Urs Neu has alerted us to an observationally-based paper that documents a change of surface radiative forcing that is consistent with our studies (as well as further refutes claims to the contrary; e.g., see).
Pielke Sr., R.A., and T. Matsui, 2005: Should light wind and windy nights have the same temperature trends at individual levels even if the boundary layer averaged heat content change is the same? Geophys. Res. Letts., 32, No. 21, L21813, 10.1029/2005GL024407
Klotzbach, P.J., R.A. Pielke Sr., R.A. Pielke Jr., J.R. Christy, and R.T. McNider, 2009: An alternative explanation for differential temperature trends at the surface and in the lower troposphere. J. Geophys. Res., in press (with edits still to be made in the final published version).
In the second paper we wrote
“…… if, for instance, there is a long-term positive trend in greenhouse gas concentrations or cloudiness over the observing site, it may introduce an upward bias in the observational record of minimum temperatures that necessarily will result in an upward bias in the long-term surface temperature record.”
The paper recommended by Urs Neu is
Philipona, R., B. Durr, C. Marty, A. Ohmura, and M. Wild (2004), Radiative forcing – measured at Earth’s surface – corroborate the increasing greenhouse effect, Geophys. Res. Lett., 31, L03202, doi:10.1029/ 2003GL018765.
The abstract reads
“The Intergovernmental Panel of Climate Change (IPCC) confirmed concentrations of atmospheric greenhouse gases and radiative forcing to increase as a result of human activities. Nevertheless, changes in radiative forcing related to increasing greenhouse gas concentrations could not be experimentally detected at Earth’s surface so far. Here we show that atmospheric longwave downward radiation significantly increased (+5.2(2.2) Watts per meter squared) partly due to increased cloud amount (+1.0(2.8) Watts per meter squared) over eight years of measurements at eight radiation stations distributed over the central Alps. Model calculations show the cloud-free longwave flux increase (+4.2(1.9) Watts per meter squared) to be in due proportion with temperature (+0.82(0.41) C) and absolute humidity (+0.21(0.10) g per meter cubed) increases, but three times larger than expected from anthropogenic greenhouse gases. However, after subtracting for two thirds of temperature and humidity rises, the increase of cloud-free longwave downward radiation (+1.8(0.8) Watts per meter squared) remains statistically significant and demonstrates radiative forcing due to an enhanced greenhouse effect.”
This paper documents that changes of 1 Watt per meter squared (or more) in the longwave fluxes that we examined in Pielke and Matsui (2005) are realistic. The Klotzbach et al. (2009) paper demonstrates that a significant bias is introduced in the land portion of the global surface temperature trend which is used in the assessment of global warming, that can be explained, at least in part, due to such changes in longwave radiative fluxes at night.