Semyorka posted a comment on my previous post that highlighted a paper that I had’t seen before. The paper is Global atmospheric downward longwave radiation over land surface under all-sky conditions from 1973 to 2008 which tries to determine (as the title might suggest) the change in downwelling longwavelength flux, over land.
The abstract concludes with
We found that daily Ld increased at an average rate of 2.2 W m-2 per decade from 1973 to 2008. The rising trend results from increases in air temperature, atmospheric water vapor, and CO2 concentration.
Ld is the global atmospheric downward longwave radiation and the observed trend (2.2Wm-2) suggests it increased by 7.7Wm-2 between 1973 and 2008. Initially I was somewhat confused by this (still am maybe 🙂 ) as it seemed rather high, but over the same time interval, land surface temperature increased by almost 1K (see the Skeptical Science trend calculator). This would increase the outgoing surface flux by
So, the outgoing surface flux over land has increased by about 5.5Wm-2, while the downward longwave flux has increased by about 7.7Wm-2. If you consider a typical forcing dataset, then the radiative forcing has increased by maybe as much as 1.5Wm-2 since the mid-1970s. If you do a simple transient temperature response calculation, that would suggest that the transient response over land is
which is somewhat higher than the expected global value of slightly below 2K (okay, maybe it should be 3.44, rather than 3.7, but that won’t change this all that much. Also, the change in forcing I’ve used is probably a bit too high anyway.). It’s possible that the system is just too complex for such a calculation to be reasonable, but given the low thermal inertia of the land – compared to the oceans – it’s not that surprising that the land-only TCR is greater than the global TCR.
However, the equilibrium response (with fast feedbacks only) shouldn’t depend on the thermal inertia (it will just take longer to reach if the thermal inertia is high, than if it is low). Therefore, if the above calculation has some merit, that the downward longwave flux over land exceeds the outgoing flux (as the paper mentioned above suggest) could suggest that the equilibrium response has to exceed 2.5K.
Admittedly, I’m ignoring uncertainties and all sorts of caveats. It’s also possible that such a calculation doesn’t really make any sense given the complexity of the system. That’s why I thought I would write this post – someone can point out where I’ve gone wrong and why this hasn’t been suggested before (assuming that it hasn’t). In my experience, when you notice something apparently simple that noone has noticed before, it’s probably because it’s not as simple as you initially thought 😀 .
Update: I knew I was going to do something silly in this post. As Chris Colose points out on Twitter, you need to close the surface energy budget using non-radiative terms too; like evaporation and convection. That the downwelling flux exceeds the upgoing flux doesn’t mean that the surface is out of energy balance. So, the latter part of this post is probably slightly nonsensical, or – rather – you can’t really use this to argue for an ECS above 2.5K.