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.

Today’s debate over global warming echoes that era. Because of politics, a post like this, intended to inform, will sway few minds. But I have spoken with skeptics who honestly want to understand, but don’t have the tools to grapple with such a large, complex field of science. And they have a point — while we talk a lot about the data, we rarely describe the path to a conclusion.

Provoked by their questions, I began to dig. 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.

Here’s the story.

Data collected by Gallup from 1954 to 2013. Image Credit: Gallup

In the 1950s, Bradford Hill kept a box of cigarettes in his desk at the London School of Hygiene and Tropical Medicine.

Professor Hill led the school’s Statistical Research Unit, and like most men of the establishment, he would open his box to respected visitors. This was hardly remarkable, save for one detail: Hill was lead statistician on the British Doctor’s Study. This was one of two large studies that, when published in 1954, led the American Cancer Society to declare that ‘the presently available evidence indicates an association between smoking, particularly cigarette smoking, and lung cancer’.

Between 1930 and 1940, the lung cancer rate among men tripled. Between 1940 and 1950, it doubled again. Between 1950 and 1960, it nearly doubled again. To quote the Surgeon’s General’s famous 1964 report linking smoking to lung cancer, “This extraordinary rise has not been recorded for cancer of any other site.”

Yet in the 1950s, even with data against smoking amassing, it was still considered rude not to offer a cigarette.

There was no singular moment when scientists realized that smoking caused lung cancer. Beginning in 1912, when the first suggestion was made, scientists slowly built multiple strands of evidence, refining experiments as they learned. As the case grew in strength, each scientist, looking at the evidence before him (it was almost all men), individually concluded the causal connection was irrefutable.

Hill embarked on the Doctor’s Study because his previous research, performed with his longtime collaborator Professor Richard Doll in 1950, found a substantial correlation between cigarettes and lung cancer in a small patient population. The 1950 link was so striking, in fact, that Doll gave up his cigarettes immediately.

Yet Hill himself held on to his pipe until the Doctor’s Study completed in 1954.

And while Hill and Doll and the American Cancer Society were in agreement by 1954, even in the late 1950s high level critics remained, including the esteemed statistician Ronald Fisher, who pronounced himself “extremely skeptical of the claim that decisive evidence has been obtained.”

Fisher was not a man to be taken lightly. As a scientist, he has been described by the scientist Richard Dawkins as “the greatest biologist since Darwin”. He provided a mathematical basis for evolutionary theory. He single-handedly created most of modern statistics and the design for the randomized controlled trial, which went on to become the primary tool of medical research.

Fisher also smoked, preached libertarian political views, and was an advisor to the Tobacco Manufacturers Standing Committee. He was not happy about the idea that scientists should inject “propaganda” onto an unsuspecting public. Especially because he believed the science was wrong.

To illustrate Hill and Doll’s folly, Fisher tore apart their data, highlighting discrepancies between cancer rates in cigarette, cigar, and pipe smokers. He described how much of the increase in cancers could be ascribed to improved methods of detection. And he inaugurated the study of spurious correlations, showing that Doll and Hill’s methods would directly tie an increase in the import of apples to an increase in the divorce rate.

Hill would eventually be proven right. But he needed to develop better tools to show this. And by creating these tools, he would define the rules of proof in epidemiology for the next fifty years.

Data credit: cancer.org, original image slightly reformatted by me in Tableau

It is simple to show correlation. But how can one prove causation?

This problem is not limited to studies of smoking — it extends through all of science. In fact, if you were to ask scientists outside epidemiology what process they use to “prove” causality, I’d wager that most would either change the subject, or stare back blankly at you. Different scientists evidently maintain different standards for proof. No one is working with a standard process.

This is vaguely unsettling.

The problem certainly infects the global warming debate. “Proof” gets thrown around by different people in different way, leaving everyone confused. Politically-driven skeptics leave the term undefined to sandbag the discussion for their own purposes — it’s easier to claim “not enough is known” when you never define “known”. Yet the lack of definition hovers like a fog over anyone trying honestly to parse out the answers for themselves. It’s hard to have faith in something so ill-defined.

And this brings us back to Professor Hill.

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.

Ronald Fisher passed away before Hill published his criteria, so he never had a chance to offer his final verdict. But the field of epidemiology has. Hill’s Criteria have effectively ended the debate over how to analyze cause, and have been used largely unchanged for the last fifty years. Fisher’s contribution was not to prove Hill wrong, but to make Hill’s arguments stronger. While Fisher’s skepticism did much damage to the public (some of whom might have stopped smoking sooner but for his efforts), the battle forced Hill to structure his thinking, to the benefit of all of science.

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.

So from here, we go off from the history of cigarettes and heath, and drive to the weeds of global warming.[1] What happens when we apply Hill’s criteria to the question:

Are humans, by adding CO2 to the air, causing the planet to warm?