“The scary thing for me is that the stuff our models is showing happening
decades from now, we’re already seeing.”

SCIENCE www.sciencemag.org
17 AUGUST2007 VOL 317


Predicting Oblivion: Are Existing Models Up to the Task?

Huge numbers of species may be at risk of extinction from climate change,
but coming up with precise estimates is proving tough

The most authoritative guide to today’s extinc-
tion crisis is a database known as the Red List.
Later this month, a group of scientists will
gather in England to consider whether the Red
List should be opened up to species that, for the
moment, show no signs of trouble. Many scien-
tists suspect that the next few decades of global
warming could push some species toward
oblivion. “The concern,” says the meeting’s
organizer, H. Resit Akí§akaya, an ecologist at
ecological software company Applied Bio-
mathematics in Setauket, New York, “is that
maybe some species that are threatened by cli-
mate are not reflected on the Red List.” But
Akí§akaya and others caution that the meeting is
unlikely to come up with firm predictions of
how many species will become extinct, let
alone which ones will be particularly at risk.
The science of predicting extinctions from
global warming is only a few years old, and the
best models are rife with uncertainties. Experts
generally agree that the models may be useful
for giving a rough idea of the potential impact
of global warming and may also offer guidance
for planning preserves. But some scientists are
concerned that policymakers will be expecting
them to provide more precise estimates than
they can deliver. “It’s worrying, says Miguel
Araíºjo, an ecologist at the Spanish National
Research Council in Madrid.

Much of the current debate over climate-
triggered extinctions focuses on what are
known as climate-envelope models. Scien-
tists analyze all the places where a species has
been recorded and look for features of the cli-
mate that those places share. The key factors
may be rainfall, for example, or the tempera-
ture during the winter.

In the early 2000s, scientists began to
look at what happened to these climate
envelopes in the scenarios climate scientists
have projected for the coming century. “A
number of us were noticing that these
envelopes seemed to be winking out
entirely,” says Lee Hannah, chief climate
change biologist at the Center for Applied
Biodiversity Science at Conservation Inter-
national, a nonprofit in Arlington, Virginia.
Concerned about the prospect of mass
extinctions, an international team of scientists,
including Hannah, combined their data into a
global analysis. They estimated the size of
future climate envelopes, assuming shrinking
climate envelopes meant an increased
risk of extinction.

Their sobering conclusion, published in Nature
in 2004: Based on a midrange climate-warming
scenario for 2050, “15-37% of species in our
sample of regions and taxa will be ‘committed
to extinction.'”

The paper was enormously influential and
figures prominently in the Intergovernmental
Panel on Climate Change’s (IPCC’s) upcom-
ing report on the impact of global warming. In
a summary for policymakers, the IPCC
authors warn that “approximately 20-30% of
plant and animal species assessed so far are
likely to be at increased risk of extinction if
increases in global average temperature
exceed 1.5-2.5°C.”

Some experts have criticized the IPCC for
implying that climate-envelope models are
more precise than they actually are. “Simply
presenting those numbers as factual, saying
this is how many species will go extinct, is
misleading,” says Richard Pearson, a post-
doctoral researcher at the American Museum
of Natural History in New York City.

Pearson and other researchers have been
testing climate-envelope models for their accu-
racy and consistency, and they’ve found some
serious causes for concern. Araíºjo and his col-
leagues studied the ranges of 116 species of
birds in England in the 1970s and 1990s. The
red-backed shrike’s range shrank dramatically
to southeast England in the 1990s, for example,
but climate-envelope models based on the
1970s data predicted that the bird’s range
would stretch all the way to the northern tip of
Scotland. “We found that there were lots of
uncertainties,” he says.

Araíºjo, Pearson, and other researchers
published a study last year in which they com-
pared the projections for a group of plant
species in South Africa from several fre-
quently used envelope models. “We found a
huge difference between the models,” says
Pearson. Their projections ranged from a
92% range reduction to a 322% expansion.
Scientists are currently debating how to
make better predictions. Climate-envelope
models are “simply mapping programs,” com-
plains Daniel Botkin, a professor emeritus at
the University of California, Santa Barbara.
“There’s no biology in that.”

To improve the performance of these mod-
els, Botkin urges researchers to include bio-
logical details about species, such as how
quickly they disperse and how they interact
with other species. Pearson and other model-
ers have already had some success at doing so,
Pearson says. In a paper in press at Global
Ecology and Biogeography, Pearson and his
colleagues report that they can do a much
better job of predicting the ranges of owls in
Finland if they also factor in where woodpeck-
ers live, as owls make their nests in wood-
pecker cavities. “In my opinion,” says Pear-
son, the role of biological interactions “is the
biggest question out there at the moment, but
we’re just nibbling on the edges of that.”

Other researchers believe that a better
strategy is to analyze existing climate-
envelope models more effectively. “You have
to find automated ways to extract information
in intelligent ways from the data you have,”
says Araíºjo. He and his colleagues have found
that averaging the results of many climate-
envelope models provides a more accurate
prediction of where species can be found than
any one model. “I think that’s a much more
useful way to go,” says Araíºjo. “This is likely
to be a closer match to the truth than anything
else we can produce so far.”

One problem with these so-called ensem-
ble forecasts, however, is that they are a huge
undertaking. Running thousands of models of
thousands of species across an entire continent
is far beyond the capacity of any existing soft-
ware. “In the next 2 to 3 years, we won’t be able
to do it,” says Araíºjo, who is now developing a
program he hopes will be up to the task.
Scientists are debating not just how to make
these models better but also the best way they
can be used to make conservation decisions.
Some researchers are trying to estimate the
percentage of species that global warming will
put at risk of extinction across entire continents
or even the entire planet. Walter Jetz, a biolo-
gist at the University of California, San
Diego, and his colleagues recently pub-
lished a study of the combined impact of
climate change and land-use change on
birds. They found that several hundred species
may lose over half of their range by 2050. Land
use will have the biggest impact on birds in the
tropics, whereas climate change will be
stronger at higher latitudes.

Araíºjo and his colleagues have a more mod-
est goal: trying to predict patterns of change in
different regions. They’ve been forecasting which parts of
Europe will be particularly vulnerable to losing species
through climate change, for instance. They’ve found that
for plants, the mountainous regions in southern Europe
will be hit hardest. For amphibians, the arid
parts of southwestern Europe are most vulnera-
ble. For now, he suggests, such estimates may be
more useful for conservation than a mislead-
ingly precise estimate of a rate of extinction for
a particular species.

Identifying these sensitive regions may
reveal how existing preserves may change and
offer hints about how to design new ones. As
the temperature warms, some preserves will
no longer have a climate suitable to the species
they are supposed to protect.

A number of researchers are using climate-
envelope models to study how preserves may
have to be altered as species shift their ranges.
For instance, existing preserves could be
linked by corridors to enable animals and
plants to disperse from one habitat to another.
Hannah believes that scientists must
move forward with this sort of planning now,
even if the models have plenty of room for
improvement. “The scary thing for me is that
the stuff our models is showing happening
decades from now, we’re already seeing,” he
says. He points to the extinction of frogs in
the Andes, where researchers suspect that a
changing climate may have fostered the
spread of a lethal fungus. “These models are
the best we’ve got at the moment, and when
we see how the complexity of the world oper-
ates, it seems that it may be worse than these
models are indicating.”

Carl Zimmer’s latest book, on E. coliand the meaning of
life, will be published next May.

www.sciencemag.org SCIENCE VOL 317 17 AUGUST 2007

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