News in Science

IPCC needs to 'use more numbers'

Matching expectations Experts should improve the way they communicate about climate science to the public, a new global study has found.

The research, published today in the journal Nature Climate Change, suggests the IPCC needs to include probability percentages alongside phrases such as 'likely' or 'unlikely'.

Although it seems that even this will not convince everyone of the scientific view of uncertainty, says Professor Michael Smithson of the Research School of Psychology at the Australian National University.

To communicate uncertainty in climate change models and predictions, the IPCC uses a range of expressions to describe the probability that a particular event will occur.

For example, in the phrase: "It is very likely that heat extremes will become more frequent in the future," the phrase 'very likely' is used to describe a likelihood of more than 90 per cent, says Smithson.

In a study of more than 12,000 people in 24 countries, he and colleagues examined what the public understands by such phrases.

The researchers used eight sample statements to test people's understanding of the IPCC phrases 'very likely', meaning more than 90 per cent; 'likely', meaning greater than 66 per cent; 'unlikely' meaning less than 33 per cent; and 'very unlikely', meaning less than 10 per cent.

They found that the public equated these phrases with greater uncertainty than intended by the IPCC.

"They gave ratings closer to the middle of the range of the scale from 0 to 100," says Smithson.

For example, the expression 'very unlikely' was equated with less than 40 per cent likelihood, and the expression 'very likely' was equated with more than 70 per cent likelihood.

"People are not interpreting these phrases in the way the IPCC is wanting them to," says Smithson.

Using numbers

In a different set up, the researchers embedded the probability percentage used by the IPCC in the sample statement itself. For example, "It's very likely (> 90 per cent) that heat extremes will become more frequent in the future."

Under these conditions, the participants gave a more accurate reflection of the scientific uncertainty, says Smithson.

However, even under these circumstances they were still not totally accurate, he adds.

"They were more accurate if we gave them the number in the sentence but they were still underestimating if it was a positive expression, and they were overestimating if it was a negative expression," says Smithson.

"We got the same pattern that seemed pretty independent of what language or culture people came from. So it is a very consistent effect, and it is rather puzzling."

Smithson says when the researchers asked the participants to describe what they themselves meant by such phrases, there was a "moderately strong correlation" between the percentages they gave and those given in the main study.

He says the findings suggest the greater uncertainty expressed by participants may be due to this personal interpretation.

"They may be anchoring on their own personal interpretations of these phrases," says Smithson, adding further investigation needed.

The researchers also found that people who have doubts about climate change equated the phrases with more uncertainty than intended by the IPCC.

Although the lay public consistently overestimate the uncertainty of climate change science, Smithson and colleagues have a recommendation they think might help reduce misinterpretation of the current scientific consensus on the topic.

"The IPCC would be wise in future reports to actually embed the intervals that they're referring to, when they use phrases such as 'very likely', in the sentences themselves," says Smithson. "That certainly gets people closer to complying with their interpretations of that."