I0
Global Warming and its possible impacts on Essex
During cold stages, the CO2 levels in the atmosphere dropped. The lost carbon dioxide cannot have
been stored on the land, as terrestrial biological activity would have been reduced. Thus it must
have been stored in the oceans, implying increased marine biological activity. In cold climates, the
loss of vegetation leads to increased dust concentrations in the atmosphere, including iron, which
may been transferred to the oceans, leading to increased plankton productivity.
Methane levels follow a similar pattern, with high values in the warm stages. Possibly the major
source of the methane is marshland, in its various forms. During cold stages, the more northerly
peatbogs and coastal marshes freeze and become inactive. In the warm stages these thaw out and
become active. Also during the warm stages, herbaceous tropical swamps expand. There are also
large quantities of methane sequestered in ooze on the ocean floors. With increased temperature,
part of this methane stock is released.
It has also been noted that the intensity of monsoons is cyclic, with a periodicity of c.21,000 years
(= precession of the equinoxes). During wetter parts of the cycle, the swamps of Sudan, the Nile
and Lake Chad would have expanded, leading to increased methane production.
Obviously human activity cannot influence the orbital forcing mechanisms. However, as there are
interactions between the forcing and the earth's atmosphere, oceans and biological activity, human
impacts may have serious consequences. Pollution of the oceans may impact on the plankton, either
increasing or decreasing its biomass. This in turn can affect the amount of CO2 transferred from the
atmosphere to the oceans; fewer plankton could mean less CO2 transfer and so suppress an ice
advance, while increased plankton activity could mean less CO2 in the atmosphere and so exaggerate
glacial activity. Similar lines of argument can be put forward for the methane.
It should be noted that C02 levels were higher in the last interglacial (known as the Ipswichian or
Oxygen Isotope Stage, OIS, 5e) than today. The main impact in Britain seems to have been a
warmer climate and larger animals (by up to 30%), presumably reflecting more luxuriant vegetation,
hence a better feedstock. Humans did not return to Britain in this interglacial, possibly because the
forest cover was too dense or because the Channel formed too soon in the warm stage to allow in-
migration.
Human impacts on climate
Currently we are undergoing climatic warming, but it is difficult to assess how much is within the
range of natural climatic instability and how much is humanly induced.
To get some idea of the characteristics of interglacial climates, such as we are experiencing at the
moment, the best analogue we have is the last interglacial (Ipswichian, OIS 5e), which started about
125-130,000 years ago and lasted about 10,000 years. Interglacials had been thought to be warm
periods with relative climatic stability. However, the GRIP (Greenland Ice-corc Project) ice core
shows that during the last interglacial there were marked temperature fluctuations on two scales
(Fig. 3). On the longer scale, measured in thousands of years, there were three warm peaks (OIS
5c5, 5c3, 5e 1) and two cool troughs (5c4 and 5c2). The shorter scale changes operated at the decade
to century level (75-750 years), particularly in OIS 5e5. The O18 variations suggest that both types
of fluctuation could be as much as 14°C. Such a switch at the decade level is a frightening thought;
a switch of 6-8° is enough to put is into another glacial episode.
Essex Naturalist (New Series) 17 (2000)