Have Past IPCC Temperature Projections/Predictions Been Accurate?
Arguments for or against doing things to slow climate change depend on what will happen if we don’t, a question the IPCC reports try to answer. That makes it important to know how reliable their predictions are. The latest report runs to almost four thousand pages, largely of detailed analysis depending on multiple scholarly articles for each step — Chapter 7, to pick one at random, has fifty editors and about nine hundred articles in its list of references. Someone with infinite time, energy and expertise might be able to go through all of the calculations that produced the predictions in the reports in order to see if they were done correctly, but that is not a practical option.
There is an alternative. The climate system is too complicated to make predictions on the basis of theory alone, hence the IPCC project largely consists of sophisticated curve fitting, picking a form for the relationship among observables suggested by physical theory, choosing parameters for the relationships, how strong each effect is, by finding the values that best fit historical data. With enough tweaking of the models and adjusting of parameters that process can fit past data, but that does not tell you whether the models fit the real system well enough to correctly predict future data. As someone is supposed to have said, with enough parameters you can fit the skyline of New York.
The solution, for both the researcher who wants to know if his model is right and someone else trying to decide whether to believe him, is to test the model against data that were not used in creating it. We do not know the future, the future eventually becomes the past, so a model constructed in 1990 can be tested in 2021 against data that did not exist when the model was constructed.
The past reports are webbed. Back in 2014 I looked at each to see what someone who read it would expect future temperature to do and reported the results on my blog. If you would like to check my conclusions about what each report implied for yourself you can find links to the reports here.
What the IPCC Predicted
The executive summary of the first report (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 next 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 .14°C/decade.[1]
From the Third Assessment Report (2001):[2]
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.
For the former period, that implies an increase of from .11 to .31 °C/decade.
The Fourth Assessment Report (2007) has[3]
For the next two decades a warming of about 0.2°C per decade is projected for a range of SRES emissions scenarios.
What Happened
When I did the calculations in 2014, I found that the IPCC had predicted high four times out of four, twice by enough so that actual warming was below the bottom of the predicted range. That looked like evidence that we should not put much weight on their predictions of future temperature.
We now have seven years more data, so I did it again. As of September of 2021, when I am writing this, the last year whose temperature is shown on the NASA page I am using is 2018; I have redone the calculations accordingly. Here are the results:
The first IPCC report was released in 1990. From then to 2018, global temperature rose .38°C for an average of .14°C/decade, well below the predicted range.
The second report was released in 1995. From then to 2018, temperature rose by .37°C, for an average rate of growth of .16 °C, a little higher than the prediction.
The third report was released in 2001. From then to 2018, temperature rose by .29°C for an average of .17°C/decade, towards the lower end of the predicted range.
The fourth report was released in 2007. From then until 2018, temperature rose by .18 degrees, .16°C/decade, below the predicted .2°C.
The predictions look better now than they did in 2014, high three times out of four, low once, and only once has actual warming been below the predicted range. They are still running a little high but the results look consistent with random error. That makes it at least possible that the IPCC researchers are now modeling the climate system well enough to produce reasonable estimates of its future behavior.
It is possible but far from certain because the test they passed is not a very strong one. A theory that correctly predicted the outcome of next year’s elections, including every house seat, every senate seat, and the total votes for each party, would be a very good theory indeed, since doing that well by chance is very unlikely, so we would have good reason to trust its future predictions. A theory which correctly predicted which party will end up with a senate majority after the 2022 election would be better than one that got it wrong but not much better, since one can get the right answer half the time by flipping a coin.
The IPCC reports rely on complicated models and a lot of data. One way to judge how impressive their results are, how much evidence that they have done a good job of modelling climate, is to compare their results with those of much simpler models. The simplest is the assumption that global temperature never changes. The IPCC did a little worse than that model in 1990, since it predicted warming from then to 2018 of .3°/decade and the actual value, .14°/decade, was closer to zero. But they did much better than that model the next three times.
The next simplest model is a straight line. From 1910, about when current warming started, to 1990, when the first IPCC report came out, warming was .11 °C/decade. The rate of warming from 1990 to 2018 was .14 °C/decade, so the straight-line prediction made in 1990 predicts about 79% of warming from then to 2018. The ratio of the IPCC prediction to what actually happened was 250% for the 1990 prediction, 81% for the 1995, 125% for the 2001 and 2007 predictions.[4] So predicting that the rate of warming would continue at its average level as of 1990 does much better than the 1990 prediction, about as well as the three later ones.
One test of how good the IPCC models are is to see how well each of them did at predicting warming from then to now. The first report fails that test, the next three pass it; actual warming was within their predicted range although not equal to their best guess. That is evidence that those models can be expect to give correct predictions in the future but not very strong evidence.
It is, however, much better than the evidence was in 2014.
[1] The figure is on page 323 of Climate Change 1995 The Science of Climate Change. When I did my calculations in 2014 I thought it was .13°/decade but measuring the graph more carefully I now think it is .14.
[2] Page 60 of Climate Change 2001 Synthesis Report.
[3] Page 7 of Climate Change 2007 Synthesis Report.
[4] I am defining each of the IPCC predictions as the predicted value if there is one or the center of the range if there isn’t.