Perceived uncertainty and the response to global warming
Jonathan Baron, Jay Schulkin, Howard
One group of college students read a description of the
greenhouse effect that emphasized scientific uncertainties. A
control group read a description containing only best estimates,
without any mention of uncertainty. In the control group, those
who originally did not strongly favor immediate action to combat
global warming became more willing to act after reading the
description, and those who strongly favored action became less
willing. In the uncertainty group, subjects maintained their
original willingness or unwillingness to act. When expressing
uncertainty, scientists might have to warn their listeners
against using it only to bolster their prior opinion.
The effects of the accumulation of greenhouse gasses in the
atmosphere are highly uncertain. Some observers argue that this
uncertainty must be resolved before action is taken. Others
argue, on the basis of worst-case scenarios, that the very
uncertainty increases the need for action.2
Both points of view are, in principle, justifiable. The argument
for inaction is that the expected cost of waiting for new
information is less than the expected cost of error from
precipitous action. The argument for action is that delay
increases expected costs.
It is possible that those who hold these contrasting positions
arrive at them not through dispassionate analysis of expectations
but, rather, on the basis of prior opinion. People may tend to
seize on the uncertainty itself as a justification for whatever
course they favored initially. When information is presented as
uncertain, people may attend mainly to the possible outcomes that
they most fear: being caught unprepared for serious warming, or
wasting money on unnecessary preparation. Although we know of no
psychological evidence for exactly this effect, a number of
findings are similar. For example, conflicting evidence is
interpreted so as to strengthen an initial view3,
and ambiguous probability information is subjectively distorted
when used in decision making 4.
To examine the effect of uncertainty, we gave each of 72 college
students a description of the basic facts about global warming as
reported in the press. A control group of 36 subjects read a
description in which the facts were presented as best guesses,
without any mention of uncertainty. An uncertainty group read an
identical description in which the facts were presented with
ranges so as to emphasize the uncertainty. Before and after
reading the description, the subjects filled out a five-question
attitude survey concerning actions about global warming.
Examples of the differences in the two forms of the description
are as follows, with material in brackets added to the
uncertainty version: `This increased carbon dioxide has already
begun to cause a warming of the average temperature of the
atmosphere of about 1 degree Celsius over the last 50 years.
[(Scientists disagree on the amount of warming that has already
occurred: estimates range from 0 to 2 degrees.)]' `The average
estimate is that the warming over the next 60 years will be about
5.5 degrees Fahrenheit, if present trends in fuel use continue,
[but the estimates range from about 2.7 to 8.1 degrees].' `The
total cost of adapting to these changes is estimated at $250
billion per year for the United States alone (about half of the
current military budget), [although estimates range from $50
billion to $500 billion].' Both versions ended with a statement
that `changes are easier to implement when more time is
available,' but the ending of the uncertainty version began with,
`The uncertainties will not be resolved in the next few years.'
The attitude survey consisted of the following questions, each
answered on a scale from +3 (strongly support) to -3 (strongly
oppose): `The U.S. should undertake a massive program of
research on new energy sources that do not yield carbon dioxide.'
`The countries of the world should agree on targets for reducing
the production of carbon dioxide.' `The U.S. should take
unilateral steps to reduce carbon dioxide through incentives for
alternative fuels' `The U.S. should take unilateral steps to
reduce carbon dioxide through taxes on fuels that produce carbon
dioxide, such as oil, gasoline, and coal.' `The U.S. should take
unilateral steps to reduce carbon dioxide through incentives for
conservation.' The total score, which could range from -15 to
15, was used in analysis.
The results are shown in Table 1, broken down according to the
initial scores on the attitude survey. In the control condition,
those with strong initial opinions tended to moderate them. In
the experimental condition, this moderation did not occur.
Mean scores before and after, by condition, for subjects grouped
by initial score.
Control group Uncertainty group
Initial Mean Mean Mean Mean Mean Mean
score N initial final change N initial final change
0 3 -2.7 3.7 6.3 3 -5.0 -2.0 3.0
1-5 4 3.5 4.3 0.8 4 3.8 4.3 0.5
6-10 9 8.0 9.3 1.3 17 8.1 10.5 2.4
11-13 13 12.4 12.1 -0.3 8 12.1 13.9 1.8
14-15 7 14.1 13.7 -0.4 4 14.3 14.3 0.0
Although these differences between conditions were small, their
statistical significance was clear. The final score was
regressed on the initial score, condition (control vs.
uncertainty), and the interaction of control and
As hypothesized, the interaction term was significant (t=2.74,
p=0.008). The effect of initial score was also significant
(t=2.37, p=0.02), as was the overall regression (R2=0.79,
p<.001). Condition itself had no significant effect.6
Another way to describe the results is to regress the change
score (final minus initial) against the initial score in each
condition. These regressions yielded the following:
(control) change = 4.0 - 0.34(initial score);
(uncertain) change = 2.4 - 0.06(initial score).
The effect of initial score was significant in the control
condition (t=4.97) but not in the uncertain condition. In sum,
facts lead to moderation of opinions when the facts are presented
as best estimates, but when they are presented as uncertain,
people seem to use the facts as support for their initial views,
whatever these views might be.
Three accounts of these results are possible. First, subjects
who favor action could be justifiably (or unjustifiably) afraid
of failing to prepare for the worst case, the upper end of the
range of effects, and subjects who oppose action could likewise
fear overreaction to the best case, the lower end.7 In the uncertain
condition, these extremes remain possible. Second, subjects in
the uncertain condition have less reason to question their prior
views (which might, or might not, be based on reliable
information). Third, subjects in the uncertain condition could
attend most strongly to the end of the range that justified their
earlier views, because they are reluctant to consider opposing
Expression of uncertainty affects the interpretation of
scientific evidence as a basis for action. Scientists should
express uncertainty, but they might also do well to warn their
audience against the possibility of biased interpretation.
1Department of Psychology, Department of Anatomy,
Department of Decision Sciences, respectively, University of
Pennsylvania, Philadelphia, PA 19104, U.S.A.
Coldglazier, Scientific uncertainties, public policy, and
global warming: How sure is sure enough? University of
Tennessee, Knoxville, 1990; E. B. Skolnikoff, Foreign
Policy, 1990, 79, 77-93.
3C. G. Lord
et al., J. Pers. Soc. Psychol., 37, 2098-2109, 1979.
4H. J. Einhorn & R. M. Hogarth,
Psychol. Rev., 92, 433-461, 1985. H. Kunreuther,
Risk Analysis, 9, 319-328, 1989.
5The groups did not differ significantly in the
distributions of initial scores, final scores, or mean change.
results were unchanged when quadratic terms were included for the
initial score. The interaction was confined to the linear term,
with t=3.01, and the quadratic component itself was not
Ecology Law Quarterly, 7, 207-243, 1978.
8J. Baron, Thinking and deciding, New York:
Cambridge University Press, 1988.
File translated from
On 12 Nov 2004, 05:25.