There’s a new paper out called A model-data comparison of the Holocene global sea surface temperature evolution by G. Lohmann, M. Pfeiffer, T. Laepple, G. Leduc, and J.-H. Kim. Hat-tip: a reader.
This is an important study because scientists need a way to check the reliability of their models. If the models can recreate the past, then there’s a chance they can be used for the future.
We have about 150 years worth of reliable instrumental records that can be used to check models. Beyond that, scientists have had to reconstruct temperatures using proxy data. That’s the only way to check models that go way back. But we also know that using proxies as thermometers is not an exact science. Yet, with enough of them, you can get a pretty good idea of how climate behaved in the past. So it should be possible to use them to test models.
In the paper’s introduction we find:
Information beyond the instrumental record covering the last 5150 yr can be obtained mainly from two strategies: on the one hand by deriving from proxies which record past climate and environmental conditions, and on the other hand by simulating climate, using comprehensive models of the climate system under appropriate external forcing.”
Relying on models to tell us what the past climate was? Sounds rather dubious. So, according to the abstract, the authors compared the ocean temperature evolution of the Holocene as simulated by climate models and reconstructed from marine temperature proxies.
The proxy dataset comprised a global compilation of marine alkenone and Mg/Ca-derived sea surface temperature (SST) estimates. The authors write what they observed (emphasis added):
Independently of the choice of the climate model, we observe significant mismatches between modelled and estimated SST amplitudes in the trends for the last 6000 years. Alkenone-based SST records show a similar pattern as the simulated annual mean SSTs, but the simulated SST trends underestimate the alkenone-based SST trends by a factor of two to five. For Mg/Ca, no significant relationship between model simulations and proxy reconstructions can be detected.”
The authors later in the abstract add:
…when modeled temperature trends are set up to allow drastic shifts in the ecological behavior of planktonic organisms, they do not capture the full range of reconstructed SST trends. These findings challenge the quantitative comparability of climate model sensitivity and reconstructed temperature trends from proxy data.”
Most of us know that determining the “appropriate external forcings” (mentioned above) has been elusive in climate science, and has involved many dubious assumptions and downright hanky panky – all to make CO2 appear as the dominant driver.
Proxies will always be the superior way of determining how climate behaved in the past. Eventually models will be able to reproduce the past, but that will not mean they will be capable of predicting the future. One can use many different combinations of forcings in models to reproduce the past climate. But finding out which one was in fact correct, if any, will remain quite the challenge.