11 October 2004The Independent
The possibility of a "feedback" impetus to global warming, greatly accelerating the process of climate change and obliging us to rip up our present forecasts as too optimistic, lies at the heart of concern about the recent rise in the rate of accumulation of carbon dioxide in the atmosphere.
Feedback is what happens when a part of the output of a process or a system returns to affect the input. Negative feedback, which occurs when what comes out lessens the strength of what subsequently goes in, tends to suppress the original process (this is what happens with the valve regulating a steam engine).
Positive feedback, on the other hand, which occurs when the output goes back to add force to the input, can magnify the whole process until it takes on a "runaway" character.
The fear of climate scientists is that just such a positive feedback might occur with global warming, in which the warming itself precipitates changes in the earth's natural systems, which themselves cause additional warming, which then causes further changes, and so on, in an unstoppable acceleration.
This fear is well founded, because records of ancient climates deduced from cores driven deep into the polar ice show that this has happened in the past: previous episodes of warming at the end of ice ages have indeed developed a runaway character, with enormous temperature rises of as much as 10 degrees centigrade in 50 years.
There are a number of possible global warming feedbacks, but the major fear associated with the current jump in CO2 is that the ability of the earth's forests and oceans to remove massive amounts of it from the atmosphere may be compromised.
For of the 6.7 billion tons of CO2 that human society is currently putting into the atmosphere each year from the burning of coal, oil and gas, only about 58 per cent remains there.
The remaining 42 per cent is absorbed in the earth's so-called "carbon cycle" - some of it by plants during photosynthesis, the means by which they grow, and some of it by the oceans, in which CO2 is soluble.
For a range of biological and chemical reasons, both of these processes may slow down in a warming world, leaving even more CO2 in the atmosphere.
That such a process may now have started is the fear sparked by the recent rise in the rate of atmospheric CO2 accumulation, shown in the Mauna Loa data.
Over the past 50 years, that rate has generally moved in parallel to the steady growth in the global consumption of fossil fuels - as might be expected, and as illustrated in the graph on the page opposite.
As the graph shows, in some years there have been sharp peaks, when the growth rate of atmospheric CO2 has shot above that of fossil fuel consumption.
But all of these - except the final one - can be explained by the fact that they occurred in the same year as an El Niño, the disruption of the atmosphere-ocean system in the tropical Pacific, which alters weather patterns around the globe in such a way that the carbon cycle takes up less CO2.
The sinister aspect of the most recent peak, on the right of the graph, is that it does not coincide with an El Niño, and there is no sudden huge jump in CO2 emissions to explain it either. The suggestion, therefore, is that it may represent the beginnings of a feedback - or as Dr Charles Keeling put it at the weekend: "The beginning of a natural process unprecedented in the record."
THE MESSAGE FROM HAWAII
A man and a mountain are the two key elements in the story of the world's gradual awakening to the threat of global warming.
The man is the US physicist, Charles Keeling, and the mountain is Mauna Loa, an 11,000ft extinct volcano on the Big Island of Hawaii.
In 1957 as a 30-year-old post-doctoral research student at California Institute of Technology, Dr Keeling was invited by one of his professors to measure the atmosphere's CO2 content as part of International Geophysical Year.
Mauna Loa, where the US weather service had an observatory, was ideal, as it was thousands of miles from any major landmass and pollution interference was minimal.
In 1958, Dr Keeling and his team found that CO2 was present in the air at a level of 315 parts per million (ppm), not hugely above the natural "background" level of before the industrial revolution, estimated at about 280ppm. However, the remorseless growth in fossil-fuel consumption as the global economy has mushroomed has produced an equally remorseless rise in the CO2 level. It now stands at 376ppm.
