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In the recent flurry of moves to ban plastic bags a frequently cited statistic is that more than 100,000 marine mammals and sea turtles die each year from entanglement in, or ingestion of, plastic bags.

The original scientific study upon which this estimate relied actually attributed these deaths to fishing tackle in the oceans, not plastic bags. Yet the terms “100,000 marine deaths” and “plastic bags” now circulate happily through our public discourse, solidified as established fact.

But when is a fact a fact? Can facts change over time? And does it matter if they do? Science is instinctively referred to as the source and authenticator of facts such as the one cited above, and rightly so.

Yet as this example shows, we need to be very careful about the veracity of the numbers we latch on to, and about what they signify. What may start out as a credible, yet qualified and provisional, scientific estimate may end up, either through distortion or mere negligence, enduring as an urban myth, apocryphal numbers – the modern equivalent of folklore.

My own area of climate change offers plenty of such examples.

In December 2005 a study in the journal Nature offered the observation that the circulation in the North Atlantic Ocean, which sustains the Gulf Stream, had weakened by up to 30 per cent over the previous few decades. This figure and its juxtapositioning alongside the melodrama of films such as The Day after Tomorrow were amplified through the cooperation of scientists and media to result in headlines such as “Alarm over dramatic weakening of Gulf Stream” ( The Guardian, Dec 1, 2005). The urban myth that emerged from this episode was that we were closer to a mini Ice Age in the UK than had previously been thought. Eighteen months later, however, and unremarked by the media, two studies in equally reputable journals pointed out that such a trend was within the range of natural variability and may signify nothing at all.

A second example concerns the claim that, “by the end of this century, climate change will have killed around 182 million people in sub-Saharan Africa” (Christian Aid, May 2006). This number – 180 million African dead – has become one of the most widely cited numbers in the litany of doom that accompanies talk of climate change. In this case, however, the number 180 million was sexed-up science. Christian Aid took the worst-case climate scenario, the highest population scenario and the scenario with the least public health intervention and conjured the number into being. And here it has stayed, a number detached from its receding scientific origins in which assumptions were overlain on scenarios that captured uncertainties.

Whether through being lost in translation, through the premature citing of provisional science or through the purposeful sexing-up of deeply uncertain numbers, the facts of science are not always to be taken at face value.

Mike Hulme is a professor in the School of Environmental Sciences at the University of East Anglia

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