Chapter 2 Global heating
Introduction
2.1
This chapter examines the scientific evidence for global heating and its
causes, and discusses the predicted effects on Australia’s climate, including
changes to temperature, rainfall, sea levels and ocean acidification.
2.2
Heating of the global climate system has already affected and will
continue to have effects across the country into the future, including
potentially serious effects upon iconic areas such as the Great Barrier Reef.
The chapter considers what level of carbon in the atmosphere is necessary to
avoid serious climate disruption.
2.3
Participants in the inquiry frequently referred to the scientific
assessments of the International Panel on Climate Change (IPCC) and
particularly its 2007 Fourth Assessment Report as a primary source for
understanding global heating.[1]
2.4
For emissions scenarios, participants largely used the main emission
scenarios described in the IPCC Special Report on Emissions Scenarios (SRES)[2]
as a starting point for their analysis. The Committee has also relied upon the
IPCC’s scientific data for global climate change.
The evidence for global heating
2.5
Evidence obtained from ice cores has demonstrated that over the last
400,000 years, global temperatures have varied in alignment with glacial and interglacial
periods. Carbon dioxide concentrations in the atmosphere over this period have
ranged from 180 parts per million[3] to around 280 parts per
million, cycling in parallel with temperature changes.[4]
2.6
In the last 150 years, the long term trend has been an increase in
global temperatures, both on the surface of the planet and in the oceans, and
an increase in carbon dioxide (CO2) concentrations in the atmosphere
to currently around 380 parts per million (ppmv).[5]
2.7
The CSIRO has explained the change in carbon dioxide concentrations as
follows:
The natural sources and sinks for CO2 have varied
widely during past glacial and interglacial cycles but have remained below
300ppmv for at least 420,000 years. With the industrial revolution, humans
began transferring carbon that was effectively locked away in the Earth’s crust
to the atmosphere, upsetting the balance of the carbon cycle. The nature sinks
for carbon (primarily terrestrial vegetation and oceans) currently sequester
~40% of emissions from fossil fuel sources. The remaining 60% remains in the
atmosphere. As a result, atmospheric CO2 concentrations have
increased from 280ppmv at the start of the industrial revolution to their
current concentration of over 370ppmv. The current rate of atmospheric CO2
accumulation (~1.8 ppmv/year) is approximately 2-3 times that of the early
1960s.[6]
2.8
In its Fourth Assessment Report, the IPCC stated that warming of the
global climate system is ‘unequivocal’ as demonstrated by increasing global
average air and ocean temperatures, widespread melting of snow and ice, and
rising global average sea level.[7]
2.9
The IPCC has presented the following evidence for global heating:
n Eleven of the years
in the twelve year period of 1995-2006 ranked amongst the warmest years in the
instrumental record of global surface temperature (that is, since 1850).
n The 100 year linear
trend in the global average surface temperature to 2005 showed an increase of
0.74 degrees Celsius. Global average surface temperature has particularly
increased since about 1950.
n Observations since
1961 have shown that the average temperature of the global ocean to a depth of
at least 3000 metres has increased and that the ocean has been taking up over
80 percent of the heat being added to the climate system. Such warming causes seawater
to expand, contributing to sea level rise.
n Global sea levels
rose at an average rate of 1.8mm per year from 1961 to 2003 and at an average
rate of about 3.1mm per year from 1993 to 2003.
n Snow and ice extent
have decreased. Satellite data since 1978 showed that the annual average Arctic
sea ice extent has shrunk by 2.7 percent per decade. Mountain glaciers and snow
cover on average have declined across the globe.
n Long term changes in
rainfall have been observed since 1900 in many regions with both significant
increases and decreases.
n Some extreme weather
events have changed in frequency and/or intensity over the last 50 years.[8]
The evidence that global heating is the result of human activity
2.10
Analysis of the global heating observed since the mid nineteenth century
has identified increased concentrations of greenhouse gases and aerosols as the
cause.[9] The IPCC considers that
there is a greater than 90 percent probability that most of the warming since
the mid twentieth century has been caused by the rapid increase in greenhouse
gas concentrations resulting from human activities since the industrial
revolution.[10]
2.11
Changes in atmospheric concentrations of greenhouse gases and aerosols,
land cover and solar radiation alter the energy balance of the climate system
by affecting the absorption, scattering and emission of radiation within the
atmosphere and at the Earth’s surface. The resulting positive or negative
changes in energy balance are expressed as radiative forcing. This is then used
to compare warming or cooling influences on the global climate.[11]
2.12
It is generally accepted that the warming effect is largely brought
about by carbon dioxide, methane and, to a lesser extent, ozone, (collectively
part of the family of greenhouse gases). The counteracting cooling influences
result from aerosols: particulate matter put into the atmosphere largely as a
result of pollution from industrialisation. The warming influences have been
found to greatly outweigh the cooling and are considered to be almost entirely
attributable to human influence.[12]
2.13
The IPCC has stated that human activities are responsible for emissions
of four long-lived greenhouse gases: carbon dioxide, methane, nitrous oxide and
halocarbons. These gases are chemically stable and persist in the atmosphere
for up to centuries or longer. Emission of these gases therefore has a long
term influence on climate.[13] Where emissions are
greater than the absorption by sinks, the gases are concentrated in the
atmosphere.
2.14
The IPCC assessed that global atmospheric concentrations of carbon
dioxide, methane and nitrous oxide:
…have increased markedly as a result of human activities
since 1750 and now far exceed pre-industrial values determined from ice cores
spanning many thousands of years … The atmospheric concentrations of CO2
[carbon dioxide] and CH4 [methane] in 2005 exceed by far the natural
range over the last 650,000 years.[14]
2.15
Global greenhouse gas emissions increased by 70 percent between 1970 and
2004.[15] The IPCC divided these
emissions into the following groups:
n carbon dioxide,
fossil fuel use: 56.6 percent;
n carbon dioxide,
deforestation, decay of biomass: 17.3 percent;
n carbon dioxide,
other: 2.8 percent;
n methane: 14.3
percent;
n nitrous oxide: 7.9
percent; and
n f-gases
(hydrofluorocarbons, perfluorocarbons and sulphurhexafluoride): 1.1 percent.[16]
2.16
Carbon dioxide is considered to be the most important human caused
greenhouse gas, with its annual emissions growing between 1970 and 2004 by
about 80 percent from 21 to 38 gigatonnes. As shown in the data above, carbon
dioxide represented 77 percent of total human caused greenhouse gas emissions
in 2004.[17]
2.17
The global atmospheric concentration of carbon dioxide increased from a
preindustrial value of about 280 parts per million to 379 parts per million in
2005. Similarly, methane increased from 715 parts per billion to 1774 parts per
billion and nitrous oxide increased from 270 parts per billion to 319 parts per
billion.[18]
2.18
As noted earlier, these changes in the composition of the atmosphere
alter the radiative balance of the Earth, that is, the balance between incoming
solar radiation and outgoing heat. Greenhouse gases reduce the radiation of
heat from the Earth’s atmosphere into space, trapping more heat in the
atmosphere and thus increasing global temperatures.[19]
2.19
The IPPC attribute the largest growth in greenhouse gas emissions
between 1970 and 2004 to energy supply, transport and industry. Residential and
commercial buildings, forestry (including deforestation) and agricultural
sectors have also grown, but at a slower rate.[20] Global increases in
carbon dioxide concentration are due primarily to fossil fuel use and land use
changes. Increases in methane and nitrous oxide are primarily due to
agriculture.[21]
2.20
Studies have shown that solar radiation, a natural form of warming, has
had a very small impact on temperature compared with human influences.[22]
Observed effects of global heating
2.21
Based upon studies since 1970, the IPCC concluded in 2007, with either
very high (greater than 90 percent) or high (greater than 80 percent)
confidence, that global heating has had discernable impacts on physical and
biological systems.[23] This includes:
n natural systems
related to snow, ice and frozen ground (eg, glacial lakes, permafrost regions
and Arctic and Antarctic ecosystems);
n hydrological systems
(eg, runoff and discharge in glacier and snow fed lakes, and warming of lakes
and rivers);
n terrestrial
biological systems (eg, the early onset of spring events); and
n marine and freshwater
biological systems (eg, range and abundance of species) associated with rising
water temperatures and changes in ice cover, salinity, oxygen levels and
circulation.[24]
2.22
While the IPCC concluded that warming had also impacted upon managed and
human systems, such as agricultural and forestry management and human health,
these impacts were more difficult to distinguish because of adaptation and
other non-climatic drivers.[25]
2.23
For Australia, the effects of global heating can be seen through changes
to temperature, rainfall, sea levels and ocean acidification. However, the
observed effects of heating differ across the country. Australia’s climate is highly variable, particularly in the context of rainfall, which is
cyclical and influenced by factors such as El Nino and La Nina. The southern
parts of Australia are dominated by a Mediterranean climate with winter
rainfall, while northern Australia experiences monsoonal influences and summer
dominant rain.[26]
Temperature, rainfall, run off and drought
2.24
The long-term trend for Australia has been an increase in temperature of
about 0.9 degrees Celsius over the last 100 years or so.[27]
Air temperature increases have been accompanied by marked declines in regional
rainfall, particularly along the east and west coasts of the continent.[28]
2.25
Warming has been associated with an increase in the frequency of hot
days, hot nights and heatwaves, and a decreased frequency of cold days and cold
nights, increases in heavy rain events, and decreases in spring-time snow depth
in the Snowy Mountains.[29]
2.26
Temperature increases have occurred not only at the surface of the planet
but also in the oceans. Substantial warming has occurred in the three oceans
surrounding Australia, but most particularly off the south-east coast and in
the Indian Ocean.[30]
2.27
The CSIRO provided evidence on annual rainfall trends for the period
1950 to 2007, which illustrated a strong drying trend in south-west Western Australia and a quite strong drying trend through eastern Australia. According to Dr Ash of the CSIRO, the causes of these drying trends are much
more complex than the temperature increases:
The drying trend in south-west Western Australia and, to a
smaller extent, in south-eastern Australia we believe is attributable to
climate change and the way that the frontal systems that go across southern
Australia in winter, which bring a fair bit of the rain, have moved southwards
as a result of a change in pressure systems from Antarctica up to the middle
latitudes.
We are confident that some of the rainfall decreases that we
are observing in southern Australia are at least in part attributable to greenhouse
gas emissions.[31]
2.28
In the same period, rainfall increased in north-western and central Australia and over the western tablelands of New South Wales.[32]
2.29
The CSIRO indicated that a significant contributor to rainfall increases
observed in northern and north-western Australia over the last 30 to 40 years
is increased aerosols over Asia as a result of industrialisation.[33]
2.30
Small changes in rainfall can lead to significant declines in runoff
into storage areas and dams. Dr Ash told the Committee that a one percent
decrease in rainfall can lead to a three to four percent decrease in runoff. A
five percent decrease in rainfall can lead to quite significant declines in run
off into storage areas and dams. [34] For example, in relation
to the Murray-Darling Basin, run off is not occurring as the ground is very dry
as a result of the current drought and any rainfall is taken up by the ground.
The combination of very low rainfalls and higher temperatures is drying out the
whole system and without run off dam levels and water levels are at historic
lows.[35]
2.31
In its submission, the Murray Darling Basin Commission highlighted that
while average annual inflows to the River Murray system are 11,100 gigalitres[36],
inflows to the River Murray system in 2006-07 were approximately 1,000
gigalitres – nine percent of average annual inflows.[37]
2.32
With increasing temperatures, the effects of drought are exacerbated
because as it gets hotter plants use more water. As a result, even with the
same amount of water in the system, ‘you are getting the impact of the drought
through increased temperatures’.[38]
2.33
While a lot of the droughts currently being observed are still largely
within the bounds of historical natural cycling in terms of rainfall, such as
that for the Murray-Darling Basin, the CSIRO considers that they have been
exacerbated significantly by the higher temperatures over the last hundred
years; that is, by global heating.[39]
Sea levels
2.34
Over a period of thousands of years, there have been large changes in
sea level as the Earth’s climate has cycled between glacial and inter-glacial
periods. These sea level changes were driven predominantly by melting and
freezing of ice on land.[40]
2.35
From about 1,000 years ago to the late nineteenth century, sea level was
relatively stable, with a variation of less than about 0.2 metres over the
whole period. This compares with changes of up to four metres in the period
from 21,000 to 2,000 years ago.[41]
2.36
However, in the twentieth century sea level again rose significantly
and, according to the Antarctic Climate and Ecosystems Cooperative Research
Centre (ACE CRC), at a rate that has not been experienced in the past 5,000
years:[42]
While sea level has varied by more than 120m during ice age
cycles, there was little net change in global average sea level from 0 AD to about
1800 AD – the time during which most of the world’s coastal development has
occurred. Over the last 130 years, the rate of sea-level rise has increased and
since the launch of satellites to measure sea-levels in the early 1990s, it is
over 3mm/year. This rate is unprecedented in the 20th century.[43]
2.37
The global average rise for the twentieth century is 0.17 metres.[44]
Unlike the sea level changes associated with earlier glacial and interglacial
cycles, recent sea-level rise is increasingly the result of a warming ocean and
the corresponding thermal expansion.[45]
2.38
The ACE CRC also notes that observed sea level is currently tracking
near the upper limit of the IPCC projections from the start date of projections
in 1990.[46]
Ocean acidification
2.39
Although carbon dioxide emissions start off in the atmosphere, a large
proportion is then absorbed by the ocean as part of the natural carbon cycle.
Greater carbon dioxide emissions have resulted in more carbon dioxide being
dissolved into the world’s oceans. Carbon dioxide forms a weak acid (carbonic
acid) therefore making the ocean more acidic.[47]
2.40
The IPCC assessed that the increase in human caused carbon emissions
since 1750 has led to an average decrease in pH[48]
of 0.1 units and therefore an increase in ocean acidification.[49]
2.41
According to the ACE CRC, approximately half the fossil fuel carbon
dioxide emitted by humans has now dissolved into the ocean. Further:
Ocean acidification is a direct consequence of CO2
emissions, so differs from the overall challenge posed by global heating in
that the inevitable and inexorable rise of dissolved CO2 in the
ocean will continue independently of whether the atmosphere is warming.[50]
Predicted effects
2.42
While the exact scale and speed of future changes in the global climate
are still uncertain, the direction of change is considered to be clear. The
IPCC concluded that continuing greenhouse gas emissions at or above current
rates would cause further warming and induce larger changes in the global
climate system during the twenty first century than those observed during the
twentieth century. Further:
…even if all greenhouse gas emissions ceased today, the Earth
would still be committed to an additional warming of 0.2 to 1.0 degrees Celsius
by the end of the century.[51]
2.43
Using SRES scenarios, the IPCC identified a ‘best estimate’ increase in
global average temperature of between 1.8 degrees Celsius and four degrees
Celsius at the end of the twenty first century depending upon the scenario. Sea
level rise, excluding future rapid dynamical changes in ice flow, using the
same scenarios, was projected to be between 0.18 and 0.59 metres.[52]
2.44
Significant effects upon ecosystems, food, coasts, industry and
settlements, health and water are considered more than 80 percent likely to
occur at a global level.[53] For Australia, the CSIRO has assessed that changes to the climate system will result in wide
ranging impacts on natural ecosystems, cropping, forestry and livestock, water
resources, public health, and infrastructure and settlement.[54]
Temperature
2.45
The CSIRO outlined a number of emissions dependent temperature scenarios
for Australia for the Committee.[55] Using a midrange SRES
emissions scenario, further warming of around one (0.6 to 1.5)[56]
degree Celsius by 2030 is predicted. Warming will be slightly less in coastal
areas and slightly more inland. There is a ten to 20 percent probability that
warming will exceed one degree Celsius on the coast, and in inland regions,
there was a strong possibility of temperatures exceeding 50 degrees
Celsius.[57]
2.46
In relation to the projected one degree Celsius increase, the CSIRO
pointed out that:
… on our current track of emissions we are heading for a
higher emissions scenario result for 2030 than that number reflects.[58]
2.47
By 2070 predicted temperature increases could be between 1.8 (1.0 to 2.5)
and 3.4 (2.2 to 5) degrees Celsius, but this is strongly dependent upon the
extent to which emissions are reduced over the next 30 to 40 years.[59]
By 2070 in the low emissions case, there is a greater than 90 percent
likelihood that warming will exceed 1 degree Celsius throughout Australia, a 20-60 percent chance of exceeding two degrees Celsius over most inland areas,
and about a ten percent chance of exceeding three degrees Celsius in more
coastal areas.
2.48
In the high emissions case, there is around a 30 percent chance of
temperature increases exceeding three degrees Celsius in southern and eastern
coastal areas and a much greater chance inland, while the chance of exceeding
four degrees Celsius is around ten percent in most coastal areas and 20 to 50
percent inland.[60]
2.49
It is expected that the number of days over 35 degrees Celsius will
substantially increase. For example, in the period 1971-2000, Perth averaged 28
days per year above 35 degrees Celsius. This is predicted to increase to
between 35 (33-39)[61] and 54 (44-67) days. In
the case of Darwin, which averages 11 days, this is predicted to increase to
between 44 (28-69) and 230 (140-308) days.[62]
2.50
The CSIRO has assessed the probable effects that changes in temperature,
ranging from less than one degree Celsius to more than five degrees Celsius, are
likely to have on the areas identified earlier.[63]
For example, at warming of one degree Celsius, areas such as the Great Barrier Reef and mountain ecosystems are considered particularly vulnerable. At
higher magnitudes of warming, the CSIRO considers that annual damage to the Great Barrier Reef will increase to the point of catastrophic failure, snow cover and
duration will decrease substantially, and animal, bird and plant species in
northern and southeast Australia will become extinct.[64]
Rainfall
2.51
While noting that rainfall is much harder to define, the CSIRO has
predicted a large range in annually averaged rainfall for southern parts of Australia by 2030, ranging from no change in rainfall down to a ten percent decline.[65]
In northern areas, the predicted change is around ten percent decline to five
percent increase.[66] These changes will vary
across regions and according to season. For example, decreases in rainfall are
likely in southern areas in the annual average and in winter, in the southern
and eastern areas in spring, and along the west coast in autumn.[67]
2.52
The CSIRO pointed out that the effect of greenhouse gases and the
climate system on the monsoonal influences (the summer dominant rain of
northern Australia) is much less clear, with more work required to understand
how emissions will affect rainfall patterns.[68]
2.53
The CSIRO considers that projected changes in rainfall and increases in
evaporation are likely to result in a decline in soil moisture over much of
southern Australia. Simulations show that droughts are likely to increase.[69]
2.54
The vulnerability of systems to climate change can be dependant upon a
number of climatic variables. For example, the impacts associated with a large
temperature increase may be more modest if precipitation also increases. This
is the case for agriculture, forestry and livestock, where it is expected that
the interaction between temperature and precipitation will influence the impact
of climate change. For warming of up to three to four degrees Celsius, wheat
yields are projected to increase provided there are sufficient increases in
precipitation. However, reductions in precipitation and increases in drought
could have severe impacts upon agriculture.[70]
Sea levels
2.55
Sea levels are predicted to continue to rise during the twenty first
century. According to the Antarctic Climate and Ecosystem Cooperative Research
Centre (ACE CRC):
Mitigation of greenhouse gas emissions might attenuate
sea-level rise in the long-term but we are already locked into significant
sea-level rise during this century.[71]
2.56
This rise is forecast to be around 0.03m in the next decade and 0.2 to
0.8m by the end of the century[72], although some
scientists have suggested that these projections may have been underestimated.[73]
2.57
Thermal expansion of the ocean and glacier and ice cap melt are the two
largest contributors to present sea level rise. It is considered that melting
ice from Greenland and the West Antarctic Ice Sheet will become increasingly
important in the future. Should these melt entirely, they contain enough ice to
raise sea levels by seven metres and six metres respectively.[74]
2.58
There are significant implications both globally and for Australia from sea level rise given the densities of populations along coastlines.
Urbanised coastal areas are particularly vulnerable to climate change because
of their exposure to sea level rise and extreme events.
2.59
Some of the specific implications of sea level rise for Australia identified by the ACE CRC include:
n More frequent extreme
sea level events. It is predicted that events that presently occur every few
years may occur every few days in 2100 if mean sea level rises by 0.5m. Sydney, Brisbane and Bass Strait are considered particularly likely to see larger
increases in the frequency of these events.[75] Sea level rise off the
east and west coast of Australia has already resulted in extreme sea level
events of a given magnitude occurring roughly three times as often in the last
half of the twentieth century compared with the first half.[76]
n Shoreline recession.
For a sea level rise of 0.5m, it is predicted that landward erosion of about
50m will occur on sandy beaches. Other high energy exposed coasts (such as
those around much of southern Australia) will experience higher rates of
recession while very low energy or embayed coasts (such as those across much of
tropical Australia) are likely to experience lower rates.[77]
n Flooding of low lying
islands. Many islands are exposed to the risks not only of flooding but also
more frequent high sea level events.[78]
2.60
In relation to extreme events, the CSIRO told the Committee that:
One of the big issues with climate change is not some of the
slow-moving trends that you see in temperature or rainfall, it is the
combination of that trend with extreme events. If there is a combination of
some sea level rise and then an extreme weather event, the storm surge that is
a result of that has a very large impact, particularly in our coastal areas.[79]
2.61
For example, in this context Kakadu is an area that is particularly
vulnerable due to salt water coming into the Kakadu wetlands.[80]
Ocean acidification
2.62
More carbon dioxide in the oceans is lowering the availability of
dissolved carbonate ions to calcifying organisms such as shelled sea animals
and corals. Calcifying organisms need this carbonate to precipitate their
shells.
2.63
According to the ACE CRC, ocean acidification can be considered an
impact ‘advancing from the south’ and one that will affect Australia’s marine
ecosystems in the Southern Ocean, marine protected areas on the southern
margins of the Australian continent (the Great Australian Bight and Tasmanian
seamounts) and eventually the Great Barrier Reef.[81]
The Southern Ocean, as a cold mass of water, contains a disproportionate amount
of human caused carbon dioxide compared with other warmer oceans. The Antarctic
polar waters are expected to be the first to experience carbonate ion
concentrations low enough that aragonite, one form of calcium carbonate, will
no longer be able to be precipitated by shell-forming organisms.[82]
2.64
In its submission, the Great Barrier Reef Marine Park Authority (GBRMPA)
cited modelling undertaken by the CSIRO and based on SRES, which forecast that
the annual sea surface temperature of the Great Barrier Reef will increase
between 1.2 degrees Celsius and 3.9 degrees Celsius by 2080. However, in
addition to increased sea surface temperatures, ocean acidification will have
far reaching effects.[83] It is predicted that
ocean pH will decrease between 0.3 to 0.5 units by 2100.[84]
GBRMPA informed the Committee that:
ocean acidification is of concern to coral reefs as calcium
carbonate binding organisms (such as corals) are unable to form their
skeletons. Scientific evidence suggests that if CO2 concentrations
rise above 500 parts per million, corals will be unable to grow. [85]
2.65
GBRMPA also told the Committee that once carbon dioxide level rises
above 450 parts per million hard corals will struggle to grow and build
skeletons strong enough to withstand storm damage. As a consequence, there will
be fewer hard corals and increasingly abundant soft corals, algae and seaweed.
This then means that the reef system will:
shift away from the system as we know it now towards a system
that has lower productivity in terms of many of the commercial species
extracted from the Great Barrier Reef and also lower values in terms of the
aesthetics that support the tourism industry.[86]
2.66
Coral bleaching is a stress response to high temperatures. Bleaching
events of a high magnitude have only been seen in the last ten to 15 years. In
the last ten years there have been three bleaching events in the Great Barrier Reef, which is a higher frequency than has ever been seen previously in a ten
year timeframe. GBRMPA considers that this is reflective of an increasing trend
in baseline temperatures. [87] In addition, there have
been unusually warm sea surface temperatures in the years when bleaching events
have occurred. [88] While these events have
affected the quality of the reef, no coral species have yet been lost because
of these warm water events.[89] However, once affected
by bleaching, coral reefs are at much greater risk, particularly if they do not
have sufficient time between bleaching events to recover.[90]
2.67
Ocean acidification is also likely to impact upon the key components of
the benthic habitats on which many fisheries in Australian waters are
dependent. It will also affect gastropods such as abalone and bivalves
including oysters and mussels. In the medium to long term there are likely to
be impacts upon fisheries and aquaculture management and practices, including
the species produced and areas suitable for production.[91]
What is an acceptable level of carbon in the atmosphere?
2.68
To maintain a stable level of carbon dioxide in the atmosphere, global
emissions of carbon dioxide must be reduced to a level that prevents any net
accumulation of carbon dioxide in the atmosphere.
2.69
In the IPCC Special Report on Emissions Scenarios (SRES), a range of
emissions scenarios were examined based upon different assumptions about future
economic, technological and social changes without specific policies to
reduce greenhouse gas emissions. All the scenarios suggested that emissions
would be at least two to four times 1990 levels by 2050 and may be as high as
five times 1990 levels by 2100.[92] Therefore:
…it is clear that achieving no net growth in atmospheric
concentrations requires reversing the current trend of increasing GHG emissions
– ultimately reducing emissions to levels well below those of 1990.[93]
2.70
The CSIRO has examined various attempts to define the global mean
temperature change and/or stabilisation level consistent with the concept of
dangerous human caused interference.[94] The stabilisation
targets identified ranged from 375 to 550 parts per million with temperature
targets ranging from 0.9 to 2.9 degrees Celsius above 1990 levels.[95]
2.71
There is growing support amongst climate scientists and experts that the
global average temperature increase should be no more than a maximum of two
degrees Celsius. This includes the average temperature increase of 0.6 degrees
Celsius which has already occurred since industrialisation.[96]
For example, the 2007 Bali Climate Declaration by Scientists stated in relation
to a future global climate treaty:
The prime goal of this new regime must be to limit global
heating to no more than 2°C above the pre-industrial temperature, a limit that
has already been formally adopted by the European Union and a number of
countries.
Based on current scientific understanding, this requires that
global greenhouse gas emissions need to be reduced by at least 50% below their
1990 levels by the year 2050. In the long run, greenhouse gas concentrations
need to be stabilised at a level well below 450ppm (parts per million; measured
in CO2-equivalent concentration). In order to stay below 2°C, global emissions
must peak and decline in the next 10 to 15 years…[97]
2.72
In its submission, Greenpeace also argued that to prevent dangerous
climate change, global temperature rise must be kept as far below two degrees
Celsius as possible, compared with pre-industrial levels. According to
Greenpeace, the goal of climate policy must be to ensure that global emissions
peak as soon as possible and are substantially reduced in the very near term,
with global emissions peaking in 2015 and reduced by more than 50 percent by
2050 with reductions continuing thereafter.[98]
2.73
The Climate Institute argued that even a two degree Celsius increase in
global temperatures would result in severe impacts for Australia. It considered that concentrations need to peak well below 500 parts per million
and then decline to levels below 400 parts per million over the coming
centuries.[99]
2.74
Figure 2.1 is reproduced from the IPCC’s 2007 report and summarises the
required emission levels to achieve different stabilisation concentrations and
the resulting global average temperature increases. As can be seen from the
table, in order to stabilise at a lower concentration and temperature level,
emissions will need to peak earlier and greater emission reductions are
required by 2050.[100]
Figure 2.1 IPCC stabilisation scenarios
Source IPCC,
Climate Change 2007: Synthesis Report, p. 67.
2.75
Global average temperature increase is expected to slow within a few
decades of greenhouse gas emissions being stabilised. However, according to
the IPCC, small increases in temperature can be expected for several centuries.[101]
2.76
Figure 2.1 also includes global average sea level rise. Sea level rise
from thermal expansion will continue for many centuries after greenhouse gas
concentrations are stabilised due to ongoing heat uptake by the oceans. In
addition to thermal expansion, the implications of Greenland ice sheet loss, as
noted earlier, are potentially several metres and would have major consequences
for world coastlines.[102]
2.77
The IPCC considered that delaying emission reductions would
significantly constrain the opportunity to achieve lower stabilisation levels
and increase the risk of more severe climate change. In contrast, it considered
that mitigation measures over the next two to three decades ‘will have a large
impact on opportunities to achieve lower stabilisation levels’.[103]
2.78
The stabilisation level that is ultimately adopted will depend upon what
climate change impacts governments are prepared to accept. Scientific evidence
presented by bodies like the IPCC and CSIRO have outlined the likely impacts
that increases in temperature will have.[104] The Garnaut Climate
Change Review has also made detailed comparisons of the likely impacts of a
‘strong’ global mitigation scenario (550 parts per million carbon dioxide
equivalent stabilisation) and ‘ambitious’ global mitigation scenario (450 parts
per million carbon dioxide equivalent stabilisation).[105]
For example, the world’s natural ecosystems are considered particularly
vulnerable to climate change with patterns of temperature and precipitation
affecting the distribution and abundance of species. The IPCC has assessed that
a one to two degrees Celsius increase in global mean temperature would pose a
significant risk to many ecosystems.[106] For Australia, the potential impacts upon one significant ecosystem, the Great Barrier Reef,
have already been discussed. In its evidence, the GBRMPA informed the Committee
that the best possible outcome for the Great Barrier Reef would be for
Australia and other countries to rapidly adopt targets that will stabilise
atmospheric carbon dioxide well below the critical threshold of 450 parts per
million:[107]
According to world-class coral reef scientists, a dramatic
loss in coral reef biodiversity is inevitable at atmospheric CO2
concentrations approaching 500 parts per million. Other ecosystems are likely
to be extremely impacted at atmospheric CO2 concentrations between
450 to 500 parts per million. Therefore, it is crucial for coral reefs and
other ecosystems that emissions are effectively managed to maintain atmospheric
CO2 well below these critical thresholds.[108]
2.79
The availability of water resources and water stresses are another
critical issue on a global basis.[109] The CSIRO has argued
that the current pressures placed on Australian water resources ‘are indicative
of their general high vulnerability to climate change’.[110]
With predicted declines in rainfall, particularly across southern Australia, climate change is likely to impact upon:
n important water
catchment areas, by impacting on the growth, species composition, frequency and
severity of fire and pest incursion of the native forest that covers many of
these areas;
n water supply and
water quality for irrigation and other uses;
n inflows into
reservoirs; and
n wetlands important
for bird breeding and other wildlife.[111]
2.80
As an example, a one to two degree Celsius increase in temperature is
projected to lead to a 12 to 25 percent decrease in flow in the Murray Darling Basin and a seven to 35 percent decrease in Melbourne’s water supply.[112]
2.81
While the CSIRO considered that limiting future increases in atmospheric
carbon dioxide to 550 parts per million could avoid the more extreme climate
changes, a lower stabilisation level, such as 450 parts per million:
…would give natural ecosystems and their associated species
greater time to adapt to changing environmental conditions, reduce the
likelihood of major adverse consequences for agriculture and forestry, help
ensure that Australia’s public health infrastructure can keep pace with
emerging health challenges, and reduce the chance of large-scale singularities.[113]
2.82
In its submission, The Climate Institute argued that:
n A stabilisation level
of 400 parts per million or below will reduce the risk of irreversible and
potentially catastrophic global impacts, although there will still be risks to
natural systems such as coral reefs.
n An objective of
stabilising atmospheric concentrations at 450 parts per million, ‘suggests that
Australia is prepared to accept the known adaptive capacity of Australian
natural ecosystems being exceeded’. It also risks the known adaptive capacity
of water security and coastal communities.
n At 550 parts per
million, the risks include severe drought constraining water supplies and
farming over large areas, catastrophic events such as collapse of the Greenland
and West Antarctic icesheets and exceeding the known adaptive capacity of
Australian natural ecosystems, water security, coastal communities,
agriculture, forestry, tourism and health systems.[114]
2.83
The projections of the IPCC, CSIRO and other scientists have identified
the likely impacts of global heating and clearly illustrate that it is in Australia’s national interest to ensure global temperatures peak at the lowest possible
level.
2.84
While it is no longer possible to avoid all the adverse impacts of
global climate change, reductions in greenhouse gas emissions can limit the
magnitude and rate of future climate change.
2.85
The Committee notes that a stabilisation target of 450 parts per million
carbon dioxide equivalent is considered to provide about a 50 percent
chance of limiting the global mean temperature increase to two degrees Celsius
above pre-industrial levels.
2.86
Stabilisation at a higher level, such as 550 parts per million is
associated with a real risk of dangerous climate disruption.
2.87
On the basis of the evidence available to it, the Committee considers
that a stabilisation target for greenhouse gas emissions that provides the best
opportunity to protect significantly threatened areas such as the Great Barrier Reef, as well as natural and human environments more broadly, from the
effects of climate change, should be adopted.
Recommendation 1 |
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|
The Committee finds that it is in Australia’s interests to
secure global agreement to deliver deep cuts in emissions so as to stabilise
concentrations of greenhouse gases in the atmosphere at 450 parts per million
or lower by 2050.
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