extent and be expected to fall significantly for a
thickness 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 deep
high latitude regions and a decrease in the oceans 143. Sea level rise is also expected to
subtropics); further ocean warming; melting continue for many centuries due to the
of mountain glaciers and polar ice sheets; ongoing melting of ice sheets and the
and rising sea levels 118. Most of these impacts gradual thermal expansion of the oceans in
have 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 the
or other regional influences. Projections Greenland ice sheet. If sustained for
of future climate for individual regions thousands of years, this would virtually
remain much less certain than global-scale eliminate the ice sheet, raising sea level
projections. Different models often disagree, by about seven metres 144. Most of the
so definitive localised projections are not yet Antarctic ice sheet, by contrast, is expected
possible 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 other
changes 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 has
aspects of climate change will likely remain been observed from Greenland and West
difficult to foresee despite continuing Antarctica. This is poorly understood, but
modelling advances, leaving open the could make these ice sheets more vulnerable
possibility of climate change “surprises” 142. to future warming 135.
Some climate change will continue Reduction of greenhouse gas
for centuries, and some change will emissions could significantly reduce
be essentially irreversible on a long-term warming
1,000-year timescale To have a better than even chance of
Stabilisation of climate requires stabilisation preventing the global average temperature
of greenhouse gas concentrations. from eventually rising more than 2°C above
However, the inertia of the climate system, pre-industrial temperatures, the world
particularly 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] and
means 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 by
for 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. 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. Figure 6.1 Climate change may have severe
has become. It is also dependent on a the world would be hotter than at any impacts across Australia. Days of extreme fire
finite 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 many
approach to understanding how a centuries. The impacts of such changes Rainfall patterns are likely to change, leading to changes in
MIKE TRENERRY www.wettropics.gov.au
river environments: the image on the far left shows stranded
sustainable planet can be attained in the are difficult to predict, but are likely to
reeds and saline mud flats in September 2007, caused by the
presence 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 image
Future 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 centre
If 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 in
global 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. 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 change
absolutely certain still quite uncertain works in both directions: there is a chance
However, a balanced assessment of the The exact amount of warming that will that climate change will be less severe than
available evidence and prior knowledge result from any particular trajectory for current best estimates, but there is also a
allows 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, climate
There is a high degree of confidence in dampen disturbances to the climate system. science has an important role to
the broad conclusions of climate science Important processes involve clouds, water play in informing public policy on
We are very confident of several fundamental vapour, ocean circulations and natural climate change
conclusions about climate change: that influences on greenhouse gas levels in the Decisions on when and how to respond to
human activities since the industrial atmosphere. However, future warming climate change involve many factors that
revolution have sharply increased greenhouse can be specified within plausible bounds, lie outside the realm of science, including
gas concentrations; that these added gases not only from climate models but also ethical and economic considerations. An
have a warming effect; and that the Earth’s from interpretations of climate changes appropriate response will depend on value
surface has indeed warmed since the in the past. judgements and an assessment of the risks
Industrial Revolution. Therefore, we are How climate change will affect individual of various courses of action. Just as in any
very confident that human-induced global regions is very hard to project in detail, other sphere of human activity, decisions
warming 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 of
supported unambiguously by all the evidence uncertain. Neither can “tipping points” or climate science is to inform these decisions
so far: “business as usual” emissions, with rapid climate transitions be projected with by providing the best possible knowledge of
continuing high reliance on fossil fuels, will any confidence, although they involve high climate outcomes and the consequences of
lead to a significantly warmer world. risks should they occur. alternative courses of action.
The Science of Climate Change