Between c. 7000 and 4000 B.C. the climate in Europe reached
its optimal level (the Hypsithermal) in the present interglacial. It was not,
however, uniform in its onset. In the British Isles the maximal warmth was
about 6000–4500 B.C., whereas in northern Europe 4000–2500 B.C. saw the highest
average temperatures. There are of course no instrumental records, but data
from fossil pollen and other organic remains, the stratigraphy of lakes and
bogs, and from tree rings suggest that temperatures were at least 1 to 2°C (1.8
to 3.6°F) above those of the late twentieth century. This implies of course
that the spread of agriculture into much of Europe and the development of all
the more complex societies of Celtic Europe and their early medieval successors
took place in periods of climatic deterioration (albeit with warmer
remissions). The hunter-gatherers had had the best of the weather.
The consequences for the natural environment are obvious to
some extent. The forest belts extended northward, so mixed deciduous forest was
dominant over much of Europe, save from mid- Scandinavia northward, where
conifers and birch predominated, and in mountainous areas. Here there were
always more conifers, though not to the extent familiar in the Alps, for
example, where there was more beech (Fagus spp.). The steppes of the east
retreated in favor of woodland cover. Within the forests, too, species that
were adapted to greater warmth flourished. The lime (Tilia spp.) is a good
example, along with ivy (Hedera sp.), holly (Ilex), and mistletoe (Viscum). The
European pond tortoise (Emys orbicularis), confined to the Mediterranean in the
twenty-first century, was found in Denmark and southern Sweden. The presence of
insect and molluscan faunas also reflected the warmth, but of greater importance
for human communities were the large mammals, such as the red and roe deer,
wild ox, wild pig, and beaver. As the optimal period peaked, agriculture became
important, and it is clearly critical that such cereals as wheat and barley
were able to ripen even in the British Isles and southern Scandinavia.
Another feature of the optimal period was its water
relations. In the early part the climate over most of Europe was drier than in
the twenty-first century, but as time passed there was a move to wetter conditions,
especially in the west. In part this change reflected the increasing influence
of the sea as its levels rose. A leading consequence of this continued eustasy
was the formation of the Dover Strait and then the submergence of the low-lying
terrain between England and the Low Countries to form the North Sea. By c. 7400
B.C. the British Isles were insulated from the rest of Europe, and it took the
completion of the Channel Tunnel in the 1990s to make it possible again to walk
from Dover, England, to Calais, France. In cultural terms this separation took
place in the Mesolithic. The adoption of agriculture in the British Isles
necessarily was preceded by a sea passage of some kind of mix of ideas, people,
seeds, and young cattle.
Wetter conditions are reflected to some extent in higher
lake levels and thus the renewal of lake fringe successions, but they are most
apparent in upland areas and the western fringe of Europe. Two processes are
notable. The first is the leaching of minerals down the profiles of many types
of soils, particularly from those on such acid substrates as sandstone and
gritstone. The redeposition of minerals, such as iron and manganese, in solid
horizons (“pans”) made the soils prone to becoming waterlogged, and hence their
floras moved away from large tree species toward wet- and acid-tolerant
species, such as birch, and to dwarf shrubs of the Ericaceae family. On some
uplands in Scandinavia and the British Isles great blankets of peat formed on
low slopes where the rainfall exceeded about 700 millimeters per year. It is
possible that there was some human involvement in the inception of these miry
spreads, whose surface often was one of the bog mosses of the genus Sphagnum.
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