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Figure 2.2 The European pond turtle ( Emys orbicularis ). (Photograph from Biopix.dk.)
activity, in clearing forests and developing agricultural practices since the
beginning of the Neolithic period, has masked the impact of climate change, and
partly because changes in plant and animal records in lake sediments, especially
sediments from shallow lakes, are also influenced by hydroseral processes that
take place naturally.
For example, the contraction of the northern boundary of the water chestnut
( Trapa natans ), a thermophilous floating leaved aquatic macrophyte, during the
Holocene, was thought to be the result of, and provide good evidence for,
Holocene cooling (cf. Alhonen 1964). Korhola and Tikkanen (1997), however,
have suggested from a detailed study of shallow lake sediments in Finland that
the loss of Trapa natans may have been caused by changes in lake habitat as lakes
gradually filled in and were transformed into peatland. Similar arguments can be
made for the apparent decline in the abundance of other aquatic macrophytes
such as Cladium mariscus (Conway 1942). The one aquatic fossil record that
seems unambiguous in indicating a range response to Holocene cooling in Europe
is the European pond turtle ( Emys orbicularis , Fig. 2.2). At the present day, the
breeding range of this species centres on the Mediterranean, and its northern
limit coincides closely with the 20°C isotherm. By contrast, its fossil record from
the mid-Holocene indicates a distribution in Europe that included eastern
England, Denmark and southern Sweden, where mean July temperatures today
are close to 18°C (Stuart 1979), providing good evidence for the potential impact
of the c. 2°C cooling over the last 5000 years suggested by climate models.
Low latitudes
In mid to low latitudes, the impact of long-term changes in insolation is registered
more through changes in moisture than temperature. The changes were especially
marked in regions influenced by the shifting position of the Intertropical
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