Have Past IPCC Temperature Projections/Predictions Been Accurate?

The question routinely shows up in climate arguments, with claims in both directions. Evaluating them is hard. The IPCC has given multiple reports with multiple projections depending on different assumptions, so one can almost certainly select reports to show either a good or bad job of predicting. The reports sometimes describe what they produce as predictions, sometimes as projections. For simplicity I will use the former term.

The past reports are webbed. To get a reasonably fair judgement, the obvious approach is to look at each, see what one would expect from reading it and how that compares with what happened. I have now done so. Skeptical readers are invited to check my summary for themselves, starting with the page that links to all of the reports.

The executive summary of the first report, from 1990, contains:

Under the IPCC Business-as-Usual (Scenario A) emissions of greenhouse gases, the average rate of increase of global mean temperature during the net century is estimated to be about 0.3°C per decade (with an uncertainty range of 0.2°C to 0.5°C).

The graph shown for the increase is close to a straight line at least from 2000 on, so it seems reasonable to ask whether the average increase from 1990 to the present is within that range.

Figure 18 from the Second Assessment Report (1995) shows the future temperature through 2020. Through that date, it rises steadily at about .13°C/decade.

The Third Assessment Report (2001) contains:

For the periods 1990 to 2025 and 1990 to 2050, the projected increases are 0.4 to 1.1°C and 0.8 to 2.6°C, respectively.

So for the former period, the average increase is supposed to be from .11 to .31 °C/decade.

The Fourth Assessment Report (2007) has "For the next two decades a warming of about 0.2°C per decade is projected for a range of SRES emissions scenarios."

Checking a graph on a NASA page, the increase from 1990 to 2013 was about .22°C, for an average rate of increase of about .1°C/decade. 

So if we judge IPCC reports by comparing what they said the average increase would be over the period from 1990 to the present with what it actually was, we find that the first report predicted a rate about three times what actually happened. The second report got it a little high. The third report got it substantially high. For both the first and third, the actual value was below the bottom of the predicted range of values.

The fourth report was written in 2007 and predicted temperature change thereafter. Looking at the graph from the NASA page, temperature from then to now has been essentially flat, with the slope positive or negative depending on your choice of end points. It's too short a time period to evaluate the prediction with much confidence, but so far as one can judge it was high. 

So it looks as though the IPCC has predicted high four times out of four, two of the four times by a lot. It would take more work than I am willing to put into the project to dig out probability distributions for their predictions and see just how unlikely it is that they would miss by that much that many times, but I would be surprised if the overall pattern was not well outside the .95 confidence interval.

All four reports show a roughly constant rate of increase from 1990 to the present. The actual pattern was an increase to about 2000 and roughly flat temperatures thereafter. That does not prove the IPCC wrong in any strong sense, since their projections are averaging out sources of temperature change that could not be readily predicted when the projections were made. But it does mean that the IPCC failed to be right. Insofar as the pattern is evidence on either side, it is evidence against the accuracy of their predictions (aka projections).

One way of judging how good a job the IPCC has done of modelling global climate is to compare its predictions with a much simpler model, a linear fit of past data. Looking at a webbed graph of the data and fitting by eye, the slope of the line from 1910, when current warming seems to have started, to 1990, when the first IPCC report came out, is about .12 °C/decade. That gives a better prediction of what happened after 1990 than any of the IPCC reports.