In an extract from his new book on global warming, Paul Brown looks at how close the planet is to irreversible damage

Guardian Unlimited

The phrase "tipping point" is heard a lot more from scientists. This is where a small amount of warming sets off unstoppable changes, for example the melting of the ice caps. Once the temperature rises a certain amount then all the ice caps will melt. The tipping point in many scientists' view is the 2˚C rise that the EU has adopted as the maximum limit that mankind can risk. Beyond that, as unwelcome changes in the earth's reaction to extra warmth continue, it is theoretically possible to trigger runaway climate change, making the earth's atmosphere so different that most of life would be threatened.

As with a lot of climate science, what used to be theory is now being seen in practice on the ground. New information makes clear that reaching the tipping point is a much more immediate threat than was previously thought.

The danger grows with the increase in average temperature above what is called the pre-industrial level - the mid-18th century. Some scientists estimate that when the temperature reaches an extra 2˚C above that equilibrium the earth's natural systems will be in serious trouble. It will affect many species' survival prospects, including our own.

Too close for comfort
So the key question is how close are we to a 2˚C rise, and when will we get there? The first thing to admit is that nobody knows for sure, but many who understand the science say the answer to this twin question is, first, that we are already very close, and second, we might get there terrifyingly soon. In fact the 2˚C threshold is much closer than almost anyone outside the specialist scientific community is prepared to acknowledge. By any standard, if you care about the future of the human race, it is too close for comfort. So to the vital question of when we might reach 2˚C above pre-industrial levels; in other words how much time do we have to curb our excess emissions? Warming is directly related to the quantities of greenhouse gases there are in the air, the chief of which is carbon dioxide.

Concentrations of carbon dioxide in the atmosphere are already at 382 parts per million (ppm). That is up from the pre-industrial level of 280ppm, a considerable increase. To get that in perspective we need to realise that the 280ppm figure had remained more or less unchanged for 10,000 years, the period which accounts for the entire span of modern human history. The benign climate that has allowed the human race to multiply, develop and prosper has remained stable through that period.

There have been minor variations: warm periods that allowed places like Greenland to be settled by the Vikings or mediaeval monks to make wine in Britain, and cold periods, known as mini-ice ages, that made it possible to have frost fairs on the frozen Thames in London during the 17th and 18th centuries. The last one was held in the winter of 1814.

These so-called natural variations in the climate have allowed those trying to rubbish global warming theories plenty of ammunition. But those changes have now been well studied and are better understood. It is no longer credible to suggest that what is happening now is a natural variation of a sort recorded in the last 2,000 years. In fact the variations in the quantities of carbon dioxide in the atmosphere have been small in that period, and other natural variations like sunspots have been the culprits for the previous warm and cool periods. The recent increases in greenhouse gases have changed all the rules and the stability in the climate system man has enjoyed so long.

Current calculations suggest that if and when the level reaches 450ppm there will be a 50% chance of the earth's temperature exceeding a rise of 2˚C - in other words an even chance of potentially catastrophic climate change. To be on the safe side (the so-called precautionary principle, which so many politicians claim they endorse) some scientists believe that the carbon dioxide in the atmosphere must be pegged back to 400ppm - a mere 18ppm above the current level. So, on their current calculations, since man began the industrial revolution, and unwittingly an experiment with the climate, the human race has already got more than 80% of the way to causing a potential disaster.

Drastic action
On this evidence it is clear that drastic action is needed. Some scientists have certainly been urging politicians to take urgent and immediate action. Recent evidence demands, according to a consensus of the world's best climate scientists, that we need to cut existing emissions by between 60% and 80% in the next 40 years to stand a chance of preventing climate change becoming unstoppable, and keeping control of our own destiny. Compare that figure with that achieved by the Kyoto protocol, to date the best effort by politicians to cut emissions. This will cut greenhouse gases from 34 of the developed countries by 5.2%, excluding the world's biggest polluter, the United States. Over the period of the agreement, which lasts only until 2012, total world emissions will rise because of the growing industries of the developing world.

What does the science tell us about how much time we have left to solve the problem? Measurements taken by Nasa Goddard Institute for Space Studies and Columbia University Earth Institute New York, released in December 2005, show that in the last 100 years the world's average temperature has increased by 0.8˚C. That seems to leave a comfortable 1.2˚C to go before the tipping point is reached, but this is where the climate plays a nasty trick. Unlike glass in a greenhouse, the extra heat-trapping gases released into the air take time to build up their full effect. This is largely because of the delaying effect of the cool oceans as they catch up with the atmosphere.

Best estimates are that there is a 25- to 30-year time lag between greenhouse gases being released into the atmosphere and their full heat-trapping potential taking effect. That wipes out any feeling of comfort. It means that most of the increase of 0.8˚C seen so far is not caused by current levels of carbon dioxide but by those already in the atmosphere up to the end of the 1970s.

Still worse, the last three decades have seen the levels of greenhouse gases increase dramatically. In this 30-year period the earth has seen the largest increase in industrial activity and traffic in history. This great burning of fossil fuels has also coincided with the mass destruction of rainforests. So on top of the extra heat we are already experiencing there is another 30 years of ever-accelerating warming built into the climate system.

Slideshow: How climate change is affecting the planet