Up
to now it has been generally assumed that global warming will
be a linear process. However evidence from the geological past
linked with climate modelling that takes into account the global
warming that is already locked into the system indicates that
there may not be a linear response to rising CO2 levels. There
is a danger that at some point we will cross a threshhold when
global warming accelerates. By continuing to increase the amount
of CO2 in the atmosphere we are getting closer to that point.
From
the analysis of the bubbles of air trapped in ice cores taken
from the Greenland icecap that are up to 500,000 years old it
has been shown that the temperature of the Earth's atmosphere
and the CO2 content has followed a regular 100,000 year cycle
of change with the CO2 content and temperature closely linked
and following the same graph line. Within this regular cycle there
are some recently discovered very short periods of approximately
only a few hundred years duration when temperatures rise dramatically
by 8 degrees centigrade above the slower rises of up to 7 degrees
centigrade. This gives a total range of 15 degrees centigrade
from peak to trough. These records show natural processes at work
before the impact of man's activities. We are currently in the
lower temperature part of the cycle.
During
the period covered by the ice core research the CO2 content of
the atmosphere has varied between 170 and 280 ppm. (parts per
million). From 1850 to today with the added input from the burning
of fossil fuels the CO2 content has risen to 350 ppm. So we are
now well outside of the historic range of values and CO2 levels
are going to continue to rise for a long time yet and temperatures
will follow.
Recent
research has shown that the Amazon rain forest is not a stable
mature forest with growth and decay in balance but is in fact
an expanding forest that is being fertilised by the excess atmospheric
CO2. The trees are getting bigger and there is a net take up of
5000 kg of carbon per hectare per year ( 1 hectare = 100 x 100
metres ). The total area of forest is 400 million hectares so
the whole forest could be absorbing 2 billion tons of carbon per
year.
Research
in the savanah lands to the east of the Amazon Basin has established
that the crucial factor determining the development of the rain
forest is the length of the dry season. The savanah to the east
of the Amazon Basin and the eastern Amazon rain forest both receive
about the same amount of rainfall, 1500 mm per year. However in
the savanah the dry season lasts 6 months but in the rainforest
the dry season only lasts 4 months.
As
a result of the longer dry season the savanah catches fire an
average of twice in ten years whereas the rainforest with the
shorter dry season does not dry out and remains damp enough to
prevent fire. In the savanah the fires destroy most of the vegetation
and this prevents the savanah developing into a rainforest. If
the dry season in the rainforest was extended to 6 months by climate
change effects then the rainforest would dry out and burn and
could not then re-establish itself. If the rainforest burnt this
would release the CO2 currently being absorbed year by year. So
the forest would change from being a buffer which for a hundred
years has absorbed our excess CO2 into a major source of CO2 releasing
tens of years build up of CO2 in a matter of weeks.
It
is accepted by all, including climate change sceptics, that increasing
the amount of CO2 in the atmosphere will lead to increased global
warming and many computer models have been constructed giving
a rise in average atmospheric temperature of between 2 and 6 degrees
centigrade by the end of the century i.e. by 2100.
But
only now are models being constructed that incorporate a feed-back
into the model of the effects of changes in world climate due
to the changes in temperature that are predicted by the model
as the model programme runs. One of the most important effects
of climate change is the release of carbon dioxide from natural
processes as atmospheric temperature rises.
Because
of the "above normal" level of CO2 already in the atmosphere
we are already committed to a certain amount of global warming
because the excess CO2 will remain effective for many years. In
addition the continued burning of fossil fuels will continue to
add to the atmospheric burden of CO2. This warming will inevitably
cause some climate change.
The
area of the world most vulnerable to the effects of global warming
induced climate change is the Amazon basin. The climate change
models show that rising sea temperatures in the Pacific Ocean
result in less rainfall and a longer dry season in the Amazon
Basin. As described above this could lead to a reversal of the
Amazon basin acting as a CO2 sink and it becoming a major source
of CO2 returning the billions of tons of carbon to the atmosphere
that have been stored there.
If
the Amazon rainforest burns and releases billions of tons of CO2
into the atmosphere in a short period then this will be a further
boost to global warming that will result in significantly higher
end of century temperatures.
The
climate change model with climate change induced feedbacks indicates
that on present trends the date for the change from CO2 sink to
source for the Amazon rainforest is about 2050. So we have not
got much time to get CO2 under control before that might happen.
The Amazon rainforest is at present still acting as a buffer and
is protecting us from the full effect of the global warming that
would be created if all the CO2 we produce remained in the atmosphere.
For
an estimate of the significance of these effects please see Article
1a. If temperatures rise too high then there could be another
natural phenomenon which would lead to the release of methane
into the atmosphere. Methane is a very powerful greenhouse gas
Current
research based on the analysis of ancient sediments from Vladivostock
and fossil evidence from Wyoming indicates that runaway methane
global warming events have occurred in the past and the conditions
prevailing on Earth now are suitable for it to happen again. The
next article in this series will deal with this issue. It will
be published soon.
For
guidance on CO2 calculations, see Article
1a
References:
Dr.
Peter Cox, Hadley Centre, The Meteorology Office
Dr.
Antonio Nobre, Instituto Nacional de Pesquisas da Amazonia
Dr.
Carlos Nobre, Instituto Nacional de Pesquisas da Amazonia
Dr.
Geoffrey Hargreaves, US Geological Survey
Dr.
Richard Corfield, Oxford University
Dr.
Philip Gingerich, University of Michigan
Santo
Bains, Oxford University
Professor
Euan Nisbet, Royal Holloway College, University of London
also:
Channel4.com/shop
(for Equinox Book of Science)
Hadley
Centre for Climate Prediction & Research
Instituto
Nacional de Pesquisas de Amazonia (site in Portugese)
Oxford
University, Earth Sciences dept
Rainforest
Action Network
Royal
Holloway, University of London
US
Geological Survey
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