"GLOBAL warming is no longer some distant threat in the next century but has already begun according to 2000 of the world's expert climate scientists meeting in Rome. . . . It means that the world is beginning to change radically. The average temperature and sea level will rise faster than at any time in the last 10,000 years, posing a risk to plants and animals trying to adapt."

Recent news? No. That was in Guardian Education, December 1995. Global warming has been on the school curriculum for years. It's just taken a long time for business and politicians to notice it. And the current "wake-up" call may be too late.

"The present is the key to the past" has been the maxim for geologists since Lyell's "Principles of Geology" was published in 1830-33. His tenet of "uniformitarianism" stated that geological remains from the past can be explained by reference to geological processes now in operation. The same principle had even been used by Leonardo da Vinci when he noticed that fossils on the top of mountains could not have been put there by the biblical "Flood."

However, searching deeper into the rocks in the twentieth century, it became quite clear that the Earth has not always been in the same state. It has gone through an evolution where the core has changed, the earth's magnetic poles have switched, and the great plates of the Earth's crust have moved and indeed resulted in the making of the continents. The atmosphere has been affected too: the length of day has increased and the chemistry of the atmosphere and oceans has radically changed since around 3,460 million years ago, when the first life was recorded. The Earth has always gone through a series of steady changes, but also through a series of catastrophes as well.

Ice Ages

There was a great worry in the 1970s that an Ice Age was imminent. That was also with great justification.

Ice Ages have affected Earth in the geological past. Glacial periods have been identified from the rock record over 2000 millions years ago, and 'Snowball Earth' may have been created in the great Sturtian-Varangian glaciation around 850-635 Ma. Glaciation is thought to have been extensive in global terms from the evidence of widespread tillites (glacial deposits) and evidence of ice age conditions which seem to have extended even into low latitudes A key factor in Ice Age initiation is the location of the land masses on the globe. Ice can only accumulate to great depths when a continent sits on a pole, like Antarctica today. It is not possible to lock up such vast quantities of water in sea ice. The present Ice Age, in which we still live, began around 2.4 million years ago with the growth of the Antarctic Ice Sheet, and is characterized by cycles of glacial advance and retreat. It is often stated that the last so called 'Ice Age' 'ended' only 10,000 year ago in Europe. Of course that was just the beginning of the last interglacial.

Ice Ages are not continuous. They are punctuated by warm periods controlled by variables such as sunspot activity, the tilt of the earth's axis towards the sun affecting the amount of heat we get, and the shape of the earth's orbit around the Sun itself. It is also now known that CO2 and CH4 concentrations changed considerably during past glacials and interglacials. During the interglacial periods there was as much as 21% more carbon dioxide, and 100% more methane. So what might happen if we were to enhance this process?

We know that even in recent centuries in the UK there have been fluctuations in temperatures which have resulted in Little Ice Ages and conversely warm periods when grapes could easily be grown. Some of these cool periods were caused by events which affected the atmosphere - outpourings of volcanic gases and dust which affected the lower atmosphere and even the stratosphere so that insolation (radiation from the Sun) was limited and the surface of the earth was cooled.

The difference now is that, in a very short time (and even a short time in human history) WE are the cause of rapid climate change which is over-riding the natural cycle. Worse still, this change comes at a point in human evolution where the population and its associated domestic animal populations are dangerously high.

At last there is a discussion by governments, and in the popular press, about population and global warming. Can the Earth sustain its current population of around 6000m at current levels of consumption? The answer in the long term is no. Not unless we adapt fast.

Thirteen percent of the population of the USA have not heard of global warming - an odd statistic quoted in a newspaper last month. Anyone who reads the press, even takes a cursory glance at the news or goes to school can't avoid the excellent expositions which must surely convince even George Bush that global warming is no longer a theory - it's a measurable fact.

All that plant and animal material conveniently locking up biogenic carbon in the rocks formed millions of years ago is now being pumped out as industrial nations burn coal and oil in unprecedented quantities. Reflect on the fact that the evolution and colonisation of the planet by plants actually created our oxygen - and subtracted CO2 over millions of years, millions of years ago. That process is now rapidly being put into reverse by us in a flurry of human activity spanning perhaps less than 200 years. What are we doing to the planet?

Since the mid nineteenth century, the burning of fossil fuels has increased carbon dioxide levels from a pre-industrial level (c. 1850) of about 280 ppm to 379 ppm in 2005. Increases are projected to reach more that 560 ppm before the end of the present century and, along with the effect of other greenhouse gases, temperature rise of 1.4- 5.6 degrees C is anticipated Modern forests continue to be burned and, perhaps less well known, the humus once locked up in the soils of the Earth is also being unlocked and released as CO2 by modern agricultural practices. It is being biodegraded in vast quantities, and not replaced by modern farming practices which 'mine' the soil - practices which exploit soil nutrients without the return of biogenic carbon.

Add to that the excess methane pumped out by the vast amounts of livestock kept, the methane emitted from rice paddy, HCFS in aerosols and those old fridges, low level ozone (also created by us and also a greenhouse gas) and one can see that there must be a cumulative effect. Other chemicals we take for granted are also culpable - halons which were in fire-fighter foam, and nitrous oxide from the breakdown of nitrogen fertilisers. In fact we are 'mining' the whole earth with little regard for the functioning of the system.

How does the 'Greenhouse Effect' work?

We do actually need a 'greenhouse effect' on Earth. The natural greenhouse effect keeps us all warm. Without the atmosphere and the greenhouse effect it causes, one calculation has put the planet around 30 degrees cooler. Daytime temperatures would be burning because there would be no atmosphere to protect us, and night time would rapidly fall to way below zero everywhere on the dark side with nothing to stop radiation from the Earth rapidly escaping to space.

It's not just CO2 which is a greenhouse gas. Water vapour, nitrous oxides, methane and other man-made gases all have the property of trapping energy. Some are more effective that others: methane is estimated as having the ability to trap 23 times more heat than CO2, and nitrous oxides 296 times more.

The atmosphere is often referred to as a 'blanket of gases' which keeps the harmful rays of the sun out, and also keeps the warmth in. It's a good analogy. But how does it actually work?

One key factor involved in the heat budget of the Earth is that the energy directed at the planet does not all get in. Around 33% of it is reflected back to space from the upper atmosphere. So, one could argue that a cloudier earth could mitigate the greenhouse effect and even reduce it (an example of negative feedback restoring the system to stability). On the other hand, temperatures could increase because of the 'blanket effect' - the extra water vapour in the clouds helps keep the heat in. In this case, it is an example of positive feedback pushing the system into a different level of activity. It's difficult to say which way the scales will tip but, on balance, positive feedback loops are thought to outweigh the negative loops which restore stability in the atmospheric system. It is because of the prevalence of the positive feedback loops that a tipping point, in the not too distant future, could become a reality.