When I started this blog, I would sometimes write posts where I’d ponder some scientific issue, mainly illustrate my own ignorance, and benefit from those who are more informed than me telling me where I was going wrong (which is different to those less informed than me telling me I’m wrong 🙂 ) . I thought I might resurrect that by writing a post about something I was considering with respect to the greenhouse effect.
A somewhat contentious issues in climate change is the role of feedbacks. A basic way to think of this is that there is some external change (Sun, volcanoes, us) that, by itself, would produce a certain amount of warming or cooling. This warming, or cooling, then produces other changes (feedbacks) that produce more warming or cooling. The final state is then set by the combination of the forcings plus feedbacks. Kate Marvel has a very amusing, and very good, blog post that explains the basics of feedbacks.
The greenhouse effect refers to the fact that our surface temperature is 33 degrees higher than it would be if our atmosphere was completely transparent to our emitted radiation (assuming a fixed albedo). A 33 degree increase in surface temperature is associated with a 155 Wm-2 increase in surface flux, or a 122 Wm-2 change in radiative forcing (33 x 3.7Wm-2). The actual number isn’t that important for this discussion, though.
Prior to the increase in anthropogenic CO2, the atmospheric CO2 concentration was about 280ppm. This would contribute a radiative forcing of about 30Wm-2, so about 20% of the the overall greenhouse effect. Other factors (water vapour, lapse rate, clouds) have to then provide about another 100Wm-2. Therefore one could argue that the feedback response is about 3Wm-2K-1 (100Wm-2 / 33 degrees), or that feedbacks amplify the warming by about a factor of 5. Given that a doubling of CO2 alone would produce a warming of about 1 degree, this would imply an ECS of around 5 degrees (and since the albedo is fixed, I think this would be an ECS and not and ESS). For comparison, an ECS of 1.65 degrees (as suggested by Lewis & Curry 2014) would require a feedback response of around 1Wm-2K-1. Most climate models suggest a feedback response of close to 2Wm-2K-1, giving an ECS of 2.5 and 3 degrees, which is also consistent with many paleo estimates.
So, does the greenhouse effect allow us to constrain the feedback response in any way? It seems clear that it illustrates that feedbacks have to be positive, but apart from that I’m not sure that there’s much more that can be said. Some issues I can think of are :
- we don’t expect feedbacks to be linear and maybe the change in climate state is simply too great compared to what we’re undergoing today.
- maybe it’s not really a situation where we can break it down into a forcing plus a feedback. However, given that the CO2 is primarily (I think) from volcanic outgassing, it seems reasonable to regard it as an external forcing and the rest as feedbacks.
- you actually need to know what the water vapour, lapse rate, and cloud conditions would be prior to any warming, as these could provide an initial forcing that I’m including as a feedback, reducing the feedback response.
Anyway, I’m not sure this post makes much sense (in fact, it’s sufficiently muddled, that I considered trashing it, but maybe someone can make some sense of this). Even if the average feedback response that contributes to the greenhouse effect is about 3Wm-2K-1, I’m not sure that this is really an argument against the low feedback response suggested by Lewis & Curry (2014). On the other hand, both the greenhouse effect, Lewis & Curry (2014), and virtually any other sensible study, suggests that feedbacks have to be positive, so maybe that’s something.