ABSTRACT
The changes in the average air temperature near the earth’s surface over the past century or so, as established from the instrumental record, are shown in Fig. 2. Over this period this temperature has increased by somewhat more than 0-5°C, although the increase has not been uniform. There are strong indications that the increase since the 1970s is linked with the growth in the atmosphere of greenhouse gases such as carbon dioxide from anthropogenic sources. The 1990s have been particularly warm in terms of this global average temperature. Not only was 1998 the warmest year on record, but the first eight months of 1998 were the warmest of those months on record. Note also the year to year variations that are a further illustration of natural climate variability. (In Fig. 6 is shown a record constructed from proxy data for the last millennium, showing that 1998 is also likely to be the wannest year in the northern hemisphere over the last millennium.)
Svante Arrhenius, a Swedish chemist, made the first calculation of the average rise in temperature to be expected at the earth’s surface if the atmospheric carbon dioxide concentration should double. His estimate of 5 or 6°C was not far out, just a little larger than current estimates that fall in the range 1-5 to 4-5°C.
