Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 285

subsequently, in 2010, Daniel Doak and William Morris cited the work of Parmesan
et al. (1999) as including a buffering response to species range shift due to climate
change.
Species' climate-induced range-shift problems are great, and complex, in a modern
landscape that is highly fragmented by human management and land use. However,
they become critical to the level of extinction if the species' thermally determined
spatial range becomes restricted and reduced to nothing. Typically this takes place at
the high-latitude margins of continents. For example, a terrestrial species in Eurasia
continuing to migrate northwards as the climate warms will ultimately reach the
continent's edge and the Arctic Ocean. Faced with nowhere else to go, extinction
becomes virtually inevitable. ('Virtually' because translocation to another part of
the globe may be an option, but species translocation and introductions to new
environments are not taken lightly.) Many of the species at risk are those that evolved
during the Pleistocene or earlier, when the Earth's climate was cooling. Of the peri-
Arctic species at risk one most often cited is the polar bear ( Ursus maritimus ). It
evolved in the mid-Pleistocene some 100 000-250 000 years ago, it is thought, from
a group of brown bears that became stranded by glaciers. They speciated rapidly,
evolving sharper canine teeth (brown bears have far more vegetation in their diet),
a longer neck (more suited for keeping their heads above water while swimming),
larger paws (for spreading weight on the ice and for swimming) and of course thicker
fur that is lighter in colour (from light brown to white). Polar bears require ice
platforms from which to hunt and here is the problem with global warming: Arctic
ice is currently reducing at a considerable rate. One early-21st-century estimate has
been a 7% reduction in ice cover in 25 years and a 40% loss of thickness. Indeed,
measurements of the minimum summer extent of sea ice since 1979 do show a
clear trend of year-on-year reduction (see Figure 6.3). This reduction in Arctic ice is
unprecedented, some suspect, in the Holocene (the past 11 700 years), and certainly
as determined in the past 1450 years as revealed by 69 circum-Arctic climate proxy
series (Kinnard et al., 2011). Because polar bears are at the top of the food chain,
changes in their numbers are likely to affect the populations of many other species.
For example, some seal populations have increased in response to fewer polar bears
and this in turn is likely to affect fish predation.
The walrus ( Odobenus spp.) is also declining, possibly because these animals too
must work harder to find food with less sea ice. Walrus mothers nurse their young on
sea-ice floes. As ice recedes the walrus do too. Further from the coast the mothers
must dive longer and deeper from the ice to the sea floor to find clams. Of the three
walrus species the population living in the Russian Arctic ( Odobenus laptevi ) has the
smallest population, thought at the turn of the century to be between just 5000 and
10 000.
In the southern hemisphere there are analogous problems. In 2001 French research-
ers Christophe Barbraud and Henri Weimerskirch reported on a long-term sur-
vey (1952-2000) of the emperor penguin ( Aptenodytes forsteri ) colony near the
Antarctic station of Dumont d'Urville and linked this to meteorological data gathered
just 500 m from the animals. They noted that the breeding population was stable until
the mid-1970s but then declined abruptly in the late 1970s to around 50% of its
former level, coinciding with a time of high winter temperatures. They contemplate