The achievement of the migration
rates given in Figure 1 depends on factors apart from temperature
change. Human landuse patterns and the increasingly fragmented
nature of temperate forests now present formidable barriers to
natural migration. One study of temperature tolerances of more
than 15,000 native plant species in the US and Canada has shown
that a temperature rise of 3°C
would place 7-11 per cent of them outside their thermal tolerance,
and therefore vulnerable to extinction, unless they could migrate
to cooler areas.
Migration is an issue for fauna as well as for flora. For example,
although most birds are extremely mobile, some species will not
cross open clearings, sometimes even as small as treefall
gaps. Others are associated with specific vegetation species or
formations, such as Kirtland's warbler and Jack pine. Alternatively,
tree migration combined with increased storms may reduce the availability
of large tree holes used as breeding sites by species such as
the redcockaded woodpecker of US oldgrowth forests,
or Australia's rare Major Mitchell cockatoo. Annual migrants,
such as Bewick's swan, depend on the availability of specific
habitats for `refuelling' stops on their long journeys, often
requiring optimum conditions for building up fat reserves prior
to setting off. Others, such as crossbill, may migrate in synchrony
with the availability of transient food sources.
Several species of shorebird, such as godwit and whimbrel, rely
on relatively few coastal refuelling stops on the Atlantic and
Pacific flyways. Alternative sites with sufficient predictability
of timing and supply of food probably do not exist. Coastal inundation
or other climate impacts could seriously affect these species.
In the US, populations of shorebirds such as semipalmated
sandpipers, knot, and sanderling depend on synchronous timing
of horseshoe crab spawning in the Delaware Bay, and on insect
emergence at their arrival in the Arctic tundra. Both these events
are probably significantly climate controlled, although scientists
do not yet fully understand the relationship.
A 3o C temperature rise could
wipe out more than 10% of North America's native plants
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Freshwater fish, despite being highly mobile,
and in many cases migratory as well, are limited by the physical
distribution of rivers and lakes. Most fish have limited tolerances
for changes in water temperature, and where water warms as a result
of climate change, there will be very significant threats for
species that are unable to find their way to cooler waters. A
recent study by the US Environmental Protection Agency suggested
that cold water fish could lose between 50 and 100 per cent of
suitable habitat in 20 states. Amongst the states likely to lose
all suitable habitat would be Maine, Massachusetts, Indiana, and
Nebraska. Amongst the species most at risk would be the already
threatened populations of cutthroat trout, brook trout, Chinook
and chum salmon.
As can be seen from the foregoing discussion, warming at the rate
implied by a CO2 doubling by around the middle of the
next century will prevent many ecosystems from adapting to climate
change. Thus, CO2 concentrations would have to be stabilized
at a level well below an equivalent doubling from preindustrial
levels in order to have a reasonable likelihood of preventing
dangerous interference with the climate system, and preventing
ecological harm.
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