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 3.2.2.1). 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 3.5.2.3, 3.5.3.1, 3.5.3.2). 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., 1997).

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.

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