In his recent State of the Union message, President Obama flatly stated that “Climate change is a fact.” These are fighting words with an influential minority of the American public, and the President may be itching to have this fight. But saying that climate change is a “fact” is a disservice to the public because it invites people to think about the issue in a misleading and unproductive way.

For one thing, climate change is not all “fact.” Scientists recognize the tremendous complexity of climate change. They use supercomputers to crunch billions and billions of numbers to make quantitative models to predict future events and they recognize that some of the model results are more trustworthy than others. There is near unanimity among scientists about some aspects of climate change, but widespread disagreement about others. So the President’s statement is oversimplifying.

Simplification is necessary for understanding complex issues. Psychological research shows that almost everyone thinks about complex issues by applying simple models that relate them to familiar personal experiences. But simple models can sometimes lead people astray. What we need are mental models that help us think about complex problems in ways that point us toward choices that will benefit ourselves, our families, our communities, and our nations.

What we need are mental models that help us think about complex problems in ways that point us toward choices that will benefit ourselves, our families, our communities, and our nations.

The “fact” model of climate change doesn’t do this. It invites us either to stop asking questions (the President’s apparent intent) or, if we want to ask questions, to continue the argument about what is or is not a fact. If we choose to argue about facts, we will be tempted to adopt a familiar model for determining truth—the one from criminal law, in which the defendant is innocent until proven guilty beyond a reasonable doubt. This way of thinking is favored by those who oppose action on climate change because it suggests that nothing should be done to change our patterns of fossil fuel consumption until we have near-absolute proof of their “guilt.”

Criminal law provides an unhelpful mental model for thinking about climate change because we are dealing with a mixture of facts, probabilities, and uncertainties; because decisions need not be final; and because there are more than two possible courses of action. A more helpful model for this kind of situation can be taken from medicine. Imagine scientists as doctors and yourself as a guardian of the planet, which is the patient. A panel of doctors has diagnosed the planet with a serious, progressive disease (anthropogenic climate change). The symptoms are not obvious, just as they are not with diabetes, hypertension, or some forms of cancer, but the disease may be just as serious. You have to decide what to do, or to ask your government to do.

In this situation, you should have a lot of questions. You will want to ask how certain the doctors are of the diagnosis, the likely course of the disease if left untreated, and about the treatment options and their possible side effects. You may want to seek out second opinions, in case the medical team is missing something or has some kind of bias. This way of thinking is helpful because it prompts us ask the right kinds of questions, think through the consequences of various courses of action and inaction, and consider ways to adapt our actions over time as we learn more.

A Medical Metaphor for Climate Change

Diagnosis. Physicians must be careful to avoid two errors: misdiagnosing the patient with a dread disease that is not present, and misdiagnosing a seriously ill patient as healthy. To avoid these types of error, physicians often run diagnostic tests or observe the patient over a period of time before recommending a course of treatment. Scientists have been doing this with Earth’s climate at least since 1959, when strong signs of illness were reported from observations in Hawaii.

Scientists now have high confidence that the patient has the disease, and they know the causes: fossil fuel consumption, certain land cover changes, and a few other physical processes that humanity has helped set in motion, mainly over the past century. We know that the disease produces a complex syndrome of symptoms involving change in many planetary systems (temperature, precipitation, sea level and acidity balance, ecological regimes, etc.). The patient is showing more and more of the syndrome, though it has its ups and downs, and although we cannot be sure that each particular symptom is due to climate change rather than some other cause, the combined evidence justifies strong confidence that the syndrome is present.

Prognosis. Fundamental scientific principles tell us that the disease is progressive and very hard to reverse. Observations tell us that the processes that cause it have been increasing, as have the symptoms. Without treatment, they will get worse. However, because this is an extremely rare disease (in fact, the first known case), there is uncertainty about how fast it will progress. It could be catastrophic, but we cannot assign a firm probability to the worst outcomes, and we are not even sure what the worst outcomes will look like. We want to avoid either seriously underestimating or overestimating the seriousness of the prognosis. One reason scientists work hard on climate models and debate about them is to improve prognostic skill.

Treatment. We want treatments that improve the patient’s chances at low cost and with limited adverse side effects and we want to avoid “cures” that might be worse than the disease. We want to consider the chances of improvement for each treatment, and its side effects, in addition to the untreated prognosis. We want to avoid the dangers both of under-treatment and of side effects. We know that some treatments (the ones limiting climate change) get at the causes and could alleviate all the symptoms if taken soon enough. But reducing the use of fossil fuels quickly could be painful. Other treatments, called adaptations—things like raising sea walls and improving preparedness for forest fires—offer only symptomatic relief. These make sense because even with strong medicine for limiting climate change, the disease will get worse before it gets better.

Choices. There are no risk-free choices. We know that the longer treatment is postponed, the ore painful it will be, and the worse the prognosis. We may choose an iterative treatment strategy, starting some treatments and monitoring their benefits and side effects before raising the dose. People will disagree about the right course of treatment, but thinking about the choices in this way might get people to talk more about choosing a course of action and less about whether climate change is proven fact.

Second Opinions. We all know that there are scientists who aren’t convinced that climate change is happening or that the causes are human in origin. What should one make of their opinions? It is always hard to evaluate the technical arguments of specialist communities when we don’t have that expertise. There are useful ways to assess these arguments from the outside—looking at the scientists’ reputations, their sources of financial support, how they deal with disagreements, and so on. In my view, these considerations strongly favor believing mainstream climate science over its detractors. But even without making such assessments, it is worth looking closely at what the detractors claim. For the most part, the argument is that of the defense attorney: that guilt has not been proved beyond reasonable doubt. Even if you believe that, it does not justify doing nothing to keep this defendant in check.

Paul C. Stern first wrote about a medical metaphor for Climate Change on the NY Times Dot Earth Blog.