How many times have we heard the apocryphal statement about global warming that "the science is settled"? Is the debate really over? It depends on who is doing the debating, and what is supposed to have been settled. There have been many climate-change debates among scientists as well as non-scientists. We need to recognize the difference between a scientific debate and other forms of disagreement. Science has ground rules. Those who don't follow the rules are entitled to their opinions, but cannot legitimately claim to be participating in a scientific debate.
Before scientific results can be fully accepted, they must be subjected to peer review and published in a scholarly scientific journal. This is a necessary, but insufficient, condition (nobody is compelled to embrace the conclusions of a paper just because it has been refereed). This rule is not intended to create a "high priesthood" of scientists or keep others from participating. On the contrary, scientists welcome dissent and encourage contrarians to publish their ideas so they can be subjected to the same harsh scrutiny that is applied to conventional thought.
Peer review is designed to screen out material that is demonstrably wrong, flawed, or illogical. Non-specialists are not always able to spot errors quickly in a highly technical piece of work, so experts are recruited to make sure any mistakes are corrected and necessary documentation is provided before peer-reviewed science can be published. Think of this as a kind of standard for all scholarly papers.
In my line of work, I'm often asked to comment on various claims about climate change. The first thing I do when I read an editorial or blog entry is to check to see if the claims have been published in the scientific literature. If not, my response is usually this: "I don't see why I should bother to read it if the authors couldn't be bothered to put it through scientific peer review." My reasoning is not that such material is necessarily wrong. But without any scientific review I have no assurance that anyone has checked to see if the equations are right, data sources correctly cited, figures properly attributed, or other workers' conclusions fairly represented.
Nobody claims that the global warming debate has ended among editorial writers, media pundits, bloggers, and politicians. The calculation of the mass of CO2 produced from burning a gallon of gasoline was the subject of a vigorous debate on the Albuquerque Journal letters page a couple years ago. This is a question that a decent high school chemistry student should be able to answer, but the highly-opinionated letter writers were not able to resolve their differences--despite the fact that reaction stoichiometry is indeed settled science.
Likewise, a competent high school physics student understands how the greenhouse effect works, which is based on the first law of thermodynamics (conservation of energy). This is also settled science. It has been known for over a hundred years that adding CO2 to the atmosphere increases its infrared opacity, and when this happens, more energy from sunlight enters the Earth's atmosphere than escapes. The atmosphere must heat up, on average. There is no scientific debate about this fact, and nobody has ever published a "zero-warming" theory to explain how it could be otherwise.
What is not settled is the degree of climate change. In the peer-reviewed scientific literature there is a healthy, open, honest, and vigorous scientific debate. The best scientific estimate of the amount of warming (when CO2 levels double, which is likely to happen this century) is about 6 ºF. There are those who disagree, and have published the basis for their disagreement. The most useful assessments are not limited to the best estimate, but include quantification of the uncertainty, which is one of the hallmarks of honesty in science. There is a broad range of possibility, from below 4 ºF to greater than 11 ºF.
One recent paper estimates a likelihood of about 2.5% that average temperature increases could exceed 14 ºF; a change that would probably lead to the collapse of global ecosystems, loss of civilization, and possible human extinction. There is no way to prove or disprove these quantitative estimates, other than to wait and see what happens. That said, it is hard to ignore a scholarly paper (emphasis on the word "scholarly") that gives longer odds for civilization than for a shuttle launch.
Recently, opinion pieces have been published that masquerade as scientific literature. Most notably is a document published by the Heartland Institute (a fossil-fuel-funded political pressure group) for an organization called "Nongovernmental International Panel on Climate Change" (NIPCC), a play on the name of the IPCC, which publishes summaries of mainstream peer-reviewed science. After reading a few sections of the document, I remembered a comment from a fellow scientist and friend of mine: "Pseudoscience is like spoiled food; you don't have to eat it all to know something is badly wrong. Just a few bites will do."
The authors' use of loaded words like "fearmonger" and "hype" were the first whiff of spoilage. Rhetorical devices are rarely if ever seen in a scholarly paper. This suspicion was borne out by close examination of figures re-plotted by NIPCC from peer-reviewed sources. The original data were mis-plotted, modified, and misrepresented. Important information was removed, and in at least one case, a data point was fabricated. The NIPCC report is an example of pseudoscholarship at its worst.
Just as serious a blunder was the unwillingness of the authors to concede any uncertainty in their beliefs. As scientists, we all have a professional obligation to be honest about what we know and what we do not know. As professionals whose work informs policy, we must always err on the side of caution. Climate change must be treated like all real but uncertain threats. To ignore that possibility is reckless.
For any debate to be called scientific, the entire spectrum of expert opinion must be taken into account. Two questions must always be asked of experts by policymakers and by the public: 1) How certain are you that you are correct? 2) What is the worst thing that can happen if you are wrong?