On climate models, the case for living with uncertainties

The council’s chairman, Harold Shapiro of Princeton, said existing IPCC guidelines on presenting uncertainty “have not been consistently followed”. In particular, its analysis of the likely impacts of climate change “contains many statements that were assigned high confidence but for which there is little evidence”. The predictions were not plucked from the air. But the charge against the IPCC is that its authors did not always correctly portray the uncertainty surrounding the predictions or present alternative scenarios.

The most notorious failure was the claim that the Himalayan glaciers could all have melted by 2035. This was an egregious error resulting from cut-and-pasting a non-peer reviewed claim from a report by a non-governmental organisation. So was a claim that 55 per cent of the Netherlands lies below sea level. But other errors were failures to articulate uncertainties. The study highlighted a claim that even a mild loss of rainfall over the Amazon could destroy 40 per cent of the rainforest, though only one modeling study has predicted this.

Another headline claim in the report, in a chapter on Africa, was that “projected reductions in [crop] yield in some countries could be as much as 50 per cent by 2020”. The only source was an 11-page paper by a Moroccan named Ali Agoumi that covered only three of Africa’s 53 countries (Morocco, Tunisia, and Algeria) and had not gone through peer review. It simply asserted that “studies on the future of vital agriculture in the region have shown ... deficient yields from rain-based agriculture of up to 50 per cent during the 2000-2020 period”. No studies were named. And even Agoumi did not claim the changes were necessarily caused by climate change. In fact, harvests in North Africa already differ by 50 per cent or more from one year to the next, depending on rainfall. In other words, Agoumi’s paper said nothing at all about how climate change might or might not change farm yields across Africa. None of this was conveyed by the report.

In general, the InterAcademy Council’s report noted a tendency to “emphasise the negative impacts of climate change,” many of which were “not supported sufficiently in the literature, not put into perspective, or not expressed clearly”. Efforts to eliminate these failings will necessarily widen the error bars on a range of predictions in the next assessment.

We are all - authors and readers of IPCC reports alike - going to have to get used to greater caution in IPCC reports and greater uncertainty in imagining exactly how climate change will play out. This is probably healthy. It is certainly more honest. But it in no way undermines the case that we are already observing ample evidence that the world is on the threshold of profound and potentially catastrophic warming. And it in no way undermines the urgent need to do something to halt the forces behind the warming.

Some argue that scientific uncertainty should make us refrain from action to slow climate change. The more rational response, given the scale of what we could face, is the precise opposite