teaching - Earth Science Teachers' Association

TEACHING EARTH SCIENCES ● Volume 31 ● Number 4, 2006
The Science of Global Warming
PROFESSOR COLIN PRENTICE
Peter Kennett’s notes on the lecture at Bristol
Conference
Changes in the Earth System are approached at three
levels:
● Detection i.e. it can be demonstrated that a change
has happened.
● Attribution i.e. the cause of the change can be
demonstrated.
● Prediction at three levels; a deduction can be drawn
that should be true in general; a statement that if x
happens then y will follow; a statement that x will
happen, i.e. a forecast.
The media do not always distinguish between these different
levels in reporting changes to the Earth!
Evidence for carbon dioxide content of the atmosphere:
● Carbon dioxide levels at Mauna Loa and at the South
Pole are rising in parallel.
● Ice cores from the Antarctic show stable levels of
CO 2 from 1000 A.D. to the start of the Industrial
Revolution around 1800, followed by a rapid rise.
● The Vostok (Antarctica) ice cores show an oscillation
in CO 2 content which can be correlated with glacial
and interglacial epochs.
Carbon stores in the Earth system are vast, but by far
the greater part lies in the lithosphere, and not simply in
fossil fuels. In the surface carbon cycle, on average,
every molecule of CO 2 goes through a plant every 6
years!
The amount of CO 2 in the atmosphere roughly correlates
with fossil fuel emissions, but not completely.
Carbon flux between ocean and atmosphere and
between land and atmosphere must also be taken into
account, although the factors are difficult to quantify
exactly at present.
The conclusion is that the rise in CO 2 since the
Industrial Revolution has been detected: It can largely
be attributed to rise in emissions from the burning of
fossil fuels: Predictions for the future are dubious.
Gases other than CO 2 also contribute to climate
change. They include methane, nitrogen oxides, CFCs,
water vapour and tropospheric ozone etc. Although less
in quantity, their radiation efficiency is very much
higher than CO 2 , so a little goes a long way in affecting
the average temperature of the Earth.
Aerosols (tiny particles in the atmosphere) also have
an effect on global temperatures. Dust and soot absorb
heat, but sulphates, sea salt and some organic compounds
reflect it back into the upper atmosphere, thus
having a cooling effect.
A reconstruction of past global temperatures from
1000 A.D. shows a generally level graph, with perturbations
attributable to solar events and to volcanic eruptions,
but since the 1980s it shows a dramatic rise. The
resultant shape of the curve is referred to as a “hockey
stick” graph.
So, temperature rise since the 1980s has been
detected and it has been attributed to human “forcing”
(accepted by the Intergovernmental Panel on Climate
Change in 2001). Predictions are very difficult
to make with any confidence, because of the uncertainty
about the effects of aerosols. For example, governments
are encouraging the reduction in emissions
of sulphates for other good reasons, and yet it is sulphates
in the atmosphere which help to keep down
rises in temperature.
Conclusions
● In order to stabilise climate, CO 2 emissions must be
reduced to very low levels.
● Large uncertainties arising from the other factors
outlined above must be reduced.
● The consequences of climate change need to be better
quantified – we still do not really know how
much global warming is dangerous for the vast range
of ecosystems and human activities on the planet.
A copy of the PowerPoint presentation that Professor
Prentice used to illustrate his lecture with at the ESTA
Conference is available on the ESTA website.
Peter Kennett
peter.kennett@tiscali.co.uk
www.esta-uk.org
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