Unformatted text preview: ange. Some species are less mobile than others, and
ecosystems involve interactions among species, so such rates of
climate change, along with habitat loss and fragmentation, new
invasive species, and other stresses are expected to have severe
impact on species survival (37).
The total distance of isotherm migration, as well as migration
rate, affects species survival. Extinction is likely if the migration
distance exceeds the size of the natural habitat or remaining habitat
fragment. Fig. 6 shows that the 21st century migration distance for
a BAU scenario ( 600 km) greatly exceeds the average migration
distance for the AS ( 100 km).
It has been estimated (38) that a BAU global warming of 3°C over
the 21st century could eliminate a majority ( 60%) of species on
the planet. That projection is not inconsistent with mid-century
BAU effects in another study (37) or scenario sensitivity of stress
effects (35). Moreover, in the Earth’s history several mass extinctions of 50–90% of species have accompanied global temperature
changes of 5°C (39).
We infer that even AS climate change, which would slow warming
to 0.1°C per decade over the century, would contribute to species
loss that is already occurring due to a variety of stresses. However,
species loss under BAU has the potential to be truly disastrous,
conceivably with a majority of today’s plants and animals headed
The pattern of global warming (Fig. 1B) has assumed expected
characteristics, with high latitude amplification and larger warming
over land than over ocean, as GHGs have become the dominant
climate forcing in recent decades. This pattern results mainly from
the ice–snow albedo feedback and the response times of ocean
Hansen et al. In assessing the level of global warming that constitutes DAI, we
must bear in mind that estimated climate sensitivity of 3 1°C for
doubled CO2, based mainly on paleoclimate data but consistent
with models, refers to a case in which sea ice, snow, water vapor, and
clouds are included as feedbacks, but ice sheet area, vegetation
cover, and non-H2O GHGs are treated as forcings or fixed boundary conditions. On long time scales, and as the present global
warming increases, these latter quantities can change and thus they
need to be included as feedbacks. Indeed, climate becomes very
sensitive on the ice-age time scale, as feedbacks, specifically ice
sheet area and GHGs, account for practically the entire global
temperature change (17).
Vegetation cover is already expanding poleward in the Northern
Hemisphere causing a positive climate feedback (42). Global warming could result in release of large amounts of GHGs, e.g., from
melting permafrost or destabilized methane clathrates on continental shelves (43). Some of the largest warmings in the Earth’s
history and mass extinctions may be associated with such GHG
releases (39, 43). Although such disastrous GHG releases may
require many centuries, our ignorance of GHG climate feedbacks
demands caution in estimating requirements to avoid DAI.
The AS is based on the rationale that positive feedba...
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