New paper finds huge erroneous assumption in climate models on how solar radiation is averaged in space & time

A paper published today in the Journal of the Atmospheric Sciences finds an astonishing bias of climate models with regard to "an important decision about how to average solar radiation in space and time" by choice of the solar zenith angle [the angle measured from directly overhead to the geometric centre of the sun's disc]. According to the authors, the bias errors in choice of solar zenith angle upon the Earth's energy budget can be as great as the effect of a 12-fold [1200%] change in CO2 concentrations.

According to the paper, "Use of daytime-average zenith angle may lead to a high bias in planetary albedo of ~3%, equivalent to a deficit in shortwave absorption of ~10 W m−2 in the global energy budget (comparable to the radiative forcing of a roughly sixfold change in CO2 concentration). Other studies that have used general circulation models with spatially constant insolation have underestimated the global-mean zenith angle, with a consequent low bias in planetary albedo of ~2-6%, or a surplus in shortwave absorption of ~7-20 W m−2 [comparable to the radiative forcing of up to a roughly 12-fold change in CO2 concentration] in the global energy budget."

As noted by Dr. Roy Spencer, a change in planetary albedo of only 1-2% alone can account for global warming or global cooling, but this paper finds climate model planetary albedo may be biased as much as 6% due to errors of the solar zenith angle alone!

Whoops

Journal of the Atmospheric Sciences 2014 ; e-View

doi: http://dx.doi.org/10.1175/JAS-D-13-0392.1

On the Choice of Average Solar Zenith Angle

Timothy W. Cronin*

Program in Atmospheres, Oceans, and Climate, Massachusetts Institute of Technology, Cambridge, Massachusetts

Abstract

Idealized climate modeling studies often choose to neglect spatiotemporal variations in solar radiation, but doing so comes with an important decision about how to average solar radiation in space and time. Since both clear-sky and cloud albedo are increasing functions of the solar zenith angle, one can choose an absorption-weighted zenith angle which reproduces the spatial- or time-mean absorbed solar radiation. Here, we perform calculations for a pure scattering atmosphere and with a more detailed radiative transfer model, and find that the absorption-weighted zenith angle is usually between the daytime-weighted and insolation-weighted zenith angles, but much closer to the insolation-weighted zenith angle in most cases, especially if clouds are responsible for much of the shortwave reflection. Use of daytime-average zenith angle may lead to a high bias in planetary albedo of ~3%, equivalent to a deficit in shortwave absorption of ~10 W m⁻² in the global energy budget (comparable to the radiative forcing of a roughly sixfold change in CO₂ concentration). Other studies that have used general circulation models with spatially constant insolation have underestimated the global-mean zenith angle, with a consequent low bias in planetary albedo of ~2-6%, or a surplus in shortwave absorption of ~7-20 W m⁻² [comparable to the radiative forcing of a roughly 12-fold change in CO₂ concentration] in the global energy budget.