were assumed to be inert, much like the Ilex trees in Silman’s plots. As
temperatures rose, they stayed put, and so, in most cases, the amount of
climatically suitable area available to them shrank, in many instances
down to zero. The projections based on this “no dispersal” scenario were
bleak. If warming were held to a minimum, the team estimated that
between 22 and 31 percent of the species would be “committed to
extinction” by 2050. If warming were to reach what was at that point
considered a likely maximum—a figure that now looks too low—by the
middle of this century, between 38 and 52 percent of the species would be
fated to disappear.
“Here’s another way to express the same thing,” Anthony Barnosky, a
paleontologist at the University of California-Berkeley, wrote of the study
results. “Look around you. Kill half of what you see. Or if you’re feeling
generous, just kill about a quarter of what you see. That’s what we could
be talking about.”
In the second, more optimistic scenario, species were imagined to be
highly mobile. Under this scenario, as temperatures climbed, creatures
were able to colonize any new areas that met the climate conditions they
were adapted to. Still, many species ended up with nowhere to go. As the
earth warmed, the conditions they were accustomed to simply
disappeared. (The “disappearing climates” turned out to be largely in the
tropics.) Other species saw their habitat shrink because to track the
climate they had to move upslope, and the area at the top of a mountain is
smaller than at the base.
Using the “universal dispersal” scenario, the team, led by Chris
Thomas, a biologist at the University of York, found that, with the
minimum warming projected, 9 to 13 percent of all species would be
“committed to extinction” by 2050. With maximum warming, the
numbers would be 21 to 32 percent. Taking the average of the two
scenarios, and looking at a mid-range warming projection, the group
concluded that 24 percent of all species would be headed toward
extinction.
tuis.
(Tuis.)
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