The Atlantic Meridional Overturning Circulation or AMOC, which we have mentioned previously including in our Pentagon Crystal Ball video, and which is better known as the “Gulf Stream” (though technically the part that hauls warm water from the Caribbean to Europe is only part of the AMOC system) has been a thorn in the side of climate modelers for a long time. It’s the sort of massive and influential component of the climate system that they really need to be able to get right if they are going to claim that the science is settled and they know what will happen to it over the next century. But they don’t get it right, and as a new study shows, the more they tweak the models, if anything the worse they get.
A new study (h/t NoTricksZone) by a pair of scientists from Ireland and Germany comparing climate model simulations of the AMOC over the post-1950 period demonstrates that, well, let’s let the authors tell us:
“We show that both the magnitude of the trend in the AMOC over different time periods and often even the sign of the trend differs between observations and climate model ensemble mean, with the magnitude of the trend difference becoming even greater when looking at the CMIP6 ensemble compared to CMIP5.”
“CMIP6” refers to the latest climate models and “CMIP5” refers to the ones before. The authors go on:
“Comparing the models’ AMOC evolution to the observational data (figure 3, lower panel), we find that neither the CMIP5 nor the CMIP6 ensemble mean are successful at representing the observational AMOC data... Overall, we find that the various AMOC reconstructions based on observational data have discrepancies but agree with each other better than with the multi-model ensemble means. While, for the post-2004 period the sign of the trend suggested by the CMIP ensemble means agrees with the one found in the observational evidence, it is off for the earlier time periods with the CMIP models suggesting an overall strengthening from 1957 to 1992 but a weakening from 1993 to 2004 and the observational data suggesting the opposite. It is also interesting to note that the CMIP6 ensemble mean tends to show greater discrepancies with the observations than the CMIP5 ensemble mean.”
So over the past 70 years there was a long weakening of the AMOC while the models said it should be strengthening. Then there was a weakening from 1993 until the middle of the past decade when the models said it should be strengthening. Other than that minor malfunction over pretty much the whole period in question, though, the models are working great.
The failure of models to get the AMOC right doesn’t necessarily mean the future will involve less warming than expected. The authors point out that maybe the weakening AMOC, by cooling Europe a bit, suppressed some warming and if the AMOC strengthens in the future instead of weakening like the models say it’s going to, that could lead to even more warming. It just means the models are no use at all in predicting the future because they can’t even explain the past.
Which does surely raise the question why the IPCC says it has such high confidence in them. Can it be that they tell it what it wants to hear?
“We finish with a pessimistic statement: if it is not possible to reconcile climate models and observations of the AMOC in the historical period, then we believe the statements about future confidence about AMOC evolution should be revised. Low confidence in the past should mean lower confidence for the future! The IPCC AR6 report ranks it as very likely that the AMOC will decline in a changing climate. But, if these models cannot reproduce past variations, why should we be so confident about their ability to predict the future?”
Why indeed? And if they know the models can’t get the AMOC right, yet the IPCC declares high confidence in their projections for that one phenomenon, why should we believe any of their projections including those that involve unreliable estimates of a series of interlocking systems?