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30 Years Of IPCC Reports: How Well Have They Done?

Clive Best

It is normal practice in science that theoretical predictions which fail experimental tests are rejected or at the very least modified. Not in climate science though.

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Over the last 30 years there have been 5 IPCC assessment reports on climate change.  I decided to compare each  warming predictions made at the time of publication to the consequently measured  temperatures. The objective is twofold. Firstly how well did the predictions fare with time?  Secondly have we learned anything new about climate change in the last 30 years?

  1. FAR

The First Assessment Report (FAR) was published in 1990. It was probably the first public alarm raised that the world was warming as a consequence of CO2 emissions. As a spinoff there was also a very good book written by the FAR chairman John Houghton – “Global Warming – The Complete Briefing”.  Here is the comparison of the “model” to resultant temperatures.

The first prediction of global warming in 1990. The data are my own 3D version of HadCRUT4.6

The trend in temperatures is confirmed but by the end of  2019 the degree of warming was less than the FAR 1990 forecast. In reality emissions have indeed followed the Business as Usual trajectory, but warming instead turned out about 0.3C less than that forecast.

2. SAR

The second assessment report (SAR) dates from 1995. Here is the SAR comparison between my 3D HadCRUT4 results shown in red with  the UK Met Office Model.

My 3D results for HadCRUT4.6 are shown in red. The results are the same as Cowtan & Way.

We see that the model which reduces the effect of CO2 by including aerosols agrees today reasonably well with the data. However since aerosols have actually fallen significantly since 1995 this result is not as good as it looks.  I would call this a moderate success for SAR but only assuming just CO2 level forcing without feedbacks. Next we look at the third assessment report (TAR).

3. TAR

The third assessment report was published in 2001. The TAR model predictions were actually lower than those of both FAR and SAR,  perhaps reflecting a real drop off in the measured temperatures. HadCRUT3 had by then showed a definite pause (hiatus) in global warming was underway following the super el Nino in 1998. Since 2001 many more stations have consequently been added (and some removed) so that that as we will see the hiatus has today essentially disappeared. Here though  is the comparison  of the TAR ‘projections’  compared to the current temperature data as of 2019. The temperature data  are again my own 3D version of the HadCRUT4.6 measurements.

The blue points are HadCRUT4.6 calculated using Spherical Triangulation.

The agreement between models and data  looks much better. However note that the absolute temperature change is only relative to that of 1990 (FAR publication date). Even then my impression is  that we are most likely following their blue B2 ensemble curve.

4. AR4

The fourth assessment report was published in 2005 and included a  shorter term prediction diagram compared to the HadCRUT data as it was available then. The updated figure below shows the updated comparison as of Jan 2020. The black squares are the latest HadCRU4.6 data as downloaded from the Hadley site. The small black circles are the old H4 data points (as available in 2005) from the original report. They differ from the current H4 results because CRU have in the meantime updated all their station data. In this respect note how the 1998 temperature has dropped a little while the 2005 value increased sufficiently so that the hiatus starts to disappear.

AR4 model comparison to HadCRUT4.6 updated for 2019

The final agreement is not too bad but the data nevertheless still lie below the model means for all reasonable SRES scenarios.

5. AR5

This finally brings us up to the Fifth Assessment report comparison. When AR5 was published in 2013 the hiatus in warming was still clearly visible in all temperature series, and as a consequence all models were running hot by comparison. Since then however many more stations have been added in the Arctic and the large el Nino of 2016 has now apparently evaporated the hiatus. Despite all this how do the models now compare with the the modern temperature data as of January 2020?

Here is the up to date comparison of Figure 11.25a to the data.

IPCC AR5 Figure 11.25a updated for 2019. The green trend is HadCRUT4.6. The Cyan trend is the 3D-version. The data is skimming along the bottom sensitivity range of all 45 CMIP5 models.

It is clear that the warming trend lies at the lower end of the CMIP5 ensemble. Only models with lower sensitivity can adequately describe the temperature data.

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