Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 265

temperature-related (water availability, for example, being another key dimension for
species response).
These studies are illustrative. Of course, discerning the biological fingerprint of
climate change all depends on what you are measuring: types of species, location and
climate change. For instance, a 2008 study examined 171 forest plant species between
1905 and 1985, and between 1986 and 2005 up to 2600 m above sea level in Western
Europe. This work not only showed, as did the above previous studies, that climate
warming has resulted in a significant upward shift in species optimum elevations
but that the upward shift averaged 29 m per decade. The shift was larger for species
restricted to mountains habitats and for grassy species: these are characterized by
faster population turnover (Lenoir et al., 2008). So why the larger altitudinal shift
compared to the above studies? Well, as noted, determinants of biotic response to
climate change are multifactorial. However, there is perhaps more than a little clue in
the warming experienced over the study period in the study area (France and Corsica).
Climatic change in France has been characterized by increases in average temperature
of far greater magnitude than increases in the world mean annual temperature, of about
0.6 ◦ C over the 20th century, reaching 0.9 ◦ C and even close to 1 ◦ C in the alpine region
since the early 1980s. The study period was divided into two: 1905-85 and 1986-
2005. (Readers may care to note that loosely speaking these two periods saw global -
not French regional - temperatures respectively below and above the 1961-90 mean;
see Figure 5.4.) In short, all the aforementioned studies represent parts of the same
terrestrial species response picture.
Nonetheless, within this overall picture of species shift there is plenty of detail.
Included here, and mentioned earlier, are the climatic cycles. To take just one example
of how a regular climate oscillation affects species, the North Atlantic Oscillation
index has been shown to correlate with UK populations of a number of butterfly
species. Specifically, the correlations are between a so-called collated index of the
butterfly species and the NAO index.
The collated index of butterflies is calculated by the UK Centre for Ecology
and Hydrology (CEH), which has been maintaining a database since 1976. CEH
researchers monitor the populations of butterfly species at 130 sites and data from
these is collated into an index. Turning to the North Atlantic Oscillation, the NAO
index is calculated from the difference in air pressure between Iceland and the Azores,
Lisbon or Gibraltar (the choice of the southern grouping of weather-station sites seems
to make little difference). The NAO index can be positive or negative. If positive then
depression systems tend to take a more northerly route across the Atlantic, so giving
the UK a warmer and wetter winter and autumn. A negative index is associated with
drier and cooler weather in the UK as moisture goes more towards the Mediterranean
basin. A number of UK butterfly populations have been shown to fluctuate with the
NAO index (Westgarth-Smith et al., 2005), including the common blue ( Polyommatus
icarus ) and small cabbage white ( Pieris rapae ).
It is important to note that these periodic fluctuations in species populations that
correlate with climatic oscillations are distinct from species population change due
to long-term climate change. This means that those studying the biological impacts
of climate change have to disentangle these two related phenomena. However, the
biological impact of climatic oscillations is relevant to the impact of longer-term