When it’s right to be reticent

[This is the pre-edited version of my latest article for muse@nature.com]

The caution of climate scientists is commendable even if caution is out of fashion.

Jim Hansen is no stranger to controversy. Ever since the 1980s he has been much more outspoken about the existence and perils of human-induced climate change than the majority of his scientific colleagues. A climate modeller at NASA’s Goddard Institute for Space Studies in New York, Hansen has flawless credentials to speak about climate change – and his readiness to do so has led to accusations of political interference and censorship (see here).

But his views haven’t only ruffled political feathers – they have dismayed other scientists too, who are uncomfortable with what they see as Hansen’s impatience with science’s inherent caution.

So in some ways, Hansen’s latest foray will surprise no one. In a preprint submitted for publication, he claims that “scientific reticence” is seriously underselling the potential danger that climate change poses – specifically, that it “is inhibiting communication of a threat of potentially large sea level rise.” Because disintegration of polar ice sheets is poorly understood, it is very difficult for scientists to make a reliable estimate of the likely future changes in sea level. As a result, Hansen charges, they have put figures on those aspects of sea-level rise they can estimate with some confidence, but have refrained from doing so for this key ingredient of the problem, giving the impression that the probable changes will be much smaller than those Hansen considers likely.

The responsibility for pronouncing on such issues falls primarily on the Intergovernmental Panel on Climate Change (IPCC), which Hansen regards as conservative. This, he admits, contributes to IPCC’s authority and is “probably a necessary characteristic, given that the IPCC document is produced as a consensus among most nations in the world and represents the views of thousands of scientists.” The most recent IPCC report has been characterised as the most strongly worded yet, but its conclusions apparently still required much negotiation and compromise.

And yet Hansen believes that “Given the reticence that IPCC necessarily exhibits, there need to be supplementary mechanisms” for communicating the latest scientific knowledge to the public and policy makers. He calls for a panel of leading scientists to “hear evidence and issue a prompt plain-written report” on the dangers – which clearly he envisages as a much more forceful statement about impending climate catastrophe and the need for immediate action to “get on a fundamentally different energy and greenhouse gas emissions path”.

This is a strange proposal, however. Basically, Hansen is calling on the scientific community to collect their scientific thoughts and then to speak out unscientifically – which is to say, without the caveats and caution that are the stock-in-trade of good science. However, Hansen points out that in fact scientists do this all the time – when they are talking among themselves. He recalls how, challenged by a lawyer acting on behalf of US automobile manufacturers to name a single glaciologist who agreed with his view that ice-sheet break-up would cause sea-level rise of more than a metre by 2100, he could not do so. Even though he had heard plenty of such scientists express deep concerns to this effect in private exchanges, none had said anything definitive in public.

Why wouldn’t they do that, if it’s really what they thought? Hansen posits what he call a “John Mercer effect”. In 1978 Mercer, a glaciologist at Ohio State University, suggested [1] that anthropogenic global warming could cause the West Antarctic ice sheet to disintegrate and sea level to surge by over 5 m within 50 years. Mercer’s paper was disputed by other scientists, who were generally portrayed as the sober and authoritative counterbalance to Mercer’s “alarmism”.

“It seemed to me”, says Hansen, “that the scientists preaching caution and downplaying the dangers of climate change fared better in receipt of research funding.” This reticence, he suggests, is encouraged and rewarded both professionally and financially.

Hansen says he experienced this himself in the early days of climate-change research. He was one of the first to point out, in a paper coauthored in 1981, that rising levels of atmospheric carbon dioxide could be linked to a warming trend throughout the twentieth century [2]. At that time the trend itself wasn’t so clear – the globe was only just emerging from a three-decade cooling spell, now known to be caused by atmospheric aerosol particles that temporarily outweighed the greenhouse-gas contributions.

But by 1989 Hansen was prepared to state with confidence that we could already see the effects of human-induced greenhouse warming in action. His colleagues felt this was jumping the gun – that it was still too early to rule out natural climate variability.

This history is instructive in the face of common claims from ‘climate sceptics’ that climate scientists play up the threat of global warming in order to secure funding. Anyone who witnessed (as I did) the slow and meticulous process that brought climate scientists from this position in the late 1980s to what is effectively a consensus today that human-induced climate change is almost certainly now evident will recognise the nonsense of the sceptics’ claim. The dogged reluctance to commit to that view in the late 1980s [3] looks rather remarkable now; but it was correct, and the community can regard its restraint with pride.

Yet it also means that Hansen was in a sense right back then. Such retrospective vindication, however, is not in itself justification. He could just as easily have been wrong. His views may have been based on sound intuition, but the science wasn’t yet there to support it.

All the same, Hansen is right to say that “scientific reticence” poses problems. He points out that, because the climate system is nonlinear (and in particular, because there are positive feedbacks to ice-sheet melting), excessive caution could end up sounding the alarm too late. Possibly it already has.

The question is what to do about that. But the real issue here is not that scientists are “reticent” – it is that the public, politicians and leaders are not accustomed to reasoning and debating as scientists do. It is within the very grain of science – Popper’s legacy, of course – that it advances by self-doubt. The contemporary culture, on the other hand (and probably it has never been very different), favours dogmatic, absolute statements, unencumbered with caveats. If they prove to be wrong, no matter – another equally definitive statement will blot out memory of the last one. Thus you can say something such as HIV does not cause AIDS, or there is no such thing as society, and still be taken seriously years later as a commentator on current affairs.

The moment it abandons its caution and claims false certainty, science loses its credibility; indeed, it ceases to be true science. This is not to say that scientists should commit to nothing for fear of being proved wrong. Nor is it by any means a call for scientists to step back from making pronouncements that guide public policy – if anything, they should do more of that. But when they are talking about scientific issues, scientists cannot afford to abandon their (public) reticence. It is as individuals, not as community spokespeople, that they should feel free, as Hansen rightly does, to voice views, intuitions and beliefs that reach beyond the strict confines that science permits.

References
1. Mercer, J. Nature 271, 321 – 325 (1978).
2. Hansen, J. et al. Science 213, 957 – 966 (1981).
3. Kerr, R. Science 244, 1041 – 1043 (1989).