The difference between climate and weather is one of scale. Climate is what happens over decades, centuries and millennia, whereas weather occurs on the human timescale of hours, days and weeks. Climate is what you expect, and weather is what we get, and this is the rub when attributing any single extreme event, such as a flooding or a drought, to global warming.
As events in Cumbria have shown, with its record 24-hour rainfall, the weather is full of surprises. This is because it is essentially a chaotic phenomenon with a wide, natural variability. It is not for nothing that the science of chaos theory is so often illustrated by the apocryphal story of a butterfly in Brazil causing a hurricane in Florida. Small perturbations in one part of the global climate system can have far-reaching consequences for weather patterns in other parts of the world. As more energy in the form of heat enters the system, climate models run on computers suggest there will be more weather extremes.
The signs of climate change are all around us, such as the melting of mountain glaciers and Arctic sea ice. But the question is whether it has already begun to trigger the extreme weather events predicted. A growing body of expertise suggests that it has.
"The evidence is growing that damaging climate and weather events – potentially intensified by global warming – are already happening and beginning to affect society and ecosystems," says a recent joint statement by the Royal Society, the Met Office and the Natural Environment Research Council.
The heatwave in western Europe in 2003, which killed more than 35,000 people, is widely considered to have been too extreme to have been part of the natural variability of the climate. By 2040, such temperatures may have become commonplace, and could be considered unusually cool by the 2060s.
Computer models also predict that, in a warmer world, Britain will be at risk of both droughts and flooding. The apparent contradiction is explained by changes to the rainfall pattern. Summer rainfall totals have already declined, as the models predict. Yet when it does rain there is a greater chance of it falling as intense downpours.
Sea levels around Britain have risen by about 10cm since 1900 and are expected to rise further by between 11cm and 76cm by the end of the century. This, combined with the threat of storm surges and swollen rivers, increases the flooding threat to coastal towns and estuaries. But the situation is far more serious in those countries that are even more vulnerable to sea-level rise, such as Bangladesh, the Maldives and other low-lying island states where there is literally nowhere to retreat from the rising salt water.
Some scientists believe that hurricanes and typhoons will become more frequent and more intense in a warmer world, since hurricanes are triggered by high sea-surface temperatures.
One of the greatest fears is that global warming could trigger large-scale changes to weather patterns affecting crop production. There are already signs in sub-Saharan Africa that semi-arid regions are becoming uninhabitable. Some scientists fear that changing patterns in the wind and ocean circulatory systems may lead to the failure of the Indian monsoon, on which hundreds of millions of people depend for food.
"In the absence of action to mitigate climate change," says the joint statement, "we can expect much larger changes in the coming decades than have been seen so far."
The weather has always broken past records and will continue to do so. But in a warmer world, we are destined for even greater extremes and more uncertainties. Above all, we must expect the unexpected.