64 / Basics of Environmental Science
North Atlantic gyre and the North Atlantic Drift which is part of it. The warming at the end of the
glaciation was associated with a retreat of the sea ice as the polar front moved as far north as
Iceland, and this disrupted the formation of NADW.
About 11000 years ago and for about 1000 years Western Europe was plunged back into ice-age
conditions. When the reversal started, Scotland may have been entirely clear of ice, but before long
much of the country lay beneath an ice sheet hundreds of metres thick (ROBERTS, 1989, pp. 50–
53). This rapid and dramatic deterioration in climate was first detected by the presence of pollen
grains of Dryas octopetala (mountain avens) in soils that could be dated to this period. Dryas octopetala
is a plant characteristic of alpine and subarctic conditions and it has given its name to the climatic
reversal, which is known as the Younger Dryas. This lasted from about 11000–10000 years BP
(before present) and it is called ‘Younger’ because there is some evidence of an earlier cooling, prior
to 12000 years BP, known as the Older Dryas (PENNINGTON, 1974, pp. 32–34), which may have
had a similar cause.
Ocean currents are clearly defined and some are fast-flowing. The Kuroshio Current, for example,
flows at up to 3 m s-1. Their climatic effect is indirect, however, in that they affect the properties of
the air in contact with them, rather than the water warming or cooling the coasts with which it makes
contact, and it is the air which brings weather to the continents. The link is obviously of the greatest
importance, and any long-term climatic prediction must be based on a much greater understanding
of it than exists at present. Is it feasible, for example, that a rapid climatic warming in the northern
hemisphere due to the greenhouse effect might disrupt the formation of NADW and trigger a severe
cooling in north-west Europe? What causes ENSO events and are they likely to become more or less
frequent should there be a global change in climatic conditions? At present such questions cannot be
answered. Until they are, all predictions of the regional implications of climate change must be
approached with great caution.
17. Weather and climate
Rain, snow, sunshine and showers, wind and storm, are among the phenomena that constitute our
weather, the conditions we experience from day to day in a particular place and which weather
forecasters aim to predict. The average weather conditions experienced over a large area year by
year constitute the climate of that area. The two concepts, of weather and climate, are distinct and
provide the subject matter of two equally distinct scientific disciplines: meteorologists study weather
and climatologists study climates. Obviously the two overlap, for you cannot understand one without
a fairly detailed knowledge of the other. When we discuss the greenhouse effect, for example, we
base our ideas on studies made by climatologists; when we wish to know whether it would be wise
to plan a picnic for the weekend we consult a meteorologist.
Weather phenomena result from interactions between bodies of air and water at different temperatures
and are governed by a small number of general principles. Because air is compressible, atmospheric
pressure decreases with height above sea level. If a ‘bubble’ of air (known technically as a ‘parcel’
of air) is made to rise, therefore, its volume will increase as it enters regions where pressure is lower.
As it expands, the air becomes less dense. This means its molecules are further apart, and in ‘making
more room’ for themselves molecules push one another aside, an activity which requires them to
expend energy. Having less energy, the molecules move more slowly and as they slow down, so the
parcel of air cools. There is no exchange of heat with the surrounding air; the cooling involves only
the expanding parcel. Similarly, if air descends it is compressed, acquires energy, and warms. This
cooling and warming is called ‘adiabatic’ and it is a version of the first law of thermodynamics