Metropolitan Melbourne Investigation Discussion Paper - Victorian ...

2.4 Climate
Meteorological measurements have been collected in
Melbourne since 1854. These long-term records are
important tools used to understand climate averages and
ranges, particularly when daily and yearly weather patterns
are so highly variable. A description of Melbourne’s climate
and the impacts of climate change are presented below.
2.4.1 MELBOURNE’S ‘NORMAL’ CLIMATE
International conventions draw upon the 30 year period
from 1961 to 1990 to defi ne normal or current climatic
averages or typical conditions. Under these conventions
Melbourne’s climate is described as temperate with
distinctly dry and warm summers using the Köppen
classifi cation scheme. 61 During this 30 year period, the
region had average maximum summer temperature of
around 22 to 24ºC near the coast and in the ranges to
the east, and 25 to 27ºC in Melbourne and inland. Winter
average maximum temperatures were mostly around 12 to
14ºC with frosts occurring inland, but rarely near the coast
and inner urban areas.
Rainfall was highly variable. The average annual rainfall
for the Melbourne city gauge was 639 millimetres (mm),
but was less than 600mm in Werribee and Laverton
west of Melbourne and more than 1300mm north and
east at Toolangi and the Dandenong Ranges. Generally,
rainfall was greatest in winter and spring. 62,63,64 Despite a
wet and grey weather reputation, Melbourne averaged
98 rainy days a year between 1961 to 1990 when
at least 1mm of rain fell (Station: Melbourne regional
offi ce). 65 In comparison, Brisbane averaged 93 rainy days
(Station: Brisbane regional offi ce) and Sydney averaged
101 rainy days over the same period (Station: Sydney:
Observatory Hill). 65
2.4.2 MELBOURNE’S CHANGING CLIMATE
Temperatures and rainfall for Melbourne have been
atypical for the last decade, even when the natural
variability of the climate is taken into account. Since 2001
Melbourne experienced fi ve of the twenty driest years
in over 150 years of records (below 472mm in the city
gauge), and annual mean temperatures are rising. 62,66
Climatic modelling suggests that this period is probably
not only a ‘normal’– albeit longer – drought cycle, but also
refl ects climate change. 67
Climate change has signifi cant implications for Australia’s
environment, economy and society through its effects on
the availability and distribution of water, higher average
temperatures and increases in the occurrence of droughts,
fi res and fl oods. 67,68,69
The Intergovernmental Panel on Climate Change (IPCC)
was established in 1988 by the World Meteorological
Organization and the United Nations Environment
Programme. The IPCC Fourth Assessment Report,
released in 2007, concluded that “warming of the climate
system is unequivocal” and “Most of the observed
increase in global average temperatures since the
mid-20th century is very likely [greater than 90 per cent
probability] due to the observed increase in anthropogenic
greenhouse gas concentrations.” 70,71
The levels of the three main greenhouse gases (carbon
dioxide, methane and nitrous oxide) have grown
signifi cantly since 1750. In the past, forest absorption,
atmospheric and oceanic processes buffered the earth
against short-term natural spikes in greenhouse gases
(e.g. from volcanic eruptions). Our increasing use of fossil
fuels and land clearing has produced an unprecedented
and sustained increase in greenhouse gas emissions,
while also undermining the natural absorption processes
of the environment.
Carbon dioxide is the most signifi cant of the
anthropogenic (human produced) greenhouse gases. 72
Present day global carbon dioxide concentrations were
recently measured as 386 parts per million (ppm). 73,74
For the 10,000 years before 1750, atmospheric carbon
dioxide levels were within the range 280± 20ppm. 75
Furthermore, the normal or natural range of carbon
dioxide during at least the last 800,000 years has
been determined by climate scientists as 170 to about
300ppm. 73,76,71,77,78,79
There has been a strong correlation between carbon
dioxide and temperature over the last 420,000 years.
A rapid rise of carbon dioxide levels beyond long term
natural variations commenced at the start of the 1800s.
But it is not just the increase of the post-industrial
greenhouse gas concentrations that is unusual; it is also
the rate at which this has occurred. From 1750 to 2005,
carbon dioxide increased from 280-379ppm (35 per
cent), methane increased from 715-1774ppb (148 per
cent) and nitrous oxide from 270-319ppb (18 per cent). 70
At no stage in the recent geological record has such a
rapid and sustained change occurred. Relatively slow
atmospheric reaction times and feedback loops will result
in climate change continuing in response to greenhouse
gases we are emitting today until at least 2030. 67 There
are potentially other long-term changes yet to be fully
realised that may be more severe and irreversible from the
perspective of human timescales. 72
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