The ongoing controversy of climate changes illustrates a rather important relationship in science and reason: The relationship between claims and the burden of proof. We’ve all heard Carl Sagan’s observation that extraordinary claims demand extraordinary proof. In the bigger picture, the issue is whether it’s reasonable to doubt climate change enough to do nothing or whether that doubt is irrational to the point we are compelled to act decisively to forestall disaster. The question is whether the fundamental concept of climate change, specifically human-caused climate change, is an ordinary claim subject to an ordinary burden of proof or an extraordinary claim requiring extraordinary evidence.

It’s all relative, to some extent. The degree to which a particular claim is extraordinary depends on our current state of knowledge. Where ordinary claims are essentially claims that build on our existing body of knowledge, extraordinary claims come with a large number of collateral assumptions that need to be verified before the claim itself can be accepted. And just to be clear: Whatever the burden of proof required, the same standards of quality evidence rigourous investigation, and methodological precisions apply whatever the nature of the claim being made.

Here’s an example. If astronomers were to detect an earth-like planet orbiting a distant star, the discovery – as astonishing as it would be – would nevertheless build on the previous detection of other planets around distant stars. (http://exoplanet.eu) Of course, hundreds of years ago this wouldn’t have been the case. To a society that couldn’t conceive of planets in orbit around a star given their lack of proper instruments and scientific knowledge, the very idea of extrasolar planets (possibly bearing life!) was profoundly radical. Today, however, the idea is not so extraordinary in terms of what we currently know about the universe.

Compare this to claims of extraterrestrial visits to our little blue marble. There is, of course, the bad reasoning that sees many people confuse “unexplained” with “extraterrestrial” – just because an incident can’t be explained by our current scientific understanding of the universe doesn’t logically entail an extraterrestrial or supernatural explanation. There is also the fact that long-term studies, like the U.S. Air Force’s infamous Project Blue Book, which ended in 1970, and the recently ended British Ministry of Defense UFO inquiry unit have not discovered any evidence of extraterrestrial visitations or potential threats to our security. (http://news.yahoo.com/s/ap/20091204/ap_on_re_eu/eu_britain_ufo_hotline) But on a conceptual level, there are many assumptions that go along with the idea of extraterrestrial visits to make the claim, indeed, extraordinary. For example: How would aliens reach us? Faster-Than-Light travel is not possible, as far as we know. There are theoretical possibilities, such as the Star Trek-like Alcubierre Drive (http://en.wikipedia.org/wiki/Alcubierre_drive), but these are too theoretical even for experimental purposes. Or, there is the notion of arks that would take, however, many tens or hundreds of years needed to travel between stars, perhaps using some sort of cryogenic system or even multiple generations of astronauts. This raises the question as to why aliens would spend so much time in space only to perform prostate exams. Interstellar cancer prevention efforts, maybe? Without the discovery of water-bearing planets outside of our solar system, however, one has to wonder about the practicality of this kind of travel, not to mention the sort of technology required to power and propel an ark ship.

We could ask similar questions of ESP, telekinesis, and other paranormal phenomena; the physics of reading thoughts or moving objects with the mind is far from established, given our current understanding. Since no experimental evidence has so far surfaced that these phenomena actually occur, they, too, like alien visits, fall under the category of extraordinary claims. To get an idea of the scientific challenges underlying these ideas, I recommend the excellent and accessible book “Physics of the Impossible” by theoretical physicist Michio Kaku.

Climate Change – Reasonable or Far-Fetched?

So what about climate change? Is the idea of humans causing a warming of the average global temperature an ordinary claim? Or is it as far-fetched as aliens and psychic powers? The answer, naturally, depends on what we currently know about the world and our actions within it. We can thus ask a few questions to set a conceptual foundation to build upon:

Can human populations impact the environment to the point of their own harm or destruction?

The answer, clearly, is yes. Consider Chernobyl, which released four hundred times more radioactive fallout than the bombs at Hiroshima and has left a harrowing legacy of radiation damage, cancers and contaminated land. Consider Bhopal, history’s worst industrial disaster, in which the release of poisonous methyl isocyanate gas into the atmosphere killed thousands of people and left behind a toxic, polluted environment. Or how about the case of Easter Island, whose civilization collapse is controversially attributed to ecosystem failure through overpopulation and deforestation? Or what about the hole in the ozone layer caused by the use of CFCs – a challenge we confronted head-on, incidentally? The list goes on.

Are there any scenarios, other than human-caused climate change, of global environmental catastrophe?

Two words: nuclear bomb. If the sheer destruction of the explosion itself isn’t enough to worry about, the radioactive fallout with smoke and soot leading to nuclear winter adds to the fear. The very notion of a nuclear winter that would blot out the sun and result in catastrophic global cooling is, of course, based on climate science. Or how about dinosaur extinction, thanks to massive asteroid impacts or volcanic activity? These are two scientifically valid examples of massive climate change, both human and natural, that have gained cultural acceptance.

Is there anything about climate change that violates what we know about how the atmosphere works?

The earth’s atmosphere is far more than something we breathe. It offers protection from meteorites. It also, crucially to life on earth, influences the quality and quantity of solar radiation we receive (i.e. absorbing ultraviolet radiation), making the difference between roasting and freezing. Compare earth to Venus and Mars.

We can add to our established, uncontroversial knowledge the familiar idea of a greenhouse from high school physics: A glass enclosure traps heat, making it possible to keep higher temperatures inside even with cooler temperatures outside. Gardeners love it. Farmers love it. Food-eaters love it. We all benefit from the physics of greenhouses. The analogy of the atmosphere as a greenhouse isn’t far-fetched, then. In fact, it’s key; the right balance of greenhouse gases in the atmosphere ensures the quality of life on earth.

The idea of human-made climate change, then, is not an extraordinary one that upends current scientific understanding. In fact, considering that the human population has expanded from roughly 791 million people in the 1750s to today’s 6.7 billion, world-wide industrialization makes light of the notion that humans aren’t badly impacting the atmosphere.

Of course, there have been countless reasonable ideas in science that were discarded entirely or reformulated in light of new evidence. For example, Jean-Baptiste Lamarck’s views on evolution, which encompassed the idea that individuals pass on acquired traits to offspring (e.g. giraffes stretch their necks to reach highly placed leaves and pass the elongated neck on to offspring.) But that’s what science is about: gathering evidence to sort through the various hypotheses. In the case of human-caused climate change, the challenge has been to improve measurement techniques and refine climate science’s understanding of the world. With gradual levels of certainty, the world’s climate scientists have come to assert the reality and dangers of human-caused climate change.

So why are deniers so hysterical about climate change if the science, in terms of philosophical framework and in practice, is reasonable? The answer obviously has nothing to do with science but everything to do with politics. That will be the topic of next week’s column.

Frédérik invites you to discuss this week's column at his blog, www.inkandashes.net