The year 1999, as we noted yesterday in Weather Eye, was cooler than 1998. But it was still an estimated 0.3 degrees warmer than the current benchmark, the 1961-90 average. In fact, 1999 was the fifth-warmest year on record, and no less than the 21st consecutive year with an above normal average global temperature. The 10 warmest years on record have all occurred since 1983, and the four warmest - those that just pipped 1999 - were all in the last decade of the century; they were 1998 at 0.6 degrees above average, and 1990, 1995 and 1997, all 0.4 degrees above the norm, give or take a little variation in the second decimal place.
All this leads to the conclusion that since 1976 the world has been warming at a rate equivalent to about 2 degrees per century, a rate unprecedented in the last 1,000 years. So when will it all end? Will planet Earth continue to simmer towards the boiling point, or will something, somewhere, turn down the heat at some point in the future?
A wide spectrum of views exists on global warming. There are some who believe that nothing very much will happen, while others might be said to belong to the catastrophe- horreur school of climatology, and cheerfully expect the worst. And even the middle ground is very wide; commonly accepted estimates of the likely rise in average global temperature range from 1.5 degrees over the next 100 years or so to a massive 4.5 degrees. There is, in a very real sense, a world of difference between the experts.
The uncertainty stems from the fact that while the greenhouse effect itself and its associated global warming are quite well understood, there are other secondary processes at work which remain something of a mystery. At present, for example, the oceans absorb more than a third of the emitted carbon dioxide, and thus act as a brake on the "greenhouse" process. But will they absorb more - or less - when they are warmer? No one really knows! And at present, ocean currents play a vital role in regulating climate; no one know what pattern might emerge if the existing thermal balance of the planet were significantly upset.
Perhaps the greatest unsolved mystery is the role of clouds in a "greenhouse world". If the world were to become a little warmer, would the wispy cirrus clouds that trap the heat increase and multiply, and thus accelerate the greenhouse process? Or would the low-lying decks of thicker clouds predominate, and by reflecting a greater amount of sunlight back to space return us to a cooler, safer world?