Historically, the greatest scientists are great precisely because they broke with the consensus. Remember this when discussing global warming and other environmental issues.

Such issues are always contentious, for they share two characteristics: They are technically complex and highly emotional. Can you think of a single environmental issue that isn’t both? Global warming tops the scale.

Advocates for dramatic action on climate change often base their appeal on the authority of scientific “consensus.” For example, “A majority of climate scientists including 99 of the world’s Nobel Prize winners, have signed a petition for the world’s leaders to act immediately to reduce greenhouse gas emissions.”

Even the most well informed citizen can’t evaluate every issue. And when experts disagree we naturally assume the majority is likely to be correct. This is a reasonable way to make sense of conflicting opinion and legitimate scientific uncertainty. But we should remember two important points.

First, science, unlike the race for homecoming queen, is not a popularity contest. While most unlikely in the climate change debate, scientific “consensus” may be overturned by a single experiment. When new results emerge and peers repeat them consistently, the old consensus crumbles. Here’s the process.

Developed in the 17th century, the scientific method is the foundation of modern science.

1) Observe some aspect of the universe.

2) Devise a hypothesis consistent with what you observe.

3) Use the hypothesis to make further predictions.

4) Test those predictions by experimentation.

5) Modify the hypothesis in light of your findings.

6) Go back to step 3 and repeat.

We use the scientific method to distinguish reality from fantasy and truth from propaganda. Appeals based on authority are especially suspect. For example, the Catholic Church condemned as heretics those who did not subscribe to their preferred model of the universe -- that the Earth, not the Sun, was at the center.

Here’s an irony: our knowledge of the world advances when we learn that something we believed to be isn’t true. For example, Aristotle asserted that heavier objects fall faster than lighter ones. This was the conventional wisdom for over 2000 years.

Galileo tested this theory by dropping different objects from certain heights and rolling balls down inclined planes. He formulated the law of falling bodies. Here it is: In the absence of air resistance, all objects accelerate at the same rate regardless of their densities. In 1971 Apollo 15 Astronaut David Scott offered a compelling demonstration when, standing on the airless lunar surface, he dropped a hammer and a falcon feather. I watched them hit at the same time.

Here’s the second point. Policy controversies involve tradeoffs among competing values. Responsible policy makers know that environmental quality is only one of several important and competing values. Just as people on fixed budgets must choose between buying medicine or more heat, societies must choose among competing goods and values (e.g., more open space, safer roads, or more funding for education). These tradeoffs are inescapable. It is irresponsible to pretend they don’t exist.

Some activists claim that “the science tells us” we must follow a particular course of action. They attack those who dissent as heretics -- individuals with suspect motives, industry flunkies, or simply reactionaries. Global warming is such a potentially important issue that it should not be hijacked by ideology of any stripe.

Recall the Russian experience under Lysenko. His pseudo-scientific theories of plant genetics where used to justify creating the “New Soviet man.” They set back Soviet science by at least a generation. His opponents were exterminated.

We count on science to help us assess whether the problems we’re told exist are in fact real and whether the solutions offered will do any good. With limited resources, choices must be made. Science can help to identify the tradeoffs, but does not tell us how to choose among them. Human values, not science, are required to rank the outcomes.

The global warming debate involves complex scientific theories supported by some good evidence. As our knowledge increases, we’ll be better able to choose a responsible course of action. We should resist being stampeded into public policies with huge immediate costs and few if any benefits.