Myth: According to Occam’s razor, the theory that our current climate change is part of a natural cycle is correct.

Fact: Occam’s razor does not apply for two reasons: the theory of natural variation is no simpler than that of anthropogenic global warming, and most importantly, natural variation is not a valid theory, which is a prerequisite for the use of Occam’s razor.

Many times, when engaged in a debate with a global warming skeptic, I will encounter an invocation of Occam’s Razor. According to this maxim, all other things being equal, the simpler of two competing theories is most likely the correct one:

Occam’s razor (sometimes spelled Ockham’s razor) is a principle attributed to the 14th-century English logician and Franciscan friar William of Ockham. The principle states that the explanation of any phenomenon should make as few assumptions as possible, eliminating those that make no difference in the observable predictions of the explanatory hypothesis or theory…This is often paraphrased as "All things being equal, the simplest solution tends to be the best one." In other words, when multiple competing theories are equal in other respects, the principle recommends selecting the theory that introduces the fewest assumptions and postulates the fewest hypothetical entities. - Wikipedia

When global warming skeptics invoke Occam’s Razor, they generally mean that the idea that climate change is caused primarily, if not solely, by natural factors is much simpler than the idea that human activity has caused such changes. Therefore, according to Occam’s Razor, the natural variations idea is the correct theory. As I stated above, this is an incorrect use of the maxim.

First, the natural variation theory is actually no simpler than anthropogenic global warming. The idea that current climate change is a natural phenomenon seems to be simpler to many skeptics because they generally don’t exercise the same amount of skepticism towards theories that agree with their ideologies, which results in a lack of any true examination of this idea. The argument I hear most often in favor of this idea is that our climate has always changed in the past, and humans weren’t around back then to influence it, so it must follow that our current climate is also due to natural factors. However, that is not a valid argument. Just because something caused changes in the past, it doesn’t mean that when a new variable is added, the new variability is not caused primarily by the new variable. So, in order to prove that climate change is indeed caused by natural factors, one must actually specify which natural factors are causing the change, and provide a statistically valid, quantified comparison of both natural and man-made causes, which shows that the natural factors outweigh human greenhouse gas emissions. Therefore, if one put the necessary effort into proving the natural variation theory, it would no longer be considered any simpler than anthropogenic global warming.

Second, Occam’s razor requires that "all things [be] equal". In other words, the prerequisite for the maxim is that both competing theories be equally verifiable. This is a major problem for skeptics who would like to use it because there is no valid theory which explains our current warming as being due to natural causes. Many of the largest changes in our planet’s temperature in the past were due to changes in the earth’s orbit and tilt, called Milankovitch cycles. This is does not explain our current warming because we are already in an interglacial period (between ice ages). Some believe the Sun is causing our warming, but the Sun’s energy output stays relatively constant. In fact, recent research has suggested that the Sun’s activity has declined in recent decades. Others blame cloud formation, or the lack thereof, due to cosmic ray flux, but this theory also remains unproven with many problems of its own. Namely, cosmic ray intensity has remained stagnant in the recent past. Conversely, anthropogenic global warming is verifiable. Nearly all research points to a warming due to anthropogenic greenhouse gases, which is what has given the IPCC the confidence to claim that it is "very likely" that global warming is due to human activity.

Simply put, this argument is yet another attempt to obfuscate the issue, rather than an honest attempt at finding the truth.