26 ISLAND ENVIRONMENTS
051015202530Bristol Channel
English Channel
Cardigan Bay
Somerset Levels
North Wales
35
10Depth below present sea level (m)98765
Radiocarbon years B.P. ('000s)43210Figure 2.8Holocene sea-level rise in southern
Britain. (Redrawn from Bell and Walker 1992,
Fig. 4.10, after Shennan 1983).Spencer 1995). However, there is some measure of
agreement that stand levels have not exceeded pres-
ent levels by more than a few metres within the past
340 000 years. The most widely accepted figure for
Pleistocene minima is of the order of 130 m,
although in places it may have been greater than this
(Bell and Walker 1992; Nunn 1994). This order of sea-
level depression, given present lithospheric configu-
ration, is sufficient to connect many present-day
islands—such as mainland Britain—to continents,
thus allowing biotic exchange between land areas that
are now disconnected. Equally important, many
islands existed which are now below sea level.
However, as will be clear, simply drawing lines on
maps in accord with present-day 130 m contour
lines is not a sufficient basis for reconstructing past
island–mainland configurations (Kayanne et al. 1993).
The rise from the late glacial minima to present
levels was not achieved overnight, nor was it a
steady or uniform pattern. As a broad generaliza-
tion, at around 14 500BP(Before Present, where
Present is by convention designated as AD1950), sea
levels stood at about 100 m, rising by some 40 m
over the next millennium (Bell and Walker 1992).
A second major phase of glacial melting around
11 000BPcaused an eustatic rise to about 40 m by
the beginning of the Holocene, when ice volumes
had been reduced by more than 50%. The pattern for
the British Isles for the past 9000 years can be seen in
Fig. 2.8. Both eustatic and isostatic elements were
involved, leaving a legacy in raised shorelines and
drowned valleys. In the North Sea, the Dogger Bank
was breached by 8700BPand the Straits of Dover by
8000 BP. The present configuration of the southern
North Sea coastlines was more or less established by
7800–7500BPand that of the British coastline by
c.6000BP, although slow adjustments continue today.
The severing of Britain from Ireland took place some
2000–3000 years before that of Britain and the rest of
Europe. In short, the switch from glacial to inter-
glacial conditions took place some 2000–3000 years
before ‘mainland’ Britain became an island once
again, with different parts of the British Isles being
separated from one another at quite different times.
These events left a legacy in the biotic composition of
the islands that has long been noted by biogeogra-
phers (Williamson 1981). Oscillations in effective sea
level over the last few thousand years, although
comparatively small, have nonetheless been suffi-
cient to have significant impacts on human societies
in island regions such as the Pacific (Nunn 2000), and
there is currently considerable concern that future
sea-level rises in association with predicted global
warming will have serious implications for island
peoples (see Chapter 12).Climate change on islands
An important point made by Williamson (1981) is
that short-term variations in climate have lower