I think there is plenty of room for honest, informed, and intelligent people to disagree on the extent to which global warming is caused by human activity. (That is why, for example, I called Tree Lobster an asshat for failing to practice the principle of reciprocity with respect to global warming skeptics.)

And here’s a pretty good example of why I think there is room for disagreement:

One of the main conclusions of the 2007 IPCC report was that the warming over the last 50 years was most likely due to anthropogenic pollution, especially increasing atmospheric CO2 from fossil fuel burning.

But a minority of climate researchers have maintained that some — or even most — of that warming could have been due to natural causes. For instance, the Pacific Decadal Oscillation (PDO) and Atlantic Multi-decadal Oscillation (AMO) are natural modes of climate variability which have similar time scales to warming and cooling periods during the 20th Century. Also, El Nino — which is known to cause global-average warmth — has been more frequent in the last 30 years or so; the Southern Oscillation Index (SOI) is a measure of El Nino and La Nina activity.

A simple way to examine the possibility that these climate cycles might be involved in the warming over the last 50 years in to do a statistical comparison of the yearly temperature variations versus the PDO, AMO, and SOI yearly values. But of course, correlation does not prove causation.

So, what if we use the statistics BEFORE the last 50 years to come up with a model of temperature variability, and then see if that statistical model can “predict” the strong warming over the most recent 50 year period? That would be much more convincing because, if the relationship between temperature and these 3 climate indicies for the first half of the 20th Century just happened to be accidental, we sure wouldn’t expect it to accidentally predict the strong warming which has occurred in the second half of the 20th Century, would we?

The resulting model (blue) fits the actual data (red) like this:

Further:

What is rather amazing is that the rate of observed warming of the Northern Hemisphere since the 1970’s matches that which the PDO, AMO, and SOI together predict, based upon those natural cycles’ PREVIOUS relationships to the temperature change rate (prior to 1960).

Again I want to emphasize that my use of the temperature change rate, rather than temperature, as the predicted variable is based upon the expectation that these natural modes of climate variability represent forcing mechanisms — I believe through changes in cloud cover — which then cause a lagged temperature response.

This is powerful evidence that most of the warming that the IPCC has attributed to human activities over the last 50 years could simply be due to natural, internal variability in the climate system. If true, this would also mean that (1) the climate system is much less sensitive to the CO2 content of the atmosphere than the IPCC claims, and (2) future warming from greenhouse gas emissions will be small.

So  he (Dr. Roy Spencer) constructed a statistical model based only on natural phenomena — the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO), and the Southern Oscillation Index (SOI).  He used data from 1900 to 1960 to construct the model, and then used the resulting model to predict what should happen from 1960 to 2010. And the model fits the actual temperature history pretty well, leaving only a relatively small difference to be explained by man-made CO2.

Now, it’s certainly possible that someone will come along and show a flaw in Dr. Spencer’s reasoning, or a mistake in the computations, or an error in the data he used, that would invalidate his model. But it’s easy to see why an analysis like this — and numerous other pieces of evidence, as well — might prompt many reasonable people to lean towards a mostly natural explanation for global warming.

So enough with the damn “global warming denier” bullshit already.