Longwave Feedbacks in Climate Models
Reference: Huang, Y. 2013. On the longwave climate feedbacks. Journal of Climate 26: 7603-7610..
In a study published in the Journal of Climate, Yi Huang - a Canadian researcher from McGill University in Montreal - addressed a pair of issues that concern the longwave climate feedbacks in transient climate change assessments. The first of these issues was that "the radiative forcing of greenhouse gases, as measured by their impact on the outgoing longwave radiation (OLR), may vary across different climate models even when the concentrations of these gases are identically prescribed," which forcing variation, as he continues, "contributes to the discrepancy in these models' projections of surface warming." The second issue was that "the stratosphere is an important factor that affects the OLR in transient climate change," in that stratospheric water vapor and temperature changes may both act as positive feedbacks during global warming and, therefore, "cannot be fully accounted as a 'stratospheric adjustment' of radiative forcing."
As the Canadian researcher goes on to demonstrate in the body of his paper, "neglecting these two issues may cause a bias in the longwave cloud feedback diagnosed as a residual term in the decomposition of OLR variations." And he notes, in this regard, that his results "and the recent results of others [e.g., the estimate of Zelinka et al. (2012) based on cloud property histograms] indicate that there is, in fact, no consensus in terms of the sign of the longwave cloud feedback among the GCMs." Such sad state of affairs -- that of no consensus, not even in terms of the sign of the longwave cloud feedback -- will likely persist for some time to come, as Huang writes that in order achieve progress, "more studies are still required to understand how clouds modify global warming, even with regard to the longwave aspect alone."
Zelinka, M.D., Klein, S.A. and Hartmann, D.L. 2012. Computing and partitioning cloud feedbacks using cloud property histograms. Part I.: Cloud radiative kernels. Journal of Climate 25: 3715-3735.