I ended up in a discussion with Roger Pielke Sr about a claim that there is a warm bias in Tmin. It ended rarely sourly when I pointed out that accusing an entire scientific discipline of being dysfunctional because they appear to disagree with you, is not very constructive. Anyway, Roger’s basic point appears to be that Tmin warms faster than Tmax. It seems that this is what Sou is discussing in this post. The planetary boundary layer is the lowest region of the atmosphere, and is a region in which vertical mixing is strong. At night it is much thinner than during the day and so the extra energy is compressed into a thinner layer and hence Tmin increases faster than Tmax; at least I think that is about right.
Roger seemed to be claiming that this produces a warm bias in Tmin. Well, this seems a bit odd if it is real; if Tmin really does increases faster than Tmax, then it’s not really a bias, it’s actually happening. Roger’s next claim was that Tmin should then not be used when assessing global warming. What he’s referring to, I think, is what he and I discussed in a joint post about assessing anthropogenic global warming. You can write down a basic 1D climate model in the following way
where , is the planetary energy imbalance, is the change in forcing, is the feedback factor, is the change in global mean temperature, and is time. If, for example, you have a forcing time series, a temperature time series, and some estimate for you can evolve the above equation in time to see how, for example, the ocean heat content should change. That way you can assess anthropogenic global warming.
Roger’s point seems to be that the global mean temperature is determined by averaging Tmin and Tmax, is therefore warm biased, and shouldn’t be used to assess global warming. Well, if Tmin really is warming faster than Tmax, then surely that doesn’t mean that the mean temperature is warm biased; it has to be some combination of minimum and maximum temperatures.
However, it is kind of true that, in the above equation, we want a mean temperature that allows us to produce a reasonable representation of the energy fluxes. However, it’s firstly not clear that the manner in which the mean global temperature is determined is not suitable. Secondly, everything in the above equation is globally averaged, and so as long as the feedback factors (such as the Planck response) are determined in a way that is consistent with the way in which the global mean temperature is computed, I can’t really see the issue.
Additionally, as Victor pointed out, if a warm bias in Tmin produces a warm bias in the mean temperature, then that would imply that the climate sensitivity estimates from energy balance models would be slightly too high. Given that these are already lower than many other estimates suggests, would seem to make this rather unlikely. So, I can’t really see what Roger is getting at. I could, of course, just be confused.