Earth Sciences / 49which temperatures are measured are higher above the sea than they used to be. In any case, ships’
thermometers are not calibrated against a standard.
Balloon readings are much more reliable. Weather balloons are released twice every day, usually
at noon and midnight Greenwich time, from about 1000 sites. These sites are located
predominantly in industrial countries, however, so records derived from them may not be typical
of the world as a whole.
Satellite measurements are by far the most reliable of all. More than 30000 measurements are
made every day.
Measurements from surface stations show temperatures are rising by 0.15°C per decade from 1979
to 1997. This is a much smaller increase than the IPCC ‘best estimate’ of 2.5°C. Over the same
period, NOAA weather balloons show temperatures are falling by 0.07°C per decade and balloon
data from the UK Meteorological Office show them falling by 0.02°C per decade. The satellite
measurements show them falling by 0.01°C per decade (science.nasa.gov/newhome/headlines/
notebook/essd13aug98_1.htm). The mean temperature measured by satellite since January 1998 has
risen by about 0.04°C per decade. This warming was caused by the very strong El Niño event in
- The warming is still very much smaller than that predicted by the IPCC.
The effects of climate forcing are being studied by teams in several countries and their results are
drawn together into a scientific consensus by the IPCC. This body, involving hundreds of specialists
from all over the world, was established in 1988 by the World Meteorological Organization and the
UN Environment Programme to advise governments. By no means all climatologists agree with the
IPCC conclusions, however.
Governments became involved following a meeting held at Villach, Austria, in 1985 under the auspices
of the International Council for Scientific Unions (ICSU, now called the International Council for
Science). There, research scientists, including ecologists and experts on climate and energy-demand
modelling, concluded that global warming was a real threat and more research was needed and,
supported by environmentalist groups, the topic quickly acquired political influence. This politicization
and resultant popular dramatization of a very complex and uncertain issue has attracted criticism
(BOEHMER-CHRISTIANSEN, 1994; BOLIN, 1995).
Studies of climate forcing begin with estimates of ways in which the chemical composition of the
atmosphere may change in the future, to produce an ‘emissions scenario’. This requires a knowledge
of the sources from which greenhouse gases are released, the sinks into which they are absorbed, and
ways the sinks may respond to increased loading. The oceans are the most important sink for carbon
dioxide, but the behaviour of the sinks is incompletely understood and no sink has been identified for
a significant fraction of the carbon dioxide known to have been emitted. Measurements of greenhouse-
gas concentrations must also distinguish between genuine changes, the ‘signal’, and natural variations,
the ‘noise’. Carbon dioxide levels vary seasonally, for example, in response to the growing season
for plants.
General circulation models (GCMs) are then constructed. These are based on a notional three-
dimensional grid placed over the entire Earth. Atmospheric behaviour is calculated according to
physical laws for every grid intersection. The input data for each calculation include the state at
adjacent grid points as well as data introduced by the modeller, and so they trace the evolution of the
atmosphere, simulating the climate. Using the known present state of the atmosphere, the model is
used to simulate the climate over several decades and its results compared with actual climate records.
If this test proves satisfactory, changes in atmospheric composition, based on the emissions scenario,
are introduced to the model and their consequences evaluated.