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How 'feedback' can suppress the earth's ability to remove greenhouse gases

By Michael McCarthy, Environment Editor

11 October 2004

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 

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 

But all of these - except the final one - can be explained by the fact 
that they occurred in the same year as an El Niņo, 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 Niņo, 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."


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.
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