Kevin Trenberth’s latest paper, which appears in Wiley Interdisciplinary Reviews: Climate Change, is uncharacteristically and refreshingly blunt right from the first few words of the introduction:
Humans are changing our climate. There is no doubt whatsoever. There are arguments about how much and how important these effects are and will be in future, but many studies (e.g., see the summary by Stott et al.1) have demonstrated that effects are not trivial and have emerged from the noise of natural variability, even if they are small by some measures. So why does the science community continue to do attribution studies and assume that humans have no influence as a null hypothesis?
That question, the nature of the null hyopthesis in climate research, is what Trenberth wants to change. So provocative is his argument, that the journal commissioned two responses to appear along with his paper and put all three online outside a firewall. Neither of the responses is supportive, but neither do they manage to definitively kill Trenberth’s proposal, which is that climatologists should turn their research design on its head. Instead of trying to disprove the idea that humans aren’t be blame for a given change in climate, he says they should be trying to disprove that we are.
Too someone not trained in the scientific process, it all seems a bit backward. But the debate goes to the heart of the science at issue, and for those interested in the nuts and bolts of climate research, it’s worth exploring. So I’ll give it a try.
Before we get into Trenberth’s paper, “Attribution of climate variations and trends to human influences and natural variability,” it will be necessary to talk about this “null hypothesis” thing. Anyone with a science degree should be able to skip the next two paragraphs.
To start, it’s important to understand that science isn’t about proving things to be true, but proving some other idea to be false. This is because we can never know for sure if the statement “all swans are white” is true; even if many, many observations fail to turn up non-white swans, there still might be a black swan out there we haven’t seen. But it is possible to know for absolute certainty that the statement is false; all we need to do is find one black swan. This “falsification” process is one of the ways science is different from other ways of knowing.
Most science that involves the standard scientific process of testing an idea requires that the researcher establish a null and an alternative hypothesis. The null hypothesis is what the research tries to falsify. The alternative is what’s left if the null hypothesis is proven wrong.
So far in climate science, the null hypothesis is that humans are not to blame for climate change. Climate research is designed to test that idea. If it finds evidence that contradicts the hypothesis that humans aren’t to blame, then it’s falsified, leaving the alternative that humans are to blame. This sort of conclusion is accompanied by a degree of confidence. In most cases, the hull hypothesis is rejected if the results shown there is less than a 5% chance that it’s correct.
But, says Trenberth, science has been pretty clear for a while now that humans are to blame, in general terms, for the global average temperature rise since the advent of the industrial revolution.
Given that global warming is ‘unequivocal’, and is ‘very likely’ due to human activities, to quote the 2007 IPCC report, should not the null hypothesis now be reversed? Should not the burden of proof be on showing that there is no human influence?
To Trenberth, a climatologist at the National Center for Atmospheric Research in Boulder, Colorado, this is not an abstract argument. There are some very real negative consequences of continuing to do things the way they’ve been done. Basically he argues that the existing null hypothesis and the nature of the changes the planet is experiences make it too easy to falsely conclude that humans aren’t to blame (what’s called a Type 2 error), and so in places where climate change is already having a serious effect (mostly the developing world), scientists aren’t able to tell politicians what they need to be hearing: that things are bad and we need to do something about it.
Georgia Tech’s Judith Curry, the first rebutter, is having none of it. The null hypothesis that we aren’t changing the global climate is “trivially false,” she writes, and so impossible to disprove. In fact, the whole idea of a null hypothesis is pointless when it comes to attributing causation for any specific change in climate, so she suggests just ditching the idea together in favor of a scientific exploration of
The key scientific question [which] is the importance of human influences relative to natural modes of climate variability.
Curry goes further, suggesting that Trenberth has strayed from science into the real of politics:
… the statement appears less about scientific analysis than about policy and winning a battle against the ‘deniers’ and reluctant politicians. In this sense, his essay comes across as a polemic. Trenberth is using the idea of reversing the null hypothesis as a metaphor for changing the political balance in the climate change debate.
… such strategies are likely to exacerbate skepticism and inflame the political debate, which can be counterproductive.
Finally, we have Myles Allen of the University of Oxford, who, while devoting a fair bit of his take on the subject to a sympathetic airing of where Trenberth is coming from, eventually decides that it’s not yet to time to reverse course.
Only when the signal of anthropogenic influence on extreme weather becomes overwhelming (which looks to be a long way off at present) will it make sense to assume human influence has increased the odds of any weather event that occurs.
Allen agrees with Curry that what we’re talking about is essentially a political issue, one revolving around the Precautionary Principle. But Curry’s solution, argues Allen, is worse that Trenberth’s.
Curry argues that because framing a scientific question in terms of hypothesis tests makes it very important where the burden of proof lies and deciding that is not a purely scientific question, the solution is to abandon hypothesis tests. This seems to be throwing the baby out with the bathwater. She questions whether the IPCC’s statement ‘most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations’ is ‘useful’ because it assigns a probability (‘very likely’, meaning less than 10% chance that the statement is false) to an imprecise statement, ‘most of the observed warming’.
There is nothing imprecise about ‘most': it means more than half. As it happens, this wording was introduced to replace the (vaguer but more evocative) phrase ‘contributed substantially’ in a nice example of the IPCC review process making its conclusions both more specific and less emotive. As Curry observes, an infinite number of statements could have been made, ranging from ‘it is extremely likely that the anthropogenic increase in greenhouse gases has caused some warming’ (not very informative, since an infinitesimally small warming is of no policy relevance) to ‘it is about as likely as not that greenhouse-gas-induced warming exceeds the total observed warming’ (which indicates the size of the greenhouse signal, but understates our confidence in attribution). Far from being a ‘poor choice’, in Curry’s words, ‘most’ was chosen for precisely the reasons she advocates: large enough to be policy relevant, while small enough for the null hypothesis ‘not most’ to be rejected at an informative confidence level.
Allen is probably correct when he writes that Trenberth’s proposal is unlikely to find a much support among his peers. By raising it, though, Trenberth may help climatologists around the world, including those charged with overseeing the next assessment from the IPCC, explore more thoughtfully the connection between how science is framed and the political context in which it will be presented. And I suspect that’s probably what Trenberth was really hoping for.
Trenberth, K. (2011). Attribution of climate variations and trends to human influences and natural variability Wiley Interdisciplinary Reviews: Climate Change, 2 (6), 925-930 DOI: 10.1002/wcc.142