We are in winter now, in London, and we enjoy or suffer the seasons (according to individual taste) because of the earth’s tilt. At 23.5⁰ the tilt of the earth enables the earth to receive more concentrated light in the summer months and les concentrate light in the winter months. If the earth titled a few degrees more the summer months would probably get more concentrated light further North and the North Pole would be ice free. If the earth titled less there would be less ice melting at the poles in the summer and if the earth did not tilt at all then the seasons would only occur to the extent that the slight elliptical orbit provide less concentrated daylight and more concentrated daylight according to the distance of the earth from the sun.
The tilt has not always been 23.5⁰; it seems to vary between 22.1⁰ and 24⁰ every 40,000 years. This accounts for some of the long term climate change that the earth has experienced. The orbit of the earth also seems to vary from a near perfect circle to an ellipse over periods of 100,000 years. There are other natural forces on the climate – the earth is not a perfect sphere and this leads to irregular light distribution.
On the whole the earth’s climatic pattern was to enjoy warmer periods interrupted by glacial periods. The last glacial period was 13,000 years ago and lasted for about a thousand years. There have been warmings and coolings since then. Greenland was warm enough to be farmed between 1000 and 1400 AD when vines grew in England and locusts descended on crops in Europe. Early in the fifteenth century temperatures fell. Settlement in Greenland was abandoned and cold wet summers created famines. This Maunder Minimum was not a little ice age, as it is sometimes referred to, but something relatively modest in climate change terms. At the time it occurred people found it hard to cope, and much suffering was caused when the winters became bitterly cold, but this was, as far as the big picture of climate change is concerned, no more than a blip that lasted less than two hundred years. As far as we can measure, the average surface temperature of the earth dropped by only one degree, possibly less.
All of the forces that create our climate and our seasons are complex; a change in the tilt of the earth will mean that some places become warmer while others become colder; rising or falling average temperatures will affect sea currents which play an important part in our climate. Sea current changes will warm and cool different places. As ice melts so the albedo lessens causing the earth to absorb more heat than it used to and creating warmer air temperatures. Sun activity, magnetic activity and gravitational forces of other planets and the moon all make or change our climate These are natural and have happened in our planet over hundreds of thousands of years.
But there is one factor which is new and it is not natural. As far as we can measure carbon dioxide concentrations 155,000 years ago were 280 ppm. They then fell gradually to about 210 ppm 20,000 years ago and since then have been rising, slowly as they do and as they rise and fall average atmospheric temperature has risen and fell with them. Since the industrial revolution atmospheric carbon dioxide has risen dramatically. It continues to rise. 300 pmm has been long past. In November 2010 it was measured at 388.59ppm and in November 2911 it was measured at 390.20ppm.
These new high levels of carbon dioxide will almost inevitably cause the planet to warm up even further and the famous two degrees of warming level, which many hoped to hold the warming at, will inevitably be exceeded. When this happens remains to be seen, but it bodes ill for our descendants.