Within recent glaciations there has been a broad rhythm, with more than 20 glacial periods interspersed with 10,000- to 15,000-year interglacials like the present. Apparently, 125,000 years ago, there were hippopotamuses in what is now Trafalgar Square. And 18,000 years ago, tundra in the same place was trodden by mammoth, woolly rhinoceros and reindeer.
Until recently, it was generally believed that all climatic change was slow. Now we are less sure. The evidence of cores drilled through parts of the Greenland and Antarctic ice-caps suggests a series of cold and warm spells that could have altered temperatures by as much as 10C over the course of only a few years. But the story of the last 12,000 years - the present interglacial - is of broad stability.
But climate change is now being affected by human interference. Since the industrial revolution, we have been using the sky as a waste unit, thereby enhancing the natural greenhouse effect with emissions of carbon dioxide, methane, nitrous oxide, chlorofluorocarbons and related molecules into the atmosphere.
In its most recent assessment, the Intergovernmental Panel on Climate Change suggested rises in average global temperature of between 1C and 3.5C by the end of the next century (an average rate of warming greater than any experienced in the last 10,000 years), and an average rise in sea-level of up to half a metre in the same period.
There remain many uncertainties, which make it difficult to quantify the risks involved, or the regions most likely to be affected. It is also difficult to disentangle natural from human-induced variability, but whatever the human contribution to global climate change, average world surface temperature is rising.
This rise may eventually cause tropical forests to die back and transform grassland into desert. Water resource stresses would be exacerbated, and some 66 million extra people would live in countries with water stress, and some 170 million people would live in countries which are extremely stressed.
Crop yields would increase in high- and mid-latitude countries, such as Canada and Europe, but decrease in lower latitudes. Although globally the food system would accommodate regional variations in yields, some regions, particularly the Tropics, would experience marked reductions in yield, lower production and higher risk of hunger. Africa would be worst affected.
Global sea-level rise by the 2050s was predicted to be 21cm. More than 20 million extra people each year could be at risk of flooding due to sea-level rise. South and South- East Asia were most vulnerable regions.
We should also consider the impacts of climate change on human health - which could be widespread. Micro-organisms respond rapidly to changes in temperature and moisture. Humans need roughly 20 years to reproduce, but bacteria can do so in 20 minutes. Old diseases, such as malaria, could return; and new ones could easily arise.
How, then, can we make a change? It sometimes seems impossibly difficult to do so, but change usually takes place for three main reasons.
Leadership can make a difference. Margaret Thatcher gave clear leadership on climate change. She was always ready to challenge the conventional wisdom, as did Al Gore and Mikhail Gorbachev.
Public pressure can also make a change. Many thought that Greenpeace's opposition to the dumping of the Brent Spar oil platform was exaggerated. But the results were positive: greater public awareness of our common global inheritance, and the need to protect it.
Useful catastrophes may also act to jerk us out of our normal inertia. Such catastrophes could include drought or flood, a major sea-level rise, millions of refugees on the march, and most likely creeping social and economic breakdown, such as can already be seen in several parts of Africa.
None of us would want to run risks of this kind if we thought they were real. But to some, they look real enough already. Scientists can illuminate them, and put them into perspective; journalists should report them to educate the public; and politicians must exercise the judgement for which we elect them. Business should take account of them and exploit the huge commercial opportunities. Everyone will have to play their part.Reuse content