The folks at Real Climate have produced a very interesting analysis that provides some useful information for the task of framing a falsification exercise on IPCC predictions of global surface temperature changes. The exercise also provides some insight into how this branch of the climate science community defines the concept of consistency between models and observations, and why it is that every observation seems to be, in their eyes, "consistent with" model predictions. This post explains why Real Climate is wrong in their conclusions on falsification and the why it is that two decades of cooling can be defined as "consistent with" predictions of warming.

In their post, RealClimate concludes:

Claims that a negative observed trend over the last 8 years would be inconsistent with the models cannot be supported. Similar claims that the IPCC projection of about 0.2ºC/dec over the next few decades would be falsified with such an observation are equally bogus.

Real Climate defines observations to be "consistent with" the models to mean that an observation, with its corresponding uncertainty range, overlaps with the spread of the entire ensemble of model realizations. This is the exact same definition of "consistent with" that I have criticized here on many occasions. Why? Because it means that the greater the uncertainty in modeling -- that is, the greater the spread in outcomes across model realizations -- the more likely that observations will be “consistent with” the models. More models, more outcomes, greater consistency – but less certainty. It is in this way that pretty much any observation becomes "consistent with" the models.

As we will see below, the assertion by Real Climate that "a negative observed trend over the last 8 years would be inconsistent with the models cannot be supported" is simply wrong. Real Climate is more on the mark when they write:

Over a twenty year period, you would be on stronger ground in arguing that a negative trend would be outside the 95% confidence limits of the expected trend (the one model run in the above ensemble suggests that would only happen ~2% of the time).

Most people seeking to examine the consistency between models and observations would use some sort of probabilistic threshold, like a 95% confidence interval, which would in this case be calculated as a joint probability of observations and models.

So let’s go through the exercise of comparing modeled and observed trends to illustrate why Real Climate is wrong, or more generously, has adopted a definition of "consistent with" that is so broad as to be meaningless in practice.

First the observations. Thanks to Lucia Liljegren we have the observed trends in global surface temperature 2001-present (which slightly longer than 8 years), with 95% confidence intervals, for five groups that keep such record. Here is that information she has presented in degrees Celsius per decade:  Read rest...

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