In Climate Change, Internal Variability is the two ton elephant in the room. Personally, I believe that most of Global Warming is due to some external driver, probably some aspect of solar activity with a little bit of Mankind. But we have to remember, climate varies all by it self. It might sound unlikely, but becuase of the very small changes we’re dealing with, it is quite possible most of the change we see is simply random. How can this be? Well, take the Global Temperature anomaly data. Temperature jumps around year after year. You can download one version here:
The up and down movements not part of the trend are probably not due to external forcings (certainly not CO2, which is monotonic) . So what causes them? probably internal oscillations, especially ENSO. There are other, pseudo-periodic patterns, like the Pacific Decadal Oscillation, etc. Models don’t do so well at replicating their behavior. Some internal phenomenon can therefore cause a change between one year and the next of about .345 C, up or down. It turns out that the temperature data fall almost entirely within a +.345 -.345 range when you include the error bars. So you can’t rule out Internal variability:
Annual Global Temperature Anomalies. Red lines represent the error bars and the blue lines the estimated internal variability.
And guess how many of the various modes of internal variability were in their warm phase during these recent warm decades? Well, PDO, AMO, NAO, AO, and on and on. Virtually all of them. It turns out that you can account for regional patterns better with these effects. What are the odds of the impacts of these modes regionally averages zero in the global mean? Virtually nil. So how can you claim to be able to determine a human influence? You can’t. There is support in the literature for such an idea, by the way.
This post was inspired by and may be taken as a sort of “update” of this article by Richard Lindzen, the Alfred P Sloan Professor of Meteorology at MIT.