- Bishop Hill blog

Is it just me that reads every paper by Andrew Dessler as an attempted rebuttal of some sceptic position or another? His latest paper was submitted to Geophysical Research Letters just three weeks after the publication of the Lewis and Crok report on climate sensitivity and reads as though it was written in direct response to it. Here's the abstract:

Estimates of the Earth's equilibrium climate sensitivity (ECS) from 20^th-century observations predict a lower ECS than estimates from climate models, paleoclimate data, and interannual variability. Here we show that estimates of ECS from 20^th-century observations are sensitive to the assumed efficacy of aerosol and ozone forcing (efficacy for a forcer is the amount of warming per unit global average forcing divided by the warming per unit forcing from CO₂). Previous estimates of ECS based on 20^th-century observations have assumed that the efficacy is unity, which in our study yields an ECS of 2.3 K (5%-95%-confidence range of 1.6-4.1 K), near the bottom of the IPCC's likely range of 1.5-4.5 K. Increasing the aerosol and ozone efficacy to 1.33 increases the ECS to 3.0 K (1.9-6.8 K), a value in excellent agreement with other estimates. Forcing efficacy therefore provides a way to bridge the gap between the different estimates of ECS.

Efficacy is not a measure with which I am familiar, and a cursory scan of Google search results suggests that it has not exactly been a research priority in the past, although there is a Hansen paper from 2005 and a section in AR4 on the subject. Hansen seems to be determining efficacy from climate models, which seems a bit back-to-front to me.

Hockey Schtick, who wrote about the paper yesterday, describes it as a "fudge factor" and I have to say that I too detect a whiff of sugar and butter on reading the abstract. If someone can send me the paper then perhaps we can see how much physics is involved.

Update on May 8, 2014 by

Bishop Hill

I've now got hold of the paper. Here is the discussion of efficacy:

It has long been expected that forcings with the same global average magnitude but different spatial patterns could evoke different responses in global surface temperature [Hansen et al., 1997; 2005; Shindell and Faluvegi, 2009; Shindell et al., 2010]. For example, forcing concentrated at high latitudes, which are less strongly restored by infrared radiation to space, will lead to more warming than well-mixed forcing agents. Certain forcing agents, particularly aerosols and tropospheric ozone, are indeed not uniformly distributed and impact the climate system differently than well-mixed constituents [Shindell et al., 2003; Feichter et al., 2004; Chung and Seinfeld, 2005; Crook et al., 2011].

Different formalisms have been adopted in the literature to account for this process [e.g. Hansen et al., 2005; Winton et al., 2010; Armour et al., 2013]. One is to account for the effect using a so-called forcing “efficacy” [Hansen et al., 1997; 2005], which is the amount of warming per unit of global average forcing divided by the amount of warming per unit of forcing from carbon dioxide. Most calculations of ECS based on 20^th-century observations assumed that the efficacy of different forcers is one, so this effect was ignored.

Recently, Shindell [2014] analyzed transient model simulations to show that the combined ozone and aerosol efficacy is about 1.5. At least some of the high efficacy of aerosols and ozone was due to nature of the transient runs he analyzed, but his analysis nevertheless clearly showed that, in the models at least, the efficacy of aerosol and ozone forcing was significantly greater than 1.0.

To test the impact of efficacy on the inferred λ and ECS in our calculations, we multiply the aerosol and ozone forcing time series by an efficacy factor in the calculation of the total forcing. We find that increasing the efficacy shifts the PDF of λ to lower values (Fig. 1), corresponding to increased climate sensitivity.

Using Shindell’s [2014] estimate of efficacy of 1.5 decreases λ to 1.1 …, corresponding to an ECS of 3.5 K … We can reasonably simulate the IPCC’s climate sensitivity range using an efficacy of 1.33, which gives an ECS of 3.0 K…

This doesn't help much, since the physics seems to be in Shindell 2014. However, the latter paper is available online.

Here I analyse results from recent climate modelling intercomparison projects to demonstrate that transient climate sensitivity to historical aerosols and ozone is substantially greater than the transient climate sensitivity to CO2. This enhanced sensitivity is primarily caused by more of the forcing being located at Northern Hemisphere middle to high latitudes where it triggers more rapid land responses and stronger feedbacks.

Ah ha. It's because the model says so.