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Thousand
years
18
15
12
9
6
3
0
Aerosol
Ice
8
S
JJA
100
4
50
0
0
-1
0
330
-2
265
4
-3
SST
S
DJF
-4
200
-8
CO
2
Figure 2.1
Schematic diagram of major changes since 18,000 years before present of
external forcing (solar radiation (S)) and internal boundary conditions (land ice, sea-
surface temperature (SST), atmospheric carbon dioxide concentration and excess glacial
aerosol) used for climate simulations. (Modified from Kutzbach & Street-Perrott 1985.)
period, approximately the last 11,500 years. The onset of the Holocene is marked
very clearly in Europe by a very rapid increase in air temperature and a rapid
recession of land ice, clearly seen in the sediments of most lakes in Northern
Europe by a switch from clays to organic lake muds. Adjustments in the 'earth
system' to post-glacial warming were more gradual: global ice cover shrank, the
land surface, previously depressed by the weight of ice, rose, sea level rose as a
result of global ice melting, the ocean circulation pattern established a new
equilibrium, new soils developed and plants and animals returned from glacial
refugia to occupy land areas similar to those occupied in the previous interglacial
period (cf. Roberts 1998). By approximately 8000 years ago, new 'boundary
conditions' for the natural world, not significantly different from those of the
present day, were established (Fig. 2.1).
Overall, the Holocene period is a warm stage, contrasting to the previous cold
(or glacial) period but similar in many respects to preceding warm (or interglacial)
periods, the last of which is referred to in Europe as the Eemian, approximately
130-105,000 years ago (cf. Drysdale
et al
. 2005). The Holocene, therefore, is
considered to be yet another interglacial period, differing from previous ones
principally because of the impact of people in practising agriculture (causing
land-cover change) and in developing industrial processes (causing pollution),
which together have significantly and progressively, over approximately the last
5000 years, strongly modified both terrestrial and aquatic ecosystems. It is now
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