Saturday, 28 January 2017

Hill’s Criteria applied to climate change

In What climate skeptics taught me about global warming Seth Miller writes that:
Long before research exposed evidence that humans cause global warming, science made another sensational claim — that smoking caused lung cancer.

That case has been proven beyond doubt. But there is a science story from this era that is mostly forgotten: The battle against cigarettes taught science how to prove.

Before linking cigarettes to lung cancer, science had no established method to prove that one thing caused another. The fields of epidemiology and statistics were new, and while they had some prior successes, the questions were so evident — think about mercury causing madness — that proof did not require the level of meticulousness that modern science expects. The need to establish a link between cigarettes and lung cancers — and the backlash that ensued — changed this. Epidemiology and cigarettes grew up together.
And I unearthed a notion that is rarely mentioned in the global warming debate: Science actually has a method for establishing that one thing causes another. Scientists don’t have to vote on the issue — the 97% consensus of climate scientists who believe that humans cause warming is telling, but only one part of a broader process. And for those who want to honestly weigh their skepticism in context of the evidence, there is a way.
The battle against smoking was the first bare-knuckles public policy debate driven by science. So over years of defending his work, Hill had to think deeply about what constitutes ‘proof’, and how to overcome the intelligent rebuttals of the world’s Ronald Fishers.

In 1965, he formally proposed a solution.

Hill recognized that there are more ways to support causation that finding that two variables track. In fact, Hill identified nine separate strands of ‘proof’, each of which makes an independent case for or against causation. The list of nine aspects — and I’ll go into details below — are now called Hill’s Criteria.

You don’t need strong support from all of the strands to prove a result. But when independent strands tell the same story, with no contradictions, the case is strong. Perhaps as importantly, by using fixed criteria, we can categorize not just data we have, but identify what data are missing as well. And with all of the possible evidence in mind, we can effectively draw a conclusion using classic, human judgment.
And while Hill’s Criteria are not commonly used outside epidemiology, they should be. The criteria take an impossibly large and complex pile of data and break them up into chunks. They make the evidence understandable. And they make the case for causality transparent — each piece of evidence is categorized, and weighed in the context of the whole. If evidence is challenged, it becomes clear just how devastating or inconsequential that challenge is. We lose any presumption that somehow a single set of data could prove the entirety of scientific understanding to be in error.

What happens when we apply Hill’s criteria to the question:
Are humans, by adding CO2 to the air, causing the planet to warm?
Miller then goes on to test this question against Hill's Criteria.

This is worth a read.

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