Tracking the average temperature of planet Earth is something of a tricky exercise. To uncover any tendency for warming or cooling, temperature values for the whole globe have to be averaged over both space and time; they have to be averaged over each year at each spot for which records are available, and then averaged over the whole planet for the year in question. In the end, any apparent difference from adjacent years is likely to be only a fraction of one degree.
It all takes time. But by now more than enough of this has elapsed for us to be able to look back with confidence on 1999. What exactly do we find?
It seems that on the global scale 1999 was warm but not as warm as 1998. The average temperature at the surface of the Earth last year was 0.3C above the standard benchmark, the 30-year 1961-90 average. This compares with an average global temperature 0.6C above the norm in 1998, the warmest year since instrumental records began a century and a half ago.
These figures are not particularly startling and merely confirm the early estimates of several months ago. But they continue a trend for above-average temperatures that some find ominous. A graph of the average global temperature for the past 150 years shows hardly any noticeable tendency until the end of the 19th century. Then the temperature climbed by half a degree between 1900 and 1940 before settling down again with little change until the middle 1970s.
Since then the average global temperature has risen sharply, and now the total rise since 1860 is almost one degree.
In these circumstances it is reasonable to ask if the fact that 1999 was not as warm as 1998 has some significance. Could it mean that the century-long trend for increasing average global temperature is now reversed? This is more than likely not the case. The drop in temperature can plausibly be attributed to La Nina, the antithesis of our old familiar friend El Nino, whom we blamed for all kinds of meteorological havoc a few years ago.
A La Nina episode developed in late 1998 and continued right through 1999, and during such periods the temperature of the surface waters of the tropical Pacific Ocean is typically several degrees lower than usual; this reflects itself in lower atmospheric temperatures, which in turn affect the global average.
La Nina years are nearly always cooler on a global scale than might otherwise have been expected. A relatively cool 1999, therefore, cannot be construed as heralding any reversal of the global warming trend.