2011-09-22 07:35:27GCMs supporting strong negative cloud feedback and small sensitivity(?)
Alex C


This came up at Y!A, it seems as though new models from the Climate Process Team on Low-Latitude Clouds Feedbacks on Climate Sensitivity indicate that cloud feedback is large, negative, and climate sensitivty is about 0.41 K/m^2 (~1.5 K/2xCO2).

To quote:


Following Cess et al. (1989), climate sensitivity was assessed by examining the TOA radiative response to a uniform SST increase of 2K, based on the difference between control and +2K 3.5 year CAMSP simulations. Fig. 2 compares the results to standard versions of the NCAR CAM3, GFDL AM2 and GMAO AGCMs. All these models have similar clear-sky responses, so we just plot the +2K changes in longwave (greenhouse) and shortwave (albedo) cloud radiative forcings ([delta]LWCF and [delta]SWCF). Since [delta]SWCF tends to be largerthan [delta]LWCF. boundary-layer cloud changes (which have little greenhouse effect compared to their albedo enhancement) appear to be particularly important.

 The CAM-SP shows strongly negative net cloud feedback in both the tropics and in the extratropics, resulting in a global climate sensitivity of only 0.41 K/(W m-2), at the low end of traditional AGCMs (e.g. Cess et al. 1996), but in accord with an analysis of 30-day SST/SST+2K climatologies from a global aquaplanet CRM run on the Earth Simulator (Miura et al. 2005). The conventional AGCMs differ greatly from each other but all have less negative net cloud forcings and correspondingly larger climate sensitivities than the superparameterization.








Anyone know about this?  Impressions?  I myself wonder why they force a temperature change to induce a change in radiative forcing, rather than induce a radiative forcing to induce a temperature change, but I don't know how that would affect results.

2011-09-22 07:58:36
Dana Nuccitelli

Somebody brought this up on Bart's blog too.  I think the document is from 2006 (note that it has no references later than 2006), for starters, so if nothing else it's rather outdated.

2011-09-22 08:07:55
Julian Brimelow

Funny how the deniers and "skeptics" are adamant that the models are BS, that is until they offer support they belief that climate sensitivity is lowish.  I do not have time to lok into this now.  This was publishe dbefore AR4, so it should be in there if it has merit.

Google "climate sensitivity was assessed by examining the TOA".  Two hits, both from a denier site.

No wonder I got no hits.  It is from a newsletter!  It was probably never published.

Update:  They did publish in GRL.  Here is the paper.  They have this caveat:

"Thus the overall climate sensitivities produced by the model must be regarded with caution. A next step with SP-CAM is to couple it to a slabocean model so that cloud responses in more realistic
climate change scenarios can be evaluated"

Not sure where this went.