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Figure 15.11 Annual mean temperatures at Santiago (33°S) and Punta Arenas (53°S) in Chile.

Figure 15.12 The variation of the 'cumulative deviation' of the annual-mean temperature from the long-term average of 17.6°C at Sydney, as in Figure 10.10.

from 26°S to 28°S, indicating an expansion of (warm) low-latitude climates. In accordance with this is the increase of summer rainfall over eastern Australia since 1953.

Reliable indications of global warming come from outside Australia too. New Zealand's surface temperatures show a rise of about 0.7 K since 1900. Sea-surface temperatures at 43°S off Tasmania have risen steadily since 1949, and a similar rise has been observed off Auckland in New Zealand.

Almost all the world's glaciers are in retreat. For instance the Franz Josef glacier in New Zealand shrank rapidly between 1935 and 1980, except for a few short-lived advances (Figure 15.13). A glacier at nearly 5 km elevation on Mount Jaya near the equator in Irian Jaya has almost disappeared, with a rise of the snowline from 4,400 m to 4,750 m between 1936 and 1993. The sea-level rose by 13 cm between 1910 and 1980, chiefly as a result of a rise of global sea-surface temperatures by about 0.5 K, with consequent thermal expansion (Note 11.B). There have been increased temperatures at Santiago in Chile, though not at Punta Arenas (Figure 15.11). Thirteen of fifteen weather stations around Antarctica have shown warming, the overall figure during the period 1956-88 being about 0.3 K/decade.

Unfortunately, establishing long-term trends is still complicated by year-to-year fluctuations. Apparent changes of climate during the past century have to be judged against the background of normal inter-annual variation. A typical standard deviation of annual mean temperatures of about 0.5 K (Section 3.2) makes it difficult to be confident about trends of smaller magnitude. Also there is the confusion caused by much larger warming temporarily in some places, as a result an El Niño, e.g. by 6 K along part of the South American Pacific coast in 1982/83, raising the global mean temperature in 1983 by 0.2 K above values in the four surrounding years. Conversely, the eruption of the volcano on Mt Pinatubo in the Philippines in June 1991 produced a veil of dust in the lower stratosphere, reflecting more sunlight than normal and therefore cooling the Earth during the following twelve months or so (Note 2.G), once more obscuring any warming trend.

Cloudiness and Rainfall

1860 1880 1900 1920 1940 1960 1980 Year

Figure 15.13 The position of the snout of the Franz Josef glacier in New Zealand, showing the distance of retreat since 1868.

1860 1880 1900 1920 1940 1960 1980 Year

Figure 15.13 The position of the snout of the Franz Josef glacier in New Zealand, showing the distance of retreat since 1868.

There has been an increase of cloudiness in many places. For example, average cloudiness over Australia increased from 39 per cent at the beginning of the century to 49 per cent in 1980 (Figure 15.14). Such an increase globally could be explained as due to more evaporation from warmer oceans. The effect of increased cloudiness (at low levels) would be to offset some of the greenhouse warming by reflecting solar radiation away to space, though increased humidity at high levels acts as a greenhouse gas, enhancing the warming. The matter is receiving much study.

There have been erratic changes of rainfall over the last century (Figure 10.8), so that the standard deviation of annual rainfalls is generally much more than any trend. Nevertheless, some

Figure 15.14 The increase of cloud in Australia.

changes are discernible. Much of southern Africa has experienced a drying trend since 1950, whereas South America has received more rain (Figure 15.15). Summer precipitation in southeast Australia increased from 1895 for the next fifty years, with an opposite trend of winter rainfall. The annual number of raindays has increased in south-eastern New South Wales (Table 15.4).

Part of the explanation of recent changes of rainfall is the abrupt alteration in the character of ENSO events since the 1970s. Prior to that, La Niña episodes lasted about as long as El Niño episodes, but El Niño episodes since 1977 are more frequent and more persistent, with only occasional excursions into the La Niña phase. The continuously negative Southern Oscillation Index from 1990 to 1995 was unprecedented in 120 years of record.

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