Wednesday, 1 September, 2004
Leonie Joubert
Dickens's 19th century Thames was a very different river from the one the Vikings sailed up during their invasions of southern England two thousand years ago. That's because England, like the rest of Europe and North America, was caught in the grips of the Little Ice Age, which lasted from about 1500 until 1850 (when Dickens was working on David Copperfield). It's an excellent illustration of just how much the planet's climate changes over time - and explains why the bones of Arctic reindeer have been found in balmy southern France. There are many theories about this change - most of them equally controversial - but the most acceptable ones read something like this:
The Milky Way, the galaxy that is our bus-ride through the heavens, spins at about 300 million years per rotation, taking us through differing patches of cosmic dust and magnetic fields. Two phases of about 150 million years each occur within this spin - and each end of the phase will be punctuated by an ice age that will last a few million years. Think of these as the 'years' of the human calendar.
Within these the Earth's climate swings between warm and cold spells on a much faster scale, like the tick-tocking of a clock, and are called glacial (cold) and interglacial (warm) periods. Miluntin Milankovitch explained how this might occur: our orbital path around the Sun is a little more elastic than we realise. In fact it stretches from less elliptical (almost circular) into very elliptical and back again over a period of 100 000 years (think of these as the 'days'). During those times when the ellipse is great, the Earth will at times be far from the Sun and receive fewer rays.
It's also been found that it's not severe winters which trigger a mini ice age, but cold summers in the high northern latitudes: each summer sufficient sunlight must melt the winter's snow, revealing green vegetation which absorbs heat and helps regulate the region's climate. If this doesn't happen, snow and ice will accumulate year on year, and as it does so it reflects the Sun's heat, preventing crucial warming - a downward spiral into a glacial period.
In addition to these 100 000 year cycles, the spin of Earth on its axis 'wobbles' and 'tilts' slightly (think of these as the 'seconds' and 'minutes' within the climatic cycle), causing additional climatic impacts every 23 000 and 41 000 years respectively.
It hurts to think about it. But consider wintertime in the north. When the Sun's rays hit the hemisphere more obliquely, it gets less heat and cools for the season. Think of this happening on a more dramatic scale every 23 000 to 41 000 years.
We're at the tail end of the 150 million year cycle's ice age, apparently. Bill Bryson says, in his excellent book A Short History of Nearly Everything, only 10 percent of the planet's landmass is under ice now but 20 000 years ago, 30 percent was. Climate change scientists agree that we're on a naturally warming phase at the moment but argue that its happening considerably faster than it should be - and that, they say, is because of greenhouse gas emissions since the industrial era burning of fossil fuels began.
Leonie Joubert is a freelance science journalist with a special interest in climate related issues.
Bill Bryson's book A Short History of Nearly Everything was published in 2003 by Doubleday.