There appears to have been erratic global warming between 1900-40 (Figure 15.10),
unlikely to have been due to any enhanced greenhouse effect, as fossil-fuel consumption was modest compared with present rates and the CO2 concentration was hardly changed from about 310 ppm. Warmth around 1920 may have been associated with a reduction of the atmospheric 'dust veil' caused by volcanoes (Note 2.G). But New Zealand experienced a relatively cold period from 1900-35, so changes were not uniform.
There followed a cooling by about 0.3 K between 1940 and 1970 in the northern hemisphere, probably due to increased pollution of the air by sulphates from the burning of coal. Sulphates convert to sulphur aerosols, which act as cloud condensation nuclei (Section 8.2), leading to more cloud droplets, i.e. a higher cloud albedo, so that more solar radiation is reflected away. This explanation is supported by the absence of any cooling trend between 1940 and 1970 in the southern hemisphere (Figure 15.10), where much less sulphate was created. Another explanation is that the apparent cooling is not real, but due to the relocation of many weather stations to airports during the 1950s, from within cities, which are often warmer (Section 3.7).
Frosts were more frequent at Quito (on the equator in Ecuador) during the period 1940-60, and temperatures remained steady or fell at Punta Arenas (53°S in Chile) until 1972 (Figure 15.11). Tree rings in Tasmania show a decline of summer temperatures during the period 190046 and then an equal rise by 1970, which continues (Note 10.D).
Surface temperatures have increased lately at most places on Earth, especially since 1979. From that date, the rise has averaged 0.09 K/ decade globally, and in eastern Australia about 0.2 K/decade between 1950 and 1990. The rise seen in Figure 15.12 for Sydney, especially since 1966, may be due partly to urban growth (Section 3.7).
The average warming during the period 1951-93 was 0.15 K/decade in the southern Pacific region, being more in winter than summer, and at night than during the day. The annual mean daily minimum rose over the forty-two years, by 0.4 K at places in Queensland away from the coast, and by more than 0.2 K over half the continent. The result is that daily ranges of temperature are now less, as in the case of South Africa (Figure 3.15) and elsewhere.
There has been a falling number of frosts within Australia, especially in outback Queensland. Tree rings in west Tasmania show unprecedented growth since 1965. Also, there was a latitudinal shift during the period 197592 of the subtropical jet in the upper troposphere,
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