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.