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
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."
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
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.