Al Gore's recent film An Inconvenient Truth has re-ignited the debate over global warming. Today, Slashdot posted a link to an article from Canada Free Press criticizing Gore's film.

One of the article's sources, a climatologist from Canada, says that one of the problems with Gore's film is that his experts are not climatologists:

Even among [the small fraction of scientists who actually work in the field of climate study], many focus their studies on the impacts of climate change; biologists, for example, who study everything from insects to polar bears to poison ivy. "While many are highly skilled researchers, they generally do not have special knowledge about the causes of global climate change," explains former University of Winnipeg climatology professor Dr. Tim Ball. "They usually can tell us only about the effects of changes in the local environment where they conduct their studies."

On the other side of the coin, we have a 2004 article from Science magazine (also linked from Slashdot), which analyzed 928 scientific papers regarding climate change:

The 928 papers were divided into six categories: explicit endorsement of the consensus position, evaluation of impacts, mitigation proposals, methods, paleoclimate analysis, and rejection of the consensus position. Of all the papers, 75% fell into the first three categories, either explicitly or implicitly accepting the consensus view; 25% dealt with methods or paleoclimate, taking no position on current anthropogenic climate change. Remarkably, none of the papers disagreed with the consensus position.

The "consensus position," according to Science, is articulated by the Intergovernmental Panel on Climate Change (IPCC), which states that human beings are modifying atmospheric constituents through the emission of "greenhouse gases," and that the increase in the Earth's temperature is due to these emissions. According to Science, we have a theory with two parts. The first part is an observation: the Earth's temperature is fluctuating now in a way that is different from how it fluctuated 50 years ago. The second part is a hypothesis about this observation: humans, through greenhouse gas emissions, are causing this temperature fluctuation.

Like evolution, global warming is a theory. Unlike evolution, there are global warming opponents even within the scientific community. (Within the scientific community, there is no one who doesn't believe that evolution exists; only outside of the scientific community is there skepticism.) The scientists who suggest that there's no such thing as global warming and that it's just a figment of everyone's imagination are most likely in the employ of multinational corporations which would benefit from looser emission restrictions. The scientists who are not affiliated with corporations or foundations, but are still skeptics, are of the opinion that we don't yet know enough about global warming to say whether or not humans are having an effect, let alone whether or not there is an effect. Accurate temperature records go back about one hundred years -- a fraction of a second in geologic time, which operates on a scale of millions of years -- but scientists have developed new ways of extracting core samples from ice caps to see how they have melted and re-frozen over the course of thousands of years.

What, then, is the truth regarding global warming? In the face of more convincing evidence, the answer appears to be, "We're not sure." Air pollution is visible outside our windows, but is this pollution causing a global climate change? And if humans are causing climate change in a negative way, is the change as dire as Al Gore might make it out to be?