The science of climate change - questions and answers - Australian Academy of Science
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The science of climate change questions and answers february 2015
page contents 3 Foreword 4 Summary 6 Q1: What is climate change? 8 Q2: How has climate changed? 12 Q3: Are human activities causing climate change? 16 Q4: How do we expect climate to evolve in the future? 20 Q5: How are extreme events changing? 22 Q6: How are sea levels changing? 24 Q7: What are the impacts of climate change? 28 Q8: What are the uncertainties and their implications? 30 Q9: What does science say about options to address climate change? For a version with full references visit www.science.org.au/climatechange Published by the Australian Academy of Science ISBN 978 0 85847 413 0 Please cite “The science of climate change: Questions and answers”, Australian Academy of Science, Canberra, 2015 www.science.org.au/climatechange Cover: Wollongong Harbour, NSW. Photo: Robert Montgomery 2 | The science of climate change
Foreword
The purpose of this booklet is to provide an understanding, based on our As in all areas of active science, uncertainties remain. However, enormous
present scientific knowledge, of some key questions about climate change. scientific progress has been made in our understanding of climate change
It is an extensively revised update of a similarly titled Academy publication and its causes and implications. Since 2010, the IPCC has prepared a new
in 2010 that summarised the state of knowledge at that time. It has been international assessment with the active involvement of many Australian
prepared by a broadly-based Working Group of Australian climate scientists researchers, including several members of the Academy Working Group. This
with review and guidance provided by an Oversight Committee composed of Q&A update is thus well informed by recent international developments in the
Academy Fellows and the former Chair of the Academy’s National Committee science as well as the most recent work by our own scientists on peculiarly
for Earth System Science. Australian aspects of the climate change problem.
Along with its sister Academies, the Australian Academy of Science has played As the summary states, ‘Societies, including Australia, face choices about how
an active role in assessing the science of climate change since the 1970s. to respond to the consequences of future climate change.’ It is incumbent on
The Academy recognises the role of the Intergovernmental Panel on Climate society to consider these choices.
Change (IPCC) as the mechanism for the international scientific assessment of I wish to thank all the members of the Working Group and Oversight
climate change science, impacts and response strategies. However, it believes Committee (whose names are listed on the back cover) for their painstaking
that it is important that Australian climate scientists explain the science, work in the preparation of this update. I also acknowledge the assistance
including its uncertainties and implications, to the Australian community in of the reviewers and others who helped with this update. The Academy is
simpler terms than can be found in most of the IPCC reports. especially grateful to the Department of the Environment, which provided the
The Working Group who prepared this update was led by Professor Michael financial support for the preparation and publication of this document.
Raupach FAA FTSE and Dr Ian Allison AO with special support, in the later On behalf of the Academy, I am pleased to commend the information in the
stages, from Professor Steven Sherwood. The views presented in the answers following pages to all those who are looking for authoritative answers to the
to the nine key questions were carefully reviewed by an Oversight Committee key questions we are all asking about the science of climate change.
and 12 independent climate scientists* who agreed to help with the
preparation of this document. The role of the Oversight Committee was to Andrew Holmes AM PresAA FRS FTSE
make sure that all reasonable review comments were properly considered by President
the Working Group in preparing their final text. While the reviewers provided Australian Academy of Science
more than 600 individual comments on the penultimate draft, neither they
nor the Oversight Committee are responsible for the final wording of the
detailed answers that represent the views of the expert members of the
Working Group.
Nevertheless the summary on pages 4 and 5 represents the fully agreed *In addition to multi-stage review carried out by the Oversight Committee, the penultimate
left: An image from space of the cloud
patterns associated with a mid-latitude
views of both the Oversight Committee and the Working Group. It has been draft of this document was reviewed by Dr G Ayers FTSE, Dr I G Enting, Professor D Griggs
cyclone off southwest Australia. endorsed by the Academy as a balanced, objective and authoritative summary FTSE, Professor D Karoly, Mr WR Kininmonth, Professor M J Manton FTSE, Dr K G McCracken
Photo: NASA of the current state of knowledge of the science of climate change. AO FAA FTSE, Professor N Nicholls, Dr N Smith FTSE and three anonymous reviewers.
The science of climate change | 3
Earth’s climate has changed over the past century. The
atmosphere and oceans have warmed, sea levels have risen,
and glaciers and ice sheets have decreased in size. The best
available evidence indicates that greenhouse gas emissions
from human activities are the main cause. Continuing increases
Summary
in greenhouse gases will produce further warming and other
changes in Earth’s physical environment and ecosystems.
The science behind these statements >> Measurements from the recent
is supported by extensive studies past (the last 150 years) tell us
based on four main lines of evidence: that Earth’s surface has warmed
>> Physical principles established as atmospheric concentrations
more than a century ago tell of greenhouse gases increased
us that certain trace gases through human activities, and
in the atmosphere, such as that this warming has led to
carbon dioxide (CO2) and water other environmental changes.
vapour, restrict the radiant flow Although climate varies from
of heat from Earth to space. decade to decade, the overall
This mechanism, known as the upward trend of average global
‘greenhouse effect’, keeps Earth’s surface temperature over the last
surface and lower atmosphere century is clear.
considerably warmer than they >> Climate models allow us
would otherwise be. The gases to understand the causes of
involved are called ‘greenhouse past climate changes, and to
gases’. An increase in greenhouse project climate change into the
gas concentrations raises the future. Together with physical
temperature of the surface. principles and knowledge of
>> The record of the distant past variations, models provide
past (millions of years) tells us compelling evidence that recent
that climate has varied greatly changes are due to increased
through Earth’s history. It has, for greenhouse gas concentrations
example, gone through ten major in the atmosphere. They tell
ice age cycles over approximately us that, unless greenhouse
the past million years. Over the gas emissions are reduced
last few thousand years of this greatly and greenhouse gas
period, during which civilisations concentrations are stabilised,
developed, climate was unusually greenhouse warming will
stable. Evidence from the past continue to increase.
confirms that climate can be This document aims to summarise
sensitive to small persistent and clarify the current scientific
changes, such as variations in understanding of climate change
Earth’s orbit. by answering nine key questions.
4 | The science of climate change
1 What is climate change? 3 Are human activities causing 5 How are extreme events 8 What are the uncertainties
The term ‘climate’, in its broadest climate change? changing? and their implications?
sense, refers to a statistical Human activities are increasing Since the mid-20th century, climate There is near-unanimous agreement
description of weather and of greenhouse gas concentrations change has resulted in increases in among climate scientists that
the related conditions of oceans, in the atmosphere. This increase the frequency and intensity of very human-caused global warming is
land surfaces and ice sheets. This is extremely likely to have caused hot days and decreases in very cold real. However, future climate change
includes consideration of averages, most of the recent observed global days. These trends will continue with and its effects are hard to predict
variability and extremes. Climate warming, with CO2 being the largest further global warming. Heavy rainfall accurately or in detail, especially
change is an alteration in the contributor. Some observed changes events have intensified over most at regional and local levels. Many
pattern of climate over a long in Australia’s climate, including land areas and will likely continue to factors prevent more accurate
period of time, and may be due to a warming throughout the continent do so, but changes are expected to predictions, and some uncertainty is
combination of natural and human- and drying trends in the southwest, vary by region. likely to remain for considerable time.
induced causes. have been linked to rising Uncertainty in climate science is no
greenhouse gas concentrations. 6 How are sea levels changing? greater than in other areas where
2 How has climate changed? Sea levels have risen during the policy decisions are routinely taken to
Global climate has varied greatly 4 How do we expect climate 20th century. The two major minimise risk. Also, the uncertainty
throughout Earth’s history. In the to evolve in the future? contributing factors are the means that the magnitude of future
final decades of the 20th century, If greenhouse gas emissions expansion of sea water as it warms, climate change could be either
the world experienced a rate of continue to grow rapidly, it is and the loss of ice from glaciers. Sea greater or less than present-day
warming that is unprecedented for expected that, by 2100, the global levels are very likely to rise more best estimates.
thousands of years, as far as we average air temperature over quickly during the 21st century than
can tell from the available evidence. the Earth’s surface will warm by the 20th century, and will continue to 9 What does science say
around 4°C above mid-19th century rise for many centuries.
about options to address
Global average temperature rise has
temperatures. There are many
climate change?
been accompanied by ongoing rises
in ocean temperatures, ocean heat likely ramifications of this warming. 7 What are the impacts Societies, including Australia, face
storage, sea levels and atmospheric However, if emissions are reduced of climate change? choices about how to respond to
water vapour. There has also been sufficiently rapidly, there is a chance Climate change has impacts on the consequences of future climate
shrinkage in the size of ice sheets that global average warming will not ecosystems, coastal systems, fire change. Available strategies include
and most glaciers. The recent exceed 2°C and other impacts will regimes, food and water security, reducing emissions, capturing CO2,
slowdown in the rate of surface be limited. health, infrastructure and human adaptation and ‘geoengineering’.
warming is mainly due to climate security. Impacts on ecosystems These strategies, which can be
variability that has redistributed and societies are already occurring combined to some extent, carry
heat in the ocean, causing warming around the world, including in different levels of environmental risk
at depth and cooling of surface Australia. The impacts will vary from and different societal consequences.
waters. Australia’s climate has one region to another and, in the The role of climate science is to
warmed along with the global short term, can be both positive and inform decisions by providing the
average warming. negative. In the future, the impacts best possible knowledge of climate
of climate change will intensify outcomes and the consequences of
and interact with other stresses. alternative courses of action.
If greenhouse gas emissions
facing page: People flocked to the
continue to be high, it is likely that
beach for respite one evening during
Melbourne’s record breaking four-day the human-induced component
heatwave in January 2014, under a sky of climate change will exceed the
made hazy by smoke from a scrub fire. capacity of some countries to adapt.
Photo: Neil O’Connor
The science of climate change | 5
Q1 left: Meteorological variables such as
wind, temperature and humidity are
space. This is called the ‘greenhouse
effect’, and the gases that cause
measured by instruments attached to it by interacting with infrared
balloons and relayed by radio to ground radiation are called greenhouse
stations on land or on ships. gases. The most important are water
Photo: Kyle D. Gahlau
vapour, carbon dioxide (CO2) and
What is climate change? by long-term influences that are
known or predictable (Box 1.1).
methane. The greenhouse effect was
identified more than a century ago;
Earth’s surface would be about
33oC cooler without it, so it keeps
Climate is determined by
many factors that influence Earth habitable.
flows of energy through the
climate system, including Changes in climate can occur
greenhouse gases through both natural and
human-induced causes
Energy from the Sun is the ultimate
driver of climate on Earth. The solar Global climate varies naturally
energy received by Earth depends over time scales from decades to
on how much the Sun emits and thousands of years and longer. These
the distance between Earth and natural variations can originate in
the Sun. Part of this sunlight is two ways: from internal fluctuations
reflected directly back to space by that exchange energy, water and
the atmosphere, clouds, and land, ice carbon between the atmosphere,
and water surfaces. Aerosols (tiny oceans, land and ice, and from
particles in the atmosphere, some external influences on the climate
coming from human activities) can system, including variations in the
increase the reflection of sunlight. energy received from the sun and
the effects of volcanic eruptions.
Eventually the solar energy absorbed
by Earth is returned to space as Human activities can also influence
infrared (heat) radiation. In the climate by changing concentrations
process it interacts with the whole of CO2 and other greenhouse gases
climate system—atmosphere, oceans, in the atmosphere (Box 1.2), altering
Climate change is a change in system’. Climate, in its broadest in the climate system, or due to land surfaces and ice sheets. The the concentrations of aerosols and
the pattern of weather, and sense, is the statistical description of human influences such as changes in flows of radiation in the atmosphere altering the reflectivity of Earth’s
related changes in oceans, the state of the climate system. the composition of the atmosphere (Figure 1.1) are very important in surface by changing land cover.
land surfaces and ice sheets, Climate change is a change in the or land use. determining climate. The main
occurring over time scales of statistical properties of the climate Weather can be forecast with gases that make up the atmosphere, A disturbance to the climate
decades or longer nitrogen and oxygen, do not interact system can trigger further
system that persists for several considerable skill up to about a week
with infrared radiation. However, changes that amplify or damp
Weather is the state of the decades or longer—usually at least in advance. Short term fluctuations
certain gases present in smaller the initial disturbance
atmosphere—its temperature, 30 years. These statistical properties in climate, such as droughts, can
humidity, wind, rainfall and so on—over quantities absorb infrared radiation There are close connections
include averages, variability and be predicted with limited skill
flowing upwards from Earth’s surface between temperature, atmospheric
hours to weeks. It is influenced by the extremes. Climate change may from season to season. In contrast,
and re-radiate it in all directions, water vapour, the extent of polar
oceans, land surfaces and ice sheets, be due to natural processes, such changes in the long-term statistics
including back downwards. By doing ice sheets and the concentrations
which together with the atmosphere as changes in the Sun’s radiation, of the climate system (climate
this they impede the outward flow of long-lived greenhouse gases
form what is called the ‘climate volcanoes or internal variability change) can be predicted if caused
of infrared energy from Earth to (especially CO2) in the atmosphere.
6 | The science of climate change
solar incoming solar reflected infrared outgoing Box 1.1: If weather can only be forecast about a week in advance, how can
we determine future climate?
The challenges of predicting weather and climate are very different.
340 100 239 Predicting the weather is like predicting how a particular eddy will move
(Units: W m-2) and evolve in a turbulent river: it is possible over short time scales by
extrapolating the previous path of the eddy, but eventually the eddy is
influenced by neighbouring eddies and currents to the extent that predicting
its exact path and behaviour becomes impossible. Similarly, the limit for
predicting individual weather systems in the atmosphere is around 10 days.
On the other hand, predicting climate is like predicting the flow of the whole
atmospheric
window river. It requires a consideration of the major forces controlling the river
such as changes in rainfall, the operation of dams, and extraction of water.
79 Projections of human-induced climate change over decades to centuries are
latent heat greenhouse possible because human activities have predictable effects on the future
solar absorbed gases atmospheric composition, and in turn a predictable effect on climate.
(atmosphere)
185
solar down
(surface) 24 Box 1.2: How do human activities enhance the ‘greenhouse effect’?
solar Today, human activities are directly increasing atmospheric concentrations
reflected
(surface)
84 20 398 342 of CO2, methane and nitrous oxide, plus some chemically manufactured
greenhouse gases such as halocarbons (Question 3). These human-
161 generated gases enhance the natural greenhouse effect and further warm
solar absorbed evaporation sensible infrared up infrared down the surface. In addition to the direct effect, the warming that results from
(surface) heat (surface) (surface) increased concentrations of long-lived greenhouse gases can be amplified
by other processes. A key example is water vapour amplification (Box 1.3).
Figure 1.1: The rates at which energy enters the Earth system from the Sun, and leaves the system, approximately balance on Human activities are also increasing aerosols in the atmosphere, which
average globally. Energy absorbed at the surface is transferred to the atmosphere via infrared radiation, conduction of sensible heat, reflect some incoming sunlight. This human-induced change offsets some
and evaporation of water whose latent heat is released later when the water condenses again. Energy leaves the system mostly via of the warming from greenhouse gases.
infrared radiation from the atmosphere. The arrows show global average energy transfer rates in units of Watts per square metre.
With more greenhouse gases in the atmosphere, but no other changes, the system must reach a higher temperature to maintain balance.
Adapted from IPCC (2013), Fifth Assessment Report, Working Group 1, Figure 2.11.
Box 1.3: If water vapour is the most important greenhouse gas, why all
the fuss about CO2?
When one of these is disturbed, An example of a slow feedback is
the others react through ‘feedback’ the ice age cycles that have taken Water vapour accounts for about half the natural greenhouse effect. Its
processes that may amplify or concentrations in the atmosphere are controlled mainly by atmospheric
place over the past million years,
temperatures and winds, in contrast with the concentrations of other
dampen the original disturbance. triggered by fluctuations in Earth’s
greenhouse gases which are directly influenced by human-induced inputs
These feedbacks occur on a wide rotation and orbit around the
of these gases to the atmosphere. When global average atmospheric
range of time scales: those involving sun. These fluctuations changed
temperatures rise, global water vapour concentrations increase, amplifying
the atmosphere are typically rapid, the distribution of solar radiation
the initial warming through an enhanced greenhouse effect. In this way,
while those involving deep oceans received by Earth, which caused human activity leads indirectly to increases in water vapour concentrations.
and ice sheets are slow and can temperatures to change, in turn
cause delayed responses. above: Surface meteorological observing The reality of the water vapour feedback is supported by recent
inducing changes in ice sheets
station at Cranbourne, Victoria, typical observations and analyses. Increased water vapour concentrations have
An example of a rapid feedback and carbon cycling that together
of stations used for observing climate been observed and attributed to warming, and this feedback approximately
is the role of water vapour as amplified the temperature response. around the world. Photo: Bureau of doubles the sensitivity of climate to human activities.
explained in Box 1.3. (Question 2). Meteorology
The science of climate change | 7
Q2
How has climate changed?
Past climate has varied in the atmosphere, the evolution about 5˚C in ice-age cycles, roughly Most past changes in global
above: Aerial view of the Norman River
enormously on a variety of of life and meteorite impacts have every 100,000 years or so (Figure temperature occurred slowly, flowing towards the Gulf of Carpentaria
time-scales also caused climate change in the 2.1a). In the coldest period of the over tens of thousands or millions in far north Queensland. Photo:
Earth’s climate has changed past. Several million years ago, for last ice age, about 20,000 years ago, of years. However, there is also ©iStockphoto.com/John Carnemolla
dramatically many times since example, global average temperature sea level was at least 120 metres evidence that some abrupt changes
the planet was formed 4.5 billion was a few degrees higher than lower than today because more occurred, at least at regional scales.
years ago. These changes have today and warm, tropical waters water was locked up on land in For example, during the last ice age,
been triggered by the changing reached much farther from the polar ice sheets. The last 8,000 temperatures in the North Atlantic
configuration of continents and equator, resulting in very different years, which includes most recorded region changed by 5°C or more over
oceans, changes in the Sun’s patterns of ocean and atmospheric human history, have been relatively as little as a few decades, likely due
intensity, variations in the orbit of circulation from today. stable at the warmer end of this to sudden collapses of Northern
Earth, and volcanic eruptions. Over the past million years, temperature range. This stability Hemisphere ice sheets or changes in
Natural variations in the Earth’s globally averaged surface enabled agriculture, permanent ocean currents.
concentrations of greenhouse gases temperature has risen and fallen by settlements and population growth.
8 | The science of climate change
(a) Changes over the last 800,000 years
5
Temp. change (°C) 0
-5
-10
400
2013 CO2 concentration (396 ppm)
CO2 (ppm)
325
250
Figure 2.1: Past changes Past records demonstrate that Global average temperatures
in temperature align with global climate is sensitive to have increased over the past
175
changes in CO2 at a variety of small but persistent influences century
800,000 700,000 600,000 500,000 400,000 300,000 200,000 100,000 0
time scales. These graphs show Climate and sea level were
Years BC Ice-age cycles were initiated by small
the changes from long‑term
average temperature (oC) variations in the rotation of Earth relatively stable over thousands of
(b) Changes over the last 2,000 years and average atmospheric CO2 and in its orbit around the sun. These years of recorded human history
0.4 concentration (parts per million) changed the seasonal and latitudinal up to the 19th century, although
Temp. change (°C)
0.2 over the last (a) 800,000 years, distribution of solar energy reaching with some variations (Figure 2.1b).
(b) 2,000 years and (c) 160 years. Earth’s surface. Measurements
0 However, globally averaged
The temperature changes in
-0.2 from climate archives such as ice near-surface air temperature rose by
-0.4 (a) are for Antarctica, while for
(b) and (c) they are global averages.
cores (Box 2.1) show that changing around 0.8°C between 1850 and 2012
-0.6
Source: Compiled from various temperatures triggered changes to (Figure 2.1c). The rate of warming
400 publicly available data sources other climate factors such as the increased in the mid-1970s, and each
(for details, see web version of concentration of carbon dioxide of the most recent three decades
360 this document) as summarised (CO2) in the atmosphere (Figure 2.1a),
CO2 (ppm)
in Box 2.1 (see page 10). has been warmer than all preceding
amplifying the initial disturbances.
320 decades since 1850. The last decade
During warm periods, the major
has been the warmest of these.
280 greenhouse gases CO2 and methane
Satellite observations and direct
0 500 1000 1500 2000 were released into the atmosphere,
measurements also show warming
Years AD and receding ice sheets reflected less
sunlight to space. These observations in the lower atmosphere over the
(c) Changes over the last 160 years confirm that the climate system past three decades. In contrast,
0.6 is sensitive to small disturbances the atmosphere above about 15 km
elevation (the stratosphere) has
Temp. change (°C)
0.4 that can be amplified by reinforcing
0.2
feedback processes. Likewise, the cooled over this time.
0.0
-0.2
climate system today is sensitive to The temperature of the oceans
-0.4 disturbances from human influences. has also risen. More than 90% of
-0.6 the total heat accumulated in the
400 climate system between 1971 and
2010 has been stored in the oceans.
360
CO2 (ppm)
The greatest ocean warming has
320 taken place close to the surface,
280
with the upper 75 m of the ocean
warming by an average of 0.11oC
1850 1870 1890 1910 1930 1950 1970 1990 2010
each decade between 1971 and 2010.
Years AD
The science of climate change | 9
Box 2.2: Has climate warming recently stopped?
According to most estimates, the rate of average surface warming
has slowed since 2001, despite ongoing rises in greenhouse gases. This
slowdown is consistent with known climate variability. Indeed, decades
of little or no temperature trend can be seen throughout the last century,
superimposed on the long-term warming trend.
Two main factors have contributed to the most recent period of slowed
surface warming. First, decadal variability in the ocean-atmosphere system
has redistributed heat in the ocean, especially in the eastern and central
Pacific. This has caused warming at depth and cooling of surface waters
and the lower atmosphere in this region. Second, several temporary
global cooling influences have come into play including unusually weak
solar activity (Box 3.1, see page 15), increased aerosol production, and
volcanic activity.
None of these influences is likely to continue over the long term. Moreover,
despite the slowdown in warming at the surface, there have been
continuing increases in heat extremes and in the heat content of the
oceans, as well as rising sea levels, shrinking Arctic sea-ice, and ongoing
Box 2.1: How do we detect climate change?
melt of ice sheets and glaciers. Some models predict that, when the
Identifying temperature change that is global in extent requires frequent current slowdown ends, renewed warming will be rapid.
observations from many locations around the world. Thermometers, rain
gauges and other simple instruments have been used to measure climate Changes are evident in many the year and particularly in
variables, starting in the mid-19th century. Over time the quality, variety parts of the climate system summer. The thickness of the ice
and quantity of observations has improved. Since the 1970s, sophisticated Changes consistent with an increase has also decreased by more than
sensors on earth-orbiting satellites have provided near global coverage in global temperature have been 30% over the last 30 years.
of many climate variables. By carefully analysing the data gathered using observed in many other components >> In the Southern Ocean, there are
top right: Scientists use ice core these techniques (with careful account for changes in instrument types,
samples to reconstruct climatic records of the climate system. strong regional differences in the
over hundreds of thousands of years.
observational practices, instrument locations and urban areas) it has been changes to areas covered by sea
>> Mountain glaciers have been
Photo: NASA/Lora Koenig possible to map the distribution of temperature and other climate changes ice, but a small increase in total
shrinking and contributing
top left: Tree rings provide one source since the late 19th century. coverage, driven by shifts in winds
to global sea-level rise since
of climate change data over hundreds To study climate changes that occurred before direct measurements were and ocean currents in a warming
of years. Photo: LandLearn NSW about 1850. Melting accelerated
made, scientists use indirect evidence from other sources that record a significantly in the 1990s. Southern Ocean. Strengthening
above left: Scientists have been using climate signal. These include climate signals encoded in the composition circumpolar winds around
specialised equipment to measure >> The Greenland and West Antarctic
and record weather and climate since
of ice cores, corals, sediments in oceans and lakes, and tree rings. All Antarctica have also been linked in
Ice Sheets have both lost ice
1850. NASA’s Global Precipitation these records are laid down sequentially over time as an organism grows part to thinning of the ozone layer.
since 1990, further contributing
Measurement (GPM) Core Observatory or as sediments accumulate. Ice cores from polar ice sheets, which >> The amount of water vapour in
satellite is designed to provide rain to sea-level rise as discussed in
are built from snow laid down over tens to hundreds of thousands of the atmosphere has increased
and snow observations worldwide. Question 6. This is from increased
years, provide records of both past CO2 and temperature. As the snow since the 1980s, which is consistent
Visualisation: NASA discharge of ice into the ocean, and
transforms into ice, it traps air in sealed bubbles that provide a sample of with warmer air (Box 1.3,
also increased surface melting in
past atmospheric composition, while the ratio of stable isotopes of either see page 7).
Greenland. The rate of loss from
oxygen or hydrogen in the water molecule is related to the temperature
Greenland appears to be increasing. >> The surface of the ocean in rainy
at the time when the snow fell. More recent historical changes can be
>> The area of the Arctic Ocean parts of the world is becoming
identified by analysing written and pictorial records, for example of
covered by sea ice has decreased less salty, which is consistent
changes in glacier extent.
significantly since 1987 throughout with freshwater dilution from
increased rainfall.
10 | The science of climate change>> Some ocean currents have MLOST 1901-2012
Figure 2.2: Surface temperature has
changed in response to changes increased across most of the world
in surface winds, ocean since 1901. This map shows the
temperature and ocean saltiness. distribution of the average temperature GISS 1901-2012
The changes include a southward change between 1901 and 2012. Adapted
from IPCC (2013), Fifth Assessment
shift of the Antarctic Circumpolar Report, Working Group 1, Figure 2.21.
Current and increasing
southward penetration of the
East Australian Current.
>> An increasing number of plants
and animals, on land and in the
oceans, are undergoing shifts in
their distribution and lifecycles Trend ( C over period)
GISS 1901-2012
o
that are consistent with
-0.6 -0.4
-0.6 -0.4 -0.2
-0.2 0.0
0 0.2
0.2 0.4
0.4 0.6
0.6 0.8 1. 1.25
0.8 1.0 1.25 1.5
1.5 1.75
1.75 2.5
2.0
observed temperature changes.
Trend (ºC over period)
There are regional differences Figure 2.3: Temperature has risen over
Australia and in the surrounding ocean
to climate change including
since the beginning of the 20th century,
within Australia
Temperature anomoly (°C)
1.00 Departures from 1961–1990 climatological average
although there are regional differences.
Over the past 100 years, temperature Plot on left shows deviations from
0.50
has increased over almost the entire the 1961–1990 average of sea surface
temperature and temperatures over land
globe; the rate of increase has in the Australian region; map on right
0.0
been largest in continental interiors shows distribution of annual average
-0.50
(Figure 2.2). The average surface temperature change across Australia
temperatures over the Australian since 1910. Adapted from BOM/CSIRO
-1.00
continent and its surrounding oceans State of the Climate 2014 p.4–5.
-0.6 1960
1910 1920 1930 1940 1950 -0.4 1970
-0.2 1980
0 0.2
1990 0.4
20000.62010
0.8 1. 1.25 1.5 1.75 2.5
have increased by nearly 1°C since
the beginning of the 20th century Sea-surface temperature Trendair(ºC
Surface over period)
temperature
(Figure 2.3 left). Seven of the ten Sea-surface temperature Surface air temperature
10-year average 10-year average
warmest years on record in Australia
have occurred since 2002. However
there are differences across Australia Figure 2.4: Recent rainfall in northern
with some regions having warmed Australia has been higher than average
faster and others showing relatively during the northern wet season, and
in southern Australia it has been drier
little warming (Figure 2.3 right).
during the southern wet season. The
Since the mid 1990s there have been maps show the relative ranking (in 10%
significant increases in wet season increments) of rainfall from July 1995 to
rainfall over northwest Australia June 2014 compared with the average
since 1900 for (left) northern Australian
(Figure 2.4 left), a declining trend wet season (Oct–Apr) and (right)
in southwest Australia, and a 15% southern Australian wet season
decline in late autumn and early (Apr–Nov). Adapted from BOM/CSIRO
winter rainfall in the southeast State of the Climate 2014, p.6–7.
(Figure 2.4 right).
The science of climate change | 11Q3 Human activities have
increased greenhouse
responses to warming and rainfall
changes (though the mix of these
gas concentrations in the mechanisms remains unclear). The
Are human activities atmosphere
Atmospheric concentrations of
carbon dioxide (CO2), methane
other 45% of emissions accumulated
in the atmosphere. These changes
to the carbon cycle are known from
causing climate change? and nitrous oxide began to rise
around two hundred years ago,
after changing little since the end of
the last ice age thousands of years
measurements in the atmosphere,
on land and in the ocean, and from
modelling studies.
The dominant cause of the
earlier. The concentration of CO2 has increasing concentration of CO2
increased from 280 parts per million in the atmosphere is the burning
(ppm) before 1800, to 396 ppm in of fossil fuels. Over the last two
2013. This history of greenhouse centuries, the growth of fossil-
gas concentrations has been fuel combustion has been closely
established by a combination of coupled to global growth in energy
modern measurements and analysis use and economic activity. Fossil-
of ancient air bubbles in polar ice fuel emissions grew by 3.2% per
(Box 2.1, see page 10). year from 2000 to 2010 (Figure 3.3),
Particularly important is CO2. a rapid growth that is dominated
Enormous amounts of it are by growth in Asian emissions and
continually exchanged between has exceeded all but the highest
the atmosphere, land and oceans, recent long-range scenarios for
as land and marine plants grow, future emissions.
die and decay, and as carbon-rich Although fossil-fuel emissions of
waters circulate in the ocean. For CO2 have grown fairly steadily,
several thousand years until around the upward march of the CO2
200 years ago, this ‘carbon cycle’ concentration in the atmosphere
was approximately in balance and varies from year to year. This is
steady. Since the 19th century, caused mainly by the effects of
human-induced CO2 emissions from weather variability on vegetation,
fossil fuel combustion, cement and also by sporadic volcanic
manufacture and deforestation have activity: major volcanic eruptions
disturbed the balance, adding CO2 to have a significant indirect influence
the atmosphere faster than it can be on atmospheric CO2 concentrations,
taken up by the land biosphere and causing temporary drawdown of
the oceans (Figures 3.1 and 3.2). On CO2 through the promotion of plant
average over the last 50 years, about growth by the light-scattering and
25% of total CO2 emissions were cooling effects of volcanic haze. By
absorbed by the ocean—making contrast, the direct contribution of
sea water more acidic—and 30% volcanic emissions to atmospheric
was taken up on land, largely by CO2 is negligible, amounting to
increased plant growth stimulated around 1% of current human-
by rising atmospheric CO2, induced emissions.
increased nutrient availability, and
12 | The science of climate changeAtmospheric CO₂ sources and accumulation
10
Emissions from fossil fuels
Figure 3.2: An ‘atmospheric CO2 budget’
Emissions from land use changes reveals the amount of carbon in the
8
Accumulation in the atmosphere net amounts of CO2 entering, leaving
Annual flow of CO₂ (billion tonnes of carbon per year)
and accumulating in the atmosphere.
6 The upper panel shows the inflows of
CO2 to the atmosphere from fossil fuel
Global carbon dioxide budget emissions (red) and net land use change
(gigatonnes of carbon per year) 4
2004-2013 (orange), together with the net annual
CO2 accumulation in the atmosphere (pale
2 blue). The lower panel shows the outflows
Fossil fuel & Atmospheric of CO2 from the atmosphere to the ocean
cement growth Land sink (dark blue) and to plants on land (green).
Land-use
8.9 ± 0.4 4.3 ± 0.1 change 2.9 ± 0.8 1850 1870 1890 1910 1930 1950 1970 1990 2010 The accumulation in the atmosphere is the
difference between the sum of the two
0.9 ± 0.5
emissions and the sum of the two sinks
Ocean sink Atmospheric CO₂ sinks Source: Working Group for this document,
2.6 ± 0.5 6 with data from the Global Carbon Project.
oceans (www.globalcarbonproject.org/)
land
4
2
1850 1870 1890 1910 1930 1950 1970 1990 2010
Prod
© Glouced by
bal C the In
arbo te
Geological
n Pro rnation
ject a
2010 l Geosp reservoirs
here-B
iosp 14
here
Prog
ramme
for th High emissions pathway Figure 3.3: CO2 emissions from burning
e Glo
bal C
arbo (billion tonnes of carbon per year) Low emissions pathway fossil fuels have continued to increase
n Pro
ject.
over recent years. The black dots show
12 Intermediate 1
observed CO2 emissions from fossil fuels
Fossil Fuel CO₂ emissions
Intermediate 2
and other industrial processes (mainly
Observed
cement manufacture); the coloured
10 lines represent four future pathways
Figure 3.1: The natural carbon cycle, in which CO2 circulates between the atmosphere, land as envisaged in 2006 for low to high
and oceans, has been changed by emissions of CO2 from human activities. In this diagram emissions. Observed emissions are
of the global carbon cycle, numbers on arrows represent carbon flows averaged over tracking the highest-emission pathway.
2004–2013, in gigatonnes (billion tonnes) of carbon per year. Source: Global Carbon 8
Source: Working Group for this document,
Project, with updated numbers. with data from the Global Carbon Project.
6
1990 1995 2000 2005 2010 2015 2020 2025 2030
facing page: Southern approach to the
Sydney Harbour Bridge, NSW.
Photo: ©iStockphoto.com/airspeed
The science of climate change | 13Most of the observed recent Of these, solar fluctuations and Using climate models, it is possible
global warming results from volcanic eruptions are entirely to separate the effects of the natural Effect on climate (Watts per square metre)
human activities natural, while the other four are and human-induced influences on
-3 -2 -1 0 1 2
Climatic warming or cooling arises predominantly caused by human climate. Models can successfully
from changes in the flows of influences. The human-induced reproduce the observed warming
CO2
energy through the climate system drivers have been dominant over the over the last 150 years when both
(Figure 1.1, see page 7) that can past century (Figure 3.4). Changes natural and human influences are
Methane + other long-lived gases
originate from a number of possible in greenhouse gas concentrations, included, but not when natural
driving factors. The main drivers that dominated by CO2, caused a large influences act alone (Figure 3.5).
warming contribution. Some of This is both an important test of the Ozone + other short-lived gases
have acted over the last century are:
this has been offset by the net climate models against observations
>> increases in atmospheric CO2 and Land use change
cooling effects of increased aerosol and also a demonstration that
other long-lived greenhouse gases
concentrations and their impact recent observed global warming
(methane, nitrous oxide and
on clouds. Black carbon or soot has results largely from human rather Aerosol
halocarbons)
probably exerted a smaller, warming than natural influences on climate.
>> increases in short-lived influence. The net effect of all aerosol It is also possible to distinguish Solar
greenhouse gases (mainly ozone) types including soot remains hard the effects of different human and
>> changes to land cover to quantify accurately. Among the natural influences on climate by
(replacement of darker forests natural influences, the effect of studying particular characteristics
with paler croplands and changes in the brightness of the Sun of their effects. For example, it was Figure 3.4: Human-induced drivers of climate change have been much larger than natural
grasslands) has been very small (Box 3.1). Volcanic predicted more than a century ago drivers over the last century. The strength of these drivers, which are changing the
influences are highly intermittent, long-term energy balance of the planet, is measured in Watts per square metre (see also
>> increases in aerosols (tiny that increases in CO2 would trap Figure 1.1). Orange and green bars respectively indicate human and natural drivers; error
particles in the atmosphere) with major eruptions (such as more heat near the surface and bars indicate 5-95% uncertainties. The solar effect (shown in green) is very small. Volcanic
Pinatubo in 1991) causing significant also make the stratosphere colder. effects are highly variable in time (see text) and are not shown here. Source: Working
>> solar fluctuations (changes in the
cooling for a year or two, but their In recent years, satellite and other Group for this document, with data from IPCC (2013), Fifth Assessment Report, Working
brightness of the sun) Group 1, Chapter 8 Supplementary Material.
average effects over the past century measurements have provided strong
>> volcanic eruptions. have been relatively small. evidence that the upper atmosphere
14 | The science of climate changeLand Land + Ocean
2.0 2.0
Figure 3.5: Climate models can correctly
replicate recent warming only if they
1.5 1.5 include human influences. Comparison
of observed changes (black lines) in
global temperatures (°C) over land (left)
Temp change (°C)
Temp change (°C)
1.0 1.0 and land plus ocean (right) with model
projections including both natural plus
human influences (red lines) and natural
0.5 0.5 influences only (blue lines). Shadings
around model results indicate 5-95%
confidence bands. Adapted from IPCC
0.0 0.0 (2013), Fifth Assessment Report, Working
Group 1, Figure 10.21.
-0.5 -0.5
-1.0 -1.0
1850 1910 1960 2010 1850 1910 1960 2010
has cooled and the lower atmosphere trends that have been observed
Box 3.1: Do changes in the Sun contribute to global warming?
has warmed significantly—the in the Australian region over the
predicted consequence of extra past two decades. These include In comparison with other influences, the effects of solar variations on
greenhouse gases. This supports the stronger westerly winds over the present global warming are small. Indirect estimates suggest that changes
inference that the observed near- Southern Ocean, strengthening in the brightness of the Sun have contributed only a few percent of the
surface warming is due primarily of the high-pressure ridge over global warming since 1750. Direct measurements show a decreasing solar
to an enhanced greenhouse effect southern Australia, and a related intensity over recent decades, opposite to what would be required to
rather than, say, an increase in the southward shift of weather systems. explain the observed warming. Solar activity has declined significantly
brightness of the Sun. These trends are consistent with over the last few years, and some estimates suggest that weak activity
climate model projections, and are will continue for another few decades, in contrast with strong activity
Some recent changes in likely to be largely human-induced through the 20th century. Nevertheless, the possible effects on warming
Australia’s climate are linked through a combination of increases are modest compared with anthropogenic influences.
to rising greenhouse gases in greenhouse gases and thinning of
Modelling studies indicate that the ozone layer.
rising greenhouse gases have made Past decadal trends in Australian There has very likely been net
facing page: Wollongong, NSW at night.
a clear contribution to the recent rainfall (Question 2) cannot yet be uptake of CO2 by Australian Photo: Jim Vrckovski
observed warming across Australia. clearly separated from natural climate vegetation, consistent with global right: Rainforest canopy,
Depletion of the ozone layer in the variations, except in southwest uptake of CO2 by vegetation on land Bellenden Ker Range, North Queensland.
upper atmosphere over Antarctica Western Australia where a significant (Figure 3.2, see page 13). This has Photo: Robert Kerton
and rising greenhouse gas observed decline in rainfall has been been accompanied by increases
concentrations are also likely to have attributed to human influences on in the greenness of Australian
contributed significantly to climate the climate system. vegetation, which is also consistent
with global trends.
The science of climate change | 15Q4
How do we expect climate With continued strong growth
in CO2 emissions, much more
warming is expected
for CO2, coupled with rises in the
other greenhouse gases, would
to evolve in the future?
be expected to result in a global-
If society continues to rely on average warming of around 4.5˚C
fossil fuels to the extent that it by 2100, but possibly as low as 3˚C
is currently doing, then carbon or as high as 6˚C. A ‘low emissions’
dioxide (CO2) concentrations in pathway, based on a rapid shift away
the atmosphere are expected to from fossil fuel use over the next
double from pre-industrial values few decades, would see warming
by about 2050, and triple by about significantly reduced later this
2100. This ‘high emissions’ pathway century and beyond (Figure 4.1).
7
40
6 30
20
5
Global temperature change (°C)
10
4 0
− 10
1980 2000 2020 2040 2060 2080 2100
3 Year
2
1
0
−1
1980 2000 2020 2040 2060 2080 2100
Year
High emissions pathway Low emissions pathway
above: Drawing on data from multiple satellite missions, Figure 4.1: Future projected climate change depends on net emissions of greenhouse
NASA scientists and graphic artists have layered land gases. Retrospective and future projected global surface air temperature changes
surface, polar sea ice, city lights, cloud cover and other (°C; relative to 1861–1880) under both high and low emissions pathways. Individual model
data in a visualisation of Earth from space. Image: NASA simulations are shown as faint lines, with bold lines indicating the multi-model average.
Goddard Space Flight Centre/Reto Stöckli The corresponding two emissions pathways, including all industrial sources, are included
in the inset. Emission units are gigatonnes (billion tonnes) of carbon per year (GtC/y).
Source: Data from Coupled Model Intercomparison Project (CMIP) 5.
16 | The science of climate changeProjections of surface air temperature and precipitation change for years 2081–2100
A model with high climate sensitivity A model with low climate sensitivity Figure 4.2: Projections of temperature
and rainfall show consistent features at
11 large scales but differ regionally, especially
for rainfall. Projected global distributions
temperature changes °C
9 of surface air temperature changes (top)
Projected surface air
and percentage precipitation change
7 (bottom) averaged for the years
2081–2100 (relative to 1981–2000), under a
5 high emissions pathway for two particular
climate models, one with relatively high
3 sensitivity to an initial disturbance to the
climate system (left hand panels) and
one with relatively low climate sensitivity
1
(right hand panels). The projections have
many similar patterns but differ in regional
−1
details, as is typical of climate projections
from different models. Source: Data from
Coupled Model Intercomparison Project 5.
50
40
30
precipitation change
20
Projected %
10
0
−10
−20
−30
−40
−50
During the next few decades and and decreases in many subtropical worldwide, broadly agree on the than under a high emissions
beyond, global warming is expected and mid-latitude regions), further patterns of global-scale warming, scenario. At more localised regional
to cause further increases in ocean warming, and further rises with greater atmospheric warming scales the models can produce
atmospheric moisture content, more in sea levels. The magnitude of over land than over the oceans, and different results: for example,
extreme heatwaves, fewer frosts, expected change depends on future greater warming at high northern some models project substantial
further decreases in the extent and greenhouse gas emissions and latitudes than in the tropics and changes to phenomena such as
thickness of sea ice, further melting climate feedbacks. Southern Ocean (Figure 4.2 top). El Niño or dramatic changes to
of mountain glaciers and ice sheets, Future projections, based on Future changes depend on the vegetation, and regional projections
shifts in rainfall (increases in most climate models operated across a emissions pathway, and will be of precipitation vary between
tropical and high-latitude regions large number of research centres less if emissions are curtailed models (Figure 4.2 bottom).
The science of climate change | 17Australia can expect further Long-term climate change is
warming and changes in water effectively irreversible
availability The decisions we make on carbon
Australian temperatures are emissions over coming decades will
expected to rise by approximately affect our climate for a long time to
half a degree or more by 2030 come, as emissions will profoundly
relative to 1990, bringing more hot impact the rate of future climate
days and nights. Average sea level is change, particularly after 2030
expected to be about 15 cm higher (Figure. 4.1, see page 16). Even if
by 2030 relative to 1990 and some emissions of greenhouse gases are
models project tropical cyclones reduced to near zero during this
becoming less frequent but more century, we will have to live with
severe in peak rainfall intensity as a warmer climate for centuries.
the world warms. For those parts of the climate
It is likely that future rainfall system that respond slowly, such
patterns across Australia will be as the deep ocean, ice sheets and
different from today. However, permafrost, change will continue
compared with temperature trends, for a long time. Many associated
changes in rainfall patterns are impacts—such as sea-level rise—
harder to predict. Regional rainfall and processes that exacerbate
projections from different climate climate change—such as releases
models are frequently different of methane and CO2 from thawing
from one another (e.g. over permafrost soils—will continue long
Australia; Figure 4.2, see page 17). after emissions are stopped.
Nevertheless, some future trends These characteristics of the climate
are projected by a majority of system mean that the only way
models, including decreases over to stop human-induced climate
southwest Western Australia coastal change (without resorting to
regions. Future rainfall trends across ‘geoengineering’—the deliberate,
the Murray Darling basin remain large-scale modification of climate)
uncertain. is to reduce net greenhouse gas
Changes in rainfall greatly affect emissions to near-zero levels. The
water availability because changes longer this takes to achieve, and the
in rainfall are amplified in the more greenhouse gases that are
resulting changes in runoff to rivers: emitted in the meantime, the larger
the runoff in typical Australian the scale of future climate change.
catchments changes by 2 to 3% for
each 1% change in rainfall.
left: Low water levels in the Cotter Dam
near Canberra, ACT. Photo: Nick Pitsas
above right: 1 Bligh St, Sydney, NSW is an
energy efficient development with six-star
green status. Improving urban energy
efficiency will help reduce emissions.
Photo: Sardaka
18 | The science of climate change7
AR5
AR4
6
Warming relative to 1860–1880 (°C)
5
4
3
2
1
0
0 500 1000 1500 2000 2500 3000
Cumulative CO2 emissions from 1870 (billion tonnes of carbon)
To keep global warming global average temperatures to no emissions were 530 billion tonnes. Figure 4.3: Global warming is closely
below any specified threshold, more than 2°C above preindustrial The remaining quota is equivalent related to cumulative CO2 emissions.
there is a corresponding levels, the total CO2 emitted from to around 30 years worth of current Points represent Intergovernmental Panel
limit on cumulative carbon human activities (accounting also emissions. To stay within such a on Climate Change projections from
dioxide emissions the Fourth and Fifth Assessments (IPCC
for effects of other gases) would carbon quota, long-term global
AR4, AR5); coloured bands represent
The amount of future global have to stay below a ‘carbon quota’ emissions reductions would have uncertainty, by showing the relationship
warming is closely related between 820 and 950 billion tonnes to average between 5.5% and if the climate were more (red) or less
to cumulative CO2 emissions of carbon. So far, humanity has 8% per year, accounting for time (blue) sensitive to disturbance than
emitted well over half of this quota: required to turn around present current best estimates. Source: Working
(Figure 4.3). For example, to
Group for this document, with data from
have a 50:50 chance of keeping between 1870 and 2013 cumulative emissions growth.
IPCC AR4 and AR5.
The science of climate change | 19Q5
How are extreme
events changing?
Australia has a variable decades, hot days and nights have For other extreme weather events
climate with many extremes become more frequent, more intense such as tropical cyclones, there
With its iconic reference to and longer lasting in tandem with are not yet sufficient good quality
‘droughts and flooding rains’, decreases in cold days and nights observational data to make
Dorothea Mackellar’s 1904 poem for most regions of the globe. conclusive statements about past
My Country highlights the large Since records began, the frequency, long-term trends. However, as above: Flooding in Darwin, NT, following
duration and intensity of heatwaves the climate continues to warm, tropical cyclone Carlos in 2011. Photo:
natural variations that occur in
have increased over large parts of intensification of rainfall from Charles Strebor
Australia’s climate, leading to
extremes that can frequently Australia, with trends accelerating tropical cyclones is expected.
cause substantial economic and since 1970. Recent scientific advances now
environmental disruption. These Because a warmer atmosphere allow us to begin ascribing changes
variations have existed for many contains more moisture, rainfall in the climate system to a set of
thousands of years, and indeed extremes are also expected to underlying natural and human
past floods and droughts in many become more frequent and intense causes. For example, it is now
regions have likely been larger than as global average temperatures possible to estimate the contribution
those recorded since the early 20th increase. This is already being of human-induced global warming hot extremes threshold
century. This high variability poses observed globally: heavy rainfall to the probabilities of some kinds
Temperature
great challenges for recording events over most land areas have of extreme events. There is a
and analysing changes in climate become more frequent and intense discernible human influence in the
extremes not just in Australia, in recent decades, although these observed increases in extremely
but the world over. Nevertheless, trends have varied notably between hot days and heatwaves. While
some changes in Australia’s climate regions and seasons. In southern the record high temperatures of
extremes stand out from that Australia, for example, the frequency the 2012/2013 Australian summer
background variability. of heavy rainfall has decreased in could have occurred naturally, they
some seasons. While there is no clear were substantially more likely to cold extremes threshold
Human-induced climate trend in drought occurrence globally, occur because of human influences
change is superimposed on indications are that droughts have on climate. By contrast, the large
natural variability increased in some regions (such as natural variability of other extremes, Time
In a warming climate, extremely cold southwest Australia) and decreased such as rainfall or tropical cyclones,
days occur less often and very hot in others (such as northwest means that there is still much less Figure 5.1: Temperature extremes change as average temperature increases. In this
days occur more often (Figure 5.1). Australia) since the middle of the confidence in how these are being schematic illustration, the increase in average temperature is shown by the sloping line
20th century. affected by human influences. on the right. The idealised temperature time series has similar variability throughout the
These changes have already been
whole record. In the latter part of the record, the hot extremes threshold is exceeded
observed. For example, in recent progressively more frequently. Source: Working Group for this document.
20 | The science of climate changeYou can also read
