Download biological response

N
E W S
F
O C U S
Consensus on Ecological
Impacts Remains Elusive
Two big new studies strengthen the case that global warming is causing biological
effects, but critics say even the additional data fall short of proof
Working Group II had a problem. The group, versity ecologist Terry Root, independently
part of the Intergovernmental Panel on embarked on a similar study. The two
Climate Change (IPCC), reviewed 44 studies papers, published in the 2 January issue of
showing that more than 400 species of plants Nature, are being touted as the most comand animals across the globe had shifted their prehensive meta-analyses to date of the
ranges or changed behaviors such as the biotic effects of global warming. To many,
timing of egg laying. To the biologists on they clinch the case. But Tol and other
the committee, this was a strong signal of working group members maintain that the
climate-induced effects
on a variety of biota. They
wanted to give the finding
a very high level of confidence, 95%. But the nonbiologists, mostly economists, advocated a confidence level of 33% to
67%, and no more.
Ultimately, IPCC’s consensus document listed a
high confidence level,
67% to 95%. But despite
that paper compromise,
the group remained split
on how certain it was that
global warming caused
the observed biological
changes. The issue: which
data should be considered Northward migration. As temperatures have warmed in Europe, the
in such an analysis.
Sooty copper butterfly has gone extinct in large parts of Spain and
“We all think we know has expanded north into Estonia.
how to analyze data, so
you’d think there wouldn’t be any disagree- two studies don’t provide any greater level
ments,” says one of the IPCC authors, of confidence than before that global
Camille Parmesan, a population biologist at warming is causing the observed biologithe University of Texas, Austin. “But we cal changes.
would look at the same data, … and one perAs part of her joint project with Yohe,
son says, ‘So what?’ and the other person Parmesan sorted species into four catesays, ‘Wow, look at that!’ ”
gories: those that changed their ranges or
Richard Tol, an environmental economist behaviors in accord with global warming
at Hamburg University in Germany, for predictions, those that did the opposite,
instance, has questioned whether the data set species that did not change, and species
represented a “fair sample.” He points out showing changes that couldn’t be ascribed
that biologists tend to do studies in regions to global warming. She found that 87% of
where impacts of climate change are expect- 484 species analyzed changed their timing
ed; in addition, he said, studies showing no as predicted by models of global warming.
effect are unlikely to be published. Others, Distributional shifts were consistent with
including economist Gary Yohe of Wesleyan predictions for 81% of 460 species. Such
University in Middletown, Connecticut, said changes would occur by chance less than
that high confidence was unwarranted one time in 10 trillion—an airtight case,
because the analysis simply showed a corre- argued Parmesan.
lation, not cause and effect.
Not so, said Yohe, pointing out that the
In an effort to persuade the skeptics, result was still based on correlations. He then
Parmesan teamed up with Yohe, also an developed a probabilistic model that could
IPCC author, to reanalyze these and other use Parmesan’s data. A key variable controldata. Another IPCC author, Stanford Uni- ling the confidence level was the likelihood
38
3 JANUARY 2003
VOL 299
SCIENCE
that a species’ observed change was properly
attributed to climate change. The model’s
result: an estimate of medium confidence, or
33% to 67%.
Parmesan then focused on effects she
calls “sign switching”; these can only be
explained by a temperature increase or
decrease—for example, spring signals such
as flowering that happen earlier during
warmer decades and later during cooler
decades. Between 80% and 100% of the 294
species examined switched as predicted by
temperature flip-flops.
Yohe found these tests to be a compelling
demonstration of causation, even though the
data sets were smaller. “To the degree to
which sign switching occurs, it’s very convincing,” he says.
In a separate study, Root attempted to
strengthen the literature review started at
IPCC by gathering a larger data set. Her
team analyzed studies in which either positive or negative changes (such as range
shifts or earlier flowering) were observed
and linked to temperature, but, unlike
Parmesan, it did not include studies that
showed no effects. In all, the team analyzed
143 studies covering 1468 species of plants
and animals around the globe; of those,
81% exhibited changes consistent with
those predicted for global warming. When
Root’s team focused on timing specifically,
it found that spring events shifted an average of 5.1 days earlier per decade.
“What we’re finding is a correlation,”
says Root. “But it’s such a robust correlation that the probability it’s by random
chance is minuscule.”
Harvard University biological oceanographer James McCarthy, co-chair of
Working Group II, says, “Before these
analyses, [the idea of] consistent response
of organisms to temperature was just a
hypothesis. Now we know that organisms
are responding globally—on all continents,
across a large range of organisms.”
But economists say that the biologists
have not yet proved their case. Tol says
Parmesan and Yohe try to deal with problems
of study selection and bias, but they fail
because the bias is inherent in the nature of
published literature. “The Root et al. paper
does not try,” he adds. Even so, he says the
papers will stimulate other groups to work on
the problem, and therefore he finds the
papers “a useful first step.”
Yohe still won’t assert with 95% confidence that global warming is causing biotic
changes. However, as more evidence accumulates that he can apply in his probabilistic
model, he expects to come around.
–MARI N. JENSEN
Mari N. Jensen is a science writer in Tucson,
Arizona.
www.sciencemag.org
CREDIT: CAMILLE PARMESAN/UNIVERSITY OF TEXAS, AUSTIN
Climate Change