As we noted yesterday in "Weather Eye", there is considerable uncertainty about what exactly the consequences of global warming may turn out to be. Like Tennyson's besotted, tragic heroine, we just get on with life:
She knows not what the curse may be,
And so she weaveth steadily,
And little other care hath she,
The Lady of Shalott.
But could it be that the mirror has already cracked? Maybe we have already gazed at Camelot, and perhaps our lines should be:
"The curse has come upon me!" cried
The Lady of Shalott.
Instrumental records tell us that the average global temperature has increased by about half a degree Celsius since 1860, that mean sea level has risen by about 20 cm, and that mountain glaciers have retreated to a significant extent.
But are these changes due to the greenhouse effect or to something else? One of the problems about answering this question is that the Earth's climate changes naturally over the decades, for a variety of reasons, and it is very difficult to establish if any observed increase in temperature might have taken place anyway, without any help from humankind.
Those who favour the latter view point to the fact that much of the warming during the last century and a half occurred between 1910 and 1940, well before the largest rise in greenhouse gas concentrations took place.
Moreover, for reasons of which we are not entirely sure, but possibly because of variations in the output of energy from the sun, there was a long cool period during the 16th, 17th and 19th centuries - the so-called "Little Ice Age" - when the northern hemisphere was considerably colder than it is today. Much of the warming over the past century, some argue, may simply be a recovery from that time.
Unfortunately scientists have no direct way of observing what would have happened if humanity had left the climate undisturbed; no direct way of comparing the greenhouse "signal" with the background "noise" of natural climate variability.
But this background variability can be estimated by running climate-change computer models with greenhouse gas concentrations held constant at some specific level, rather than assuming that they will increase steadily, as in fact they have done.
Results from such experiments, analysed statistically, indicate that some 20 to 30 per cent of the warming since 1860 may well be a recovery, as it were, from the Little Ice Age, but that the full rise of half a degree Celsius is unlikely to have been a chance fluctuation due entirely to the natural variability of our climate.