4.2.2. Trends in Climate, Sea Level, and CO2
Summary: The region's climatic trends are consistent with those of other
parts of the world. Mean temperatures have risen by up to 0.1°C per decade over
the past century; nighttime temperatures have risen faster than daytime temperatures;
and the past decade has seen the highest mean annual temperatures ever recorded.
Increases in the frequency of heavy rainfalls and average rainfall have been
reported for large areas of Australia. Sea levels have risen on average by about
20 mm per decade over the past 50 years.
Climate exhibits a variety of natural variations-from those occurring over
geological time (McGlone et al., 1996) to those measured on decadal, year to
year, and seasonal time scales. Long-term climate fluctuations in the Australasia
region, assessed from historical data measured over the past century, are summarized
in regional reviews by Hobbs (1988) and Salinger et al. (1996) and the regional-mean
series in Figure A-6 of Annex A. The region's trends
are broadly consistent with global trends reported in the recent Intergovernmental
Panel on Climate Change (IPCC) second assessment (IPCC 1996, WG I, Chapter 3).
The impacts of such long-term variations on land use in parts of southern Australia
are documented by Heathcote (1994, 1996).
Air temperatures in the region have risen by 0.5-0.9°C since the beginning
of the century (Salinger et al., 1996), and New Zealand temperatures show a
trend of +0.11°C per decade (Zheng et al., 1997). These figures are somewhat
higher than the global mean trend (IPCC 1996, WG I, Section 3.2.1). There are
large variations in decadal averages (Figure A-6), presumably
of natural origin. The largest increases have occurred since about 1950. Regional
trends in intraseasonal and interannual temperature variability were mixed and
generally not statistically significant (Plummer, 1996). Nineteenth-century
Australian temperature data may be systematically too high, owing to instrumentation
error, which may partially mask the real temperature rise for Australia derived
from these early data (IPCC 1996, WG I, Section 18.104.22.168). Ocean temperatures
in the region also are generally rising (Folland and Salinger, 1995; Salinger
et al., 1996; Holbrook and Bindoff, 1997; Zheng et al., 1997).
Analyses have shown that, along with other parts of the world, the region's
minimum daily temperatures have tended to rise faster than the maximum daily
temperatures, which means that the day-night difference (the diurnal temperature
range) has decreased noticeably in most places, by up to 1°C over the past 40
years (Karl et al., 1992; Salinger, 1995; Torok and Nicholls, 1996; Zheng et
al., 1997). The trend in diurnal temperature range has been strongest since
the 1950s and appears to be a result of increasing cloudiness-the source of
which is unclear at present (IPCC 1996, WG I, Section 8.5.3). In Australia,
there has been a decrease in the number of clear days since at least the 1950s,
with the largest declines occurring in spring (Plummer et al., 1997), and cloud
cover has increased by 5% overall since 1910 (Jones, 1991).
As expected, the trend toward rising temperatures has resulted in an increase
in the frequency of very warm days and nights and a decrease in the frequency
of very cool days and nights. Plummer et al.'s (1997) study of Australian data
for 1961 to 1995 showed that the decrease in the frequency of cool nights exhibited
the strongest trend, whereas the decrease in the frequency of cool days was
a relatively weak trend.
Rainfall is intrinsically more variable than temperature, both from place to
place and time to time; thus, the detection of trends in rainfall is more difficult
and uncertain. The picture for the region is not straightforward. There are
seasonal and subregional differences, the trends are affected by the ENSO phenomenon,
and the results depend on the period chosen for analysis. Some of the observed
trends, notably those in recent decades in the southWest of Western Australia
(Allan and Haylock, 1993), have been related to changes in regional atmospheric
circulation. The time series of regional average annual rainfall is shown in
Figure A-6. There are marked interdecadal variations,
which are dominated by ENSO-induced variations in summer half-year rainfall
over northern and eastern Australia.
Recent studies (Nicholls and Lavery, 1992; Suppiah and Hennessy, 1996, 1997;
Lavery et al., 1997) demonstrate an increase in heavy rainfall and average rainfall
over large areas of Australia from 1910 to 1990. The largest increases have
occurred along the east coast, particularly in New South Wales, but decreases
are evident in southWest Western Australia and inland Queensland. In the summer
half-year, the all-Australia average rainfall (based on areal weighting of station
data) increased by 14%, heavy rainfall increased by 10-20%, and the number of
dry days decreased by 4%. In the winter half-year, the changes were about half
these figures. The trends in heavy rainfall are partially but not totally explained
by ENSO fluctuations over recent decades.
The evidence on global trends in tropical cyclones, ENSO behavior, mid-latitude
storms, and other atmospheric circulation is inconclusive (IPCC 1996, WG I,
Sections 22.214.171.124, 126.96.36.199, 188.8.131.52). However, there is evidence of more frequent
depressions along the east coast of Australia, which may be due to local circulation
changes (Hopkins and Holland, 1997; Leighton et al., 1997). Increased numbers
of tropical cyclones over the past few decades have been reported in the southWest
Pacific region (north of New Zealand), with the greatest increases for the stronger
cyclones, but there are doubts about the homogeneity of the available database,
partly owing to improvements in observation capabilities in recent decades (Thompson
et al., 1992; Radford et al., 1996). In the Australian region (105-160°E), the
total number of cyclones from the 1969-70 to the 1995-96 seasons has decreased,
but the number of strong cyclones has increased slightly, reflecting an ENSO
influence, as has the total duration of tropical cyclones (Nicholls et al.,
Although sea-level measurements are strongly influenced by local vertical land
movement and other factors (and must be carefully interpreted), the evidence
points to an average rise in sea level in the Australasia region over the past
50 years of about 2 mm per year (Rintoul et al., 1996; Salinger et al., 1996).
This figure is within the range of the current estimate of global sea-level
rise (IPCC 1996, WG I, Section 7.2.1).
Higher CO2 concentration has direct biological effects on plants through the
increased CO2 "fertilization" effect and increased plant water-use efficiency
(IPCC 1996, WG II, Chapter A), in addition to its effect on the climate system.
For this reason, we note that present-day atmospheric CO2 concentrations are
about 30% higher than in pre-industrial times (IPCC 1996, WG I, Section 2.1.1;
Manning et al., 1996) and are currently increasing at about 0.4% per year.