6
THE 2000 AGM ADDRESS
Global warming and its possible impacts on Essex
PETER ALLEN
13 Churchgate, Cheshunt, Herts ENS 9NB
Introduction
Global warming has to be viewed in the light of climatic change and in this context it should be said
that climate is not, and never has been, stable. Climate is controlled by a number of factors, each
working on a different time scale.
Latitudinal effects
On the longest time scale, of hundreds of millions of years, latitudinal position on the earth is the
controlling factor. The climate zones of the earth can be described in simplified terms as :
North Pole - polar regions (glacial/periglacial)
- temperate regions
- northern deserts, e.g. Sahara
Equator - humid tropics - rainforest
- southern deserts, e.g. Kalahari
- temperate regions
South Pole - polar regions (glacial/periglacial)
Over time, the continents have wandered and 400 million years ago, in Devonian Old Red Sandstone
times, Britain was in the southern desert (e.g. Kalahari) latitudes. As Britain traversed northwards,
it experienced moist tropical conditions during the Carboniferous (300 million years ago) as we
migrated past the Equator, with the Coal Measures swamps; and then northern (e.g. Saharan) desert
conditions during Permian and Triassic times (200-250 million years ago). Since then we have
migrated into the northern temperate zone.
This process is still in train, but as it is measured in tens and hundreds of millions of years, the rate
of movement is sufficiently slow that we need not take it into account.
Tectonic activity and ice ages
The above is actually a tcctonically driven process. Tectonics also affect us in other ways.
Past ice ages have broadly coincided with periods of continental collision. These collisions could
affect climate in two ways. The collisions were accompanied by mountain building. The new
mountains could have been sufficiently high to disturb the global airflow patterns and so cause a
climatic deterioration in sensitive near-polar regions. Secondly the collisions were accompanied by
volcanic activity and major eruptions can throw out sufficient dust into the atmosphere that the
particles physically block part of the incoming solar energy causing a temperature drop, and also act
as nuclei for water evaporation, leading to increased cloudiness and increased precipitation. Depressed
temperatures and increased precipitation in polar regions could lead to glacier expansion.
Essex Naturalist (New Series) 17 (2000)