I started this blog because I was getting frustrated by how often I saw scientific claims being made, with regards to global warming, that were just simply wrong. I was following, and partially involved with, a brief exchange between Joanne Nova and Michael Brown, an astronomer at Monash University. Joanne eventually responded to Michael Brown with the following suggestion
— Joanne Nova (@JoanneNova) October 15, 2013
So, yes, the radiative imbalance is what matters. The problem, though, is that the article to which Joanne links is simply wrong. I know it’s more than a year old, but I thought I would briefly discuss it again here as I think it illustrates a major issue with this whole debate.
The basic premise of the article, written by someone called David Stockwell, is that one can use ocean heat content data (from, for example, Levitus et al. 2012) to determine how much energy has accrued in the climate system in the last 50 years. Dividing this by the surface area of the Earth and the time (in seconds) then gives an estimate of the average top-of-the-atmosphere (TOA) energy imbalance (in Wm-2). This is essentially correct and the calculation gives a TOA energy imbalance of about 0.27 Wm-2.
The article then goes on to basically say that this is considerably smaller than the continuous top-of-atmosphere forcing of 1 Wm-2 – presented by the IPCC – and, hence, that this mild forcing is consistent with the lower estimates from Lindzen, Spencer, Loehle and himself. Here’s the problem (and this was explained extremely clearly by Tom Curtis in the comments to Stockwell’s article): the 1 Wm-2 is not the continuous top-of-atmosphere forcing, it is the change in radiative forcing (anthropogenic only) over the time interval considered. If the surface temperature did not change over that time interval, then the TOA energy imbalance would indeed be at least as big as this change (likely bigger because of additional forcings and feedbacks), but the surface temperature does change. Hence, the actual TOA imbalance is much smaller than the change in radiative forcing. As also pointed out by Tom Curtis in the comments, the actual TOA imbalance, at time t, is given by (from a paper by Winton et al. 2010)
where ΔF is the change in radiative forcing, ΔT is the change in surface temperature, and λ is the climate feedback factor (basically representing how much a certain change in forcing will influence surface temperatures). Therefore, that the surface temperature also rises while the radiative forcing increases means that the TOA imbalance does not (and should not) match the expected change in radiative forcing.
So, Joanne, did I find your link helpful? Not really; it, sadly, just further confirmed my view that a number of those who are skeptical of the science of global warming don’t actually understand the underlying physics particularly well. What was quite remarkable about this particular article was that Tom Curtis very clearly explained why the comparison was wrong, and yet even Roger Pielke Sr (who was also commenting) would not acknowledge the error. Maybe the author has since acknowledged his mistake (the article was written last year) but, if not, I find this incredibly frustrating. The comparison is very obviously wrong and any credible scientist should be willing to at least consider this and, ideally, actually accept their error.
In truth this whole issue may not actually be trivial (the concept of radiative balance can get confusing and I’ve certainly made some mistakes myself) but it’s not that complicated. If we even disagree about the basics (and this is remarkable in that it really shouldn’t be something we’re disagreeing about) how can we hope to actually have sensible discussions about those aspects that are much more complicated than this. Furthermore, that someone can’t even get this right does beg the question of why anyone should take anything they say (with respect to global warming at least) seriously? And, as usual, if I’ve made some kind of mistake, feel free to point it out in the comments.