One of the problems posed by the debate on global warming is the lack of detailed weather data before the second half of the 19th century. The main argument of the anti-pollution lobby is that the earth's average temperature has risen by about half a degree since 1860, and the recorded changes broadly correlate with the level of greenhouse gases in the atmosphere. The let's-keep-burning-fossil-fuels lobby, on the other hand, say that the importance of greenhouse gases has been overrated and that the earth was warming up anyway.
The question both sides have to answer, however, is what the earth's climate would have been doing without human interference, and to answer that, we need to know what happened in the distant past.
For the last thousand years, we have the evidence of recorded history. While we cannot rely on supposedly precise weather observations - there is great doubt over whether early thermometers and other instruments were correctly calibrated - there are plenty of other data that provide a picture of changing weather. Tree rings, lake levels, movement of glaciers, accounts of frozen rivers and even pollen distribution enable estimates to be made of average temperatures during certain periods.
In England, for example, the popularity of frost fairs on the frozen Thames - the last was held in 1814 - tell us that London was considerably chillier than normal during the period of the "Little Ice Age" between about 1400 and 1850. Only a moment before that - on a geological time scale - we know that grapevines were growing in Yorkshire during a warm period that lasted from about 1100 to 1300.
Proceeding back to the time before recorded history, some of the most reliable evidence comes from the ice-caps of Greenland and Antarctica. These are formed from compacted snow, each year's deposit being squashed by the following one. By drilling deep into the ice and analysing air bubbles trapped in it, a picture may be obtained of the composition of the atmosphere ages ago. One core recently drilled in Antarctica reached a depth of a mile and a half, to reach snow that had fallen some 200,000 years ago.
Further evidence comes from the presence of certain isotopes of oxygen in the ice itself, from which prevailing temperatures near the earth's surface may be calculated.
Finally, and perhaps most important of all, we have variations in the Earth's path round the sun. The shape of the elliptic orbit changes in a regular manner, with a cycle of about 95,000 years; the tilt of its axis of spin varies over a period of about 42,000 years; and the time of year at which the earth is nearest to the sun varies according to a cycle of about 23,000 years.
Putting all these data together, it seems that these cycles appear to coincide with the most extreme variations in the earth's climate. And that is why we can confidently predict a new ice age within the next 10,000 years - barring the unpredictable results of human intervention.Reuse content