Basics of Environmental Science

Earth Sciences / 79

activity and climate so close he described it ‘almost that of a key in a lock’, extending to 3000 BC
(EDDY, 1977).

Again, the solar influence may be overwhelmed by that from greenhouse gases. David Thomson, a
skilled statistician, has analysed data since 1659 and concluded that global temperatures are now
linked more closely to atmospheric carbon dioxide concentrations than to sunspot activity or orbital
effects (THOMSON, 1985), although his interpretation has been questioned by some climatologists,
who think it too simple (KERR, 1995). The idea is now gaining ground that present changes in the
atmosphere and climate are more likely to be due to changes in solar output and volcanic eruptions
than to human intervention (CALDER, 1999).

Debate will continue for some time over what is forcing present climate change, but at least in the
past it has clearly been triggered by astronomical events, and when the climate changes it can do
so very quickly. At one time it was thought that ice ages begin and end gradually, it taking centuries
or longer for the ice sheets to spread. This may be incorrect. According to the ‘snowblitz’ theory,
a slight fall in summer temperatures in high latitudes might allow some of the winter snow to
survive where in previous years it had melted. The affected areas would then be white, when
previously they had been dark, thus increasing albedo and lowering temperature further. In
succeeding years, the snow-covered area would increase and temperatures would continue to fall,
climatic forcing by the increased albedo accelerating the change by a strongly positive feedback.
It might take very little time to move from our present interglacial climates to a full glaciation.
Warming can also proceed rapidly, the change from glacial to interglacial perhaps taking no more
than a few decades.

Stability of the polar ice sheets

If the polar ice caps were to melt, the volume of water released into the oceans

would be sufficient to raise sea levels substantially. The stability of the ice

caps is therefore of great importance and their condition is monitored closely.

The ice caps comprise three major ice sheets: in Greenland, West Antarctica,

and East Antarctica. The Greenland ice cap is growing thicker in some areas,

thinner in others, and is shrinking slightly overall. The reduction in its size is

due to the rate of flow of its outflow glaciers and is not thought to be due to

climatic change.

In Antarctica, the ice sheet on the eastern side of the Transantarctic Mountains

is about twice the size of that on the western side. The East Antarctic ice sheet

is very firmly grounded on the underlying rock. Its size remains constant and

there is not considered to be any risk of it decreasing in thickness.

The West Antarctic ice sheet is less firmly grounded and the line marking the

edge of the grounded sheet is retreating. It is doing so very slowly, at a

constant rate, and has been retreating at this rate for about 7500 years. The

retreat is due to the way glaciers within the ice sheet are moving and not to

climatic change.

Evidence from the past indicates that despite minor fluctuations, the climate throughout the present
interglacial, the Flandrian, has been very stable. During the last two glaciations and the Eemian
Interglacial separating them, temperatures rose and fell rapidly, by 3°C or more, bringing warmer or