Cross-posted from NextGen Journal

It is common for one to fail to grasp the difference between “consensus” and “unanimity”.

A consensus does not require agreement from absolutely every member involved. Rather, it is a more general measure of extremely high agreement, high enough to accept and base decisions on. It’s stronger than a majority-rules style of democracy, but does not necessarily equal unanimity. In fact, in the area of science, where the concept of consensus is particularly important, unanimity is nearly impossible.

With the exception of pure mathematics, scientific theories cannot be proven beyond a doubt. Every physical process that researchers study has some amount of irreducible uncertainty – because there is always, no matter how small, a chance that our understanding could be completely wrong. Additionally, science is never “settled”, because there is always more to learn, whatever the field. Even a law as basic as gravity is still being studied by physicists, and it turns out that it gets more complicated the more you look at it.

Despite this inherent uncertainty, scientists have developed consensuses around all sorts of topics. The Earth is approximately oblate-spherical in shape. Smoking cigarettes increases one’s risk of lung cancer. HIV causes AIDS. There’s a tiny chance that these statements are incorrect, but researchers can still have confidence in their accuracy. Incomplete knowledge is not the same as no knowledge.

However, when there is room for doubt, there will usually be doubters. Physicist Richard Lindzen continues to dispute the health risks of smoking (a conversation is recounted in a recent book by James Hansen). Peter Duesberg, an active molecular and cell biologist, prominently opposes the link between HIV and AIDS. Believe it or not, the Flat Earth Society was alive and well until the death of its leader in 2001 – and signs of the society’s renewal are emerging.

As these examples suggest, for a layperson to wait for scientific unanimity before accepting a topic would be absurd. When consensus reaches a certain point, the null hypothesis shifts: the burden of proof is on the contrarians, rather than the theory’s advocates.

Another case study that may seem surprising to many is that of anthropogenic global warming. A strong scientific consensus exists that human activity, mainly the burning of fossil fuels, is exerting a warming influence on the planet’s temperature, which is already beginning to show up in the instrumental record. This phenomenon is contested by less than 3% of publishing climatologists, a negligible amount of peer-reviewed scientific studies (so few that not one showed up in a 2004 survey’s random sample of almost one thousand papers), and no major scientific societies internationally. Additionally, scientists who dispute the existence or causes of climate change tend to have lower academic credibility than those who do not. It becomes apparent that this scientific question warrants “consensus” standing: never quite settled, never quite unanimous, but certainly good enough to go by. The mainstream media does not always reflect this consensus accurately, but it nonetheless exists.

As world leaders meet in Cancun this week to discuss a global policy to prevent or limit future climate change – a prospect that looks less likely by the day – science can only offer so much advice. Climatologists can approximate what levels of emissions cuts are required to prevent unacceptable consequences, but only when the governments of the world decide which consequences they are willing to accept. Can we deal with worldwide food shortages? Rising sea levels? What about a mass extinction? Even after we define “dangerous consequences”, scientists are unsure of exactly how much temperature change will trigger these consequences, as well as how much greenhouse gas emissions will need to be cut, and how quickly, to prevent the temperature change. All they can offer is a range of probabilities and most likely scenarios.

But remember, incomplete and uncertain knowledge is not the same as no knowledge. Of one thing climate scientists are sure: the more greenhouse gas emissions we emit, the more the world will warm, and the harder it will be to deal with the consequences. There’s no reason for you and I to doubt that simple correlation any longer.