Uncertain times call for strong voices
Arthur Petersen: Chief scientist at the PBL Netherlands Environmental Assessment Agency and visiting professor at CCCEP and the Grantham Research Institute
Experts charged with assessing the scientific evidence on global climate change are entering the final straight in their efforts to sum up the relevant research and offer predictions for the decades ahead.
Although content remains under wraps, the physical science part of the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC) is due to be published next year, followed by two more overviews – of impacts, adaptation and vulnerability and mitigation.
But as the authors head into their final year of drafting, they should be reminded not to be afraid of fully expressing their expert judgment on the potential future impacts of climate change.
Controversies over the IPCC’s last assessment in 2007 focused on the overstated rate of melting of the Himalayan glaciers and overemphasis of the most negative projected regional impacts of global warming.
It is less well appreciated that the report also provided estimates of some of the most important future risks that many scientists think were too conservative – but they did not include estimates of these risks in the official document.
For example, the estimates of possible rises in global sea level by the end of the century neglected the possibility of a rapid destabilisation of the land-based ice sheets in West Antarctica and Greenland, which could cause a much bigger increase.
The report openly acknowledged that this impact was excluded because current models of ice sheets cannot produce reliable projections. Yet it would have been far more helpful if the scientists had used their expert judgment to estimate how much more sea levels might rise than the 59 centimetres presented in the worst case scenario in the report.
Similarly, the climate models cited by the IPCC in estimates of temperature change by the end of the century did not fully take into account uncertainties in the relationship between climate and the carbon cycle. And even though this shortcoming was openly acknowledged in the 2007 assessment, the researchers who prepared the report’s summary for politicians did not attempt to quantify how it might affect future projections of temperature.
As a result, policy-makers and the public are not being fully informed of the worst potential consequences.
Why has this come about? A key reason is that the scientists involved in producing the assessments have been reluctant to over-rule conservative estimates by computer models. This is largely due to the insufficient guidance on communicating uncertainty given by the IPCC in the past, as I point out in my book ‘Simulating Nature: a philosophical study of computer-simulation uncertainties and their role in climate science and policy advice’.
The IPCC has issued improved guidance on how authors should express uncertainty surrounding such model predictions, but it remains to be seen how effective this will be in avoiding the issues seen in 2007. Use of these guidelines in a recent document resulted in an assessment of uncertainty that was technical and hence may not help provide clarity for politicians and the public.
As the next major assessment takes shape, the message is that researchers must be encouraged to speak up and feel free to convey uncertainties over the risks ahead in a way that allows politicians and the public to get the bigger picture – one that encompasses the full range of potential impacts of climate change.