I’m trying, and failing, to work from home while looking after the kids (the ice cream van has just arrived outside). I thought I might just discuss a paper that I became aware of yesterday (forget how, but it might have been a tweet from Oliver Bothe). The paper is The Holocene temperature conundrum by Liu et al. and which is essentially a comparison between Holocene temperature reconstructions and climate models.
The basic issue seems to be that the temperature reconstructions (Shakun et al. 2012; Marcott et al. 2013) suggest that after the Holocene maximum (8000 – 9000 years ago) we cooled by about 0.5oC. Climate models, however, suggest that we should have gradually warmed (by about 0.5oC to 1oC) over the last 11000 years. The basic result is shown in the figure below. The blues lines are the two reconstructions (Shakun and Marcott), the yellow and black are from two different ways of stacking the climate models runs, and the red is from the climate models, but biased in a way that tries to compensate for potential biases in the sea surface temperature reconstructions.
In the paper they try to compensate for various biases that may be (I assume) present in the reconstructions and do (as the red line in the above figure shows) get a better comparison between the models and the reconstructions. They show more detailed tests in the Supplementary Information. However, they seem to argue that although this may help to improve the global comparison, it doesn’t help to resolve inconsistencies are individual sites.
The other thing the paper discusses is the sensitivity of climate models to different forcings. The forcings that are relevant here are those due to greenhouse gases (GHGs), ice sheet changes, variations in insolation, and meltwater flux. What they ignore is variations in volcanic activity or solar variability. So, they argue that it is possible that the difference between the reconstructions and climate models is to do with the sensitivity of the climate models to one or more of these forcings. The paper says
Whatever the biases, the model biases have to exhibit a common warming bias across all of the current models with a total magnitude of at least ∼1 °C, such that removal of this model bias can generate a global cooling of ∼1 °C, which overcomes the 0.5 °C warming by GHGs and ice sheets to leave a net cooling of 0.5 °C as in the M13 reconstruction.
So, the one thing that struck me was that this difference can’t really be an indication of the models being over-sensitive to greenhouse gas (GHG) or ice sheet forcings. The CO2 concentration increases by about 20ppm over the Holocene, and the ice sheets retreat. Both of these should cause warming. Even if the climate models are over-sensitive, they can’t be so over-sensitive as to turn a warming trend into a cooling trend. That’s physically implausible. It doesn’t mean that climate models aren’t over-sensitive to GHG and/or ice sheet forcing; it just means that this discrepancy is too big to say either way.
Solar insolation, on the other hand, remains reasonably constant over the Holocene, but the distribution changes. What struck me about this is that the Milankovitch cycles are thought to be associated with orbital variations in which the total insolation doesn’t change much, but the distribution does. So, it is possible – I guess – that climate models are insufficiently sensitive to how the solar flux is distributed across the planet. This seems to be what the paper concludes by saying
The biases in current models, if they exist, are more likely to be related to their sensitivity to the orbital forcing and additional feedbacks in climate models,
where by “additional feedbacks” it means feedbacks not currently included in most climate models and which may be triggered by variations in solar insolation (I think).
Of course, the other possibility is that there are biases in the reconstructions that amplify – or produce – a cooling trend. I saw a few tweets, with links that I didn’t follow, today that seemed to suggest that there are disagreements about details of these reconstructions.
To be honest, one reason I thought I might write about this was that I was a little concerned that some might use this paper to argue that it is further evidence that climate models are too sensitive, but I think it did a fairly good job of covering that. I also think that comparisons between changes in our past climate and climate models is an interesting topic. I did, however, get somewhat distracted in the middle of writing it, then made dinner, and almost decided to bin it. So, I don’t know if I’ve given this as much thought as I should have, but I thought I’d post it anyway. Some may find it interesting and maybe others will have more insight into this than I currently have.