80 / Basics of Environmental Science
cooler periods lasting several centuries or a few thousand year. (GROOTES ET AL., 1993). These
oscillations have since been linked to changes in ocean circulation (ZAHN, 1994).
Ancient climates are reconstructed mainly from evidence obtained from ice cores, those refer-ring
to the Eemian Interglacial and the glaciations to either side having been obtained from Greenland.
Ice sheets form by the compaction of snow under the weight of overlying snow, so the ice forms in
seasonal layers that can be dated by counting, much like tree rings. Temperatures are inferred by
oxygen-isotope analysis. There are three isotopes of oxygen,^16 O,^17 O, and^18 O, but only^16 O and
(^18) O are of importance in climatic studies. Being lighter, water containing (^16) O evaporates more
readily than H
2
(^18) O, so fresh water is enriched in (^16) O as compared with sea water. The degree of
enrichment depends on the temperature at which the water evaporated, because the higher the
temperature, the greater the rate of evaporation and the more H
2
(^18) O that enters the air with the
H
2
(^16) O. This allows mean surface temperature to be calculated from analyses of the ratio of (^16) O: (^18) O
in dated samples of ice trapped in cores as ‘fossil precipitation’, the present ratio of^18 O:^16 O=1:500
providing a standard.
Astronomical climate forcing can be predicted, but volcanic eruptions are wholly unpredictable, at
least at present. Some eruptions, but not all, have a climatic influence, although its scale is small and
it is of short duration. If it is to affect climate, a volcanic eruption must inject material into the
stratosphere, where it will remain for some time; tropospheric material is adsorbed on to surfaces or
removed by precipitation in a matter of hours, days, or at most weeks. The eruption should also be in
a low latitude. The convection cells governing the movement of low-latitude air allow only minor
exchanges of tropospheric air between the northern and southern hemispheres. Stratospheric air is
less affected and there is some interchange. Material injected into the stratosphere near the equator
will be carried around the Earth and may also spill into higher latitudes in both hemispheres.
On 15 June 1991, the eruption of Mount Pinatubo on the island of Luzon, in the Philippines (latitude
15° N) caused the greatest stratospheric perturbation this century. The plume reached a height of
about 30 km and released into the stratosphere some 30 million tonnes of aerosol composed of
sulphuric acid and water. Within 14 days the material had spread across the equator, to about 10° S,
and carried westward; within 22 days it had circled the planet. Eventually it spread as a blanket
between about 30° N and 20° S. The presence of so much fine-particulate matter in the upper
atmosphere increased the planetary albedo and thus reduced the amount of solar radiation reaching
the surface, with the result that surface temperatures were depressed during the remainder of 1991
and for the whole of 1992; it was 1993 before they began to recover. In 1992, the mean global
temperature was 0.2°C lower than the 1958–91 average and it would have been lower still were it not
for the warming influence of the 1992 ENSO event. The eruption ended the run of warm years.
Because the aerosol engaged in chemical reactions, the eruption also contributed to the greatest
depletion of stratospheric ozone recorded up to that time (MCCORMICK ET AL., 1995).
Mount Pinatubo was the biggest eruption this century, but it was not the only one. Five other eruptions
were large enough to have had some climatic effect: those of Katmai (1912), Agung (1963), Fuego
(1974), El Chichón (1982), and Cerro Hudson (1991), releasing 20, 16–30, 3–6, 12, and 3 million
tonnes of aerosol respectively. In the last century there were two even larger eruptions, of Tambora
(1815) and Krakatau (1883); these released more than 100 and about 50 million tonnes of aerosols
respectively. The year 1815 was known as ‘the year with no summer’ and in Britain the summers of
1816 and 1817 were also wet and cold; the 1816 harvest was disastrous and there were food riots
(STRATTON AND BROWN, 1978).
Our climate is changing constantly, driven by factors over which we have no hope of control. It
is affected by cyclical variations in the Earth’s orbit and rotation and apparently erratic fluctua-