Wednesday, August 8, 2012

New paper shows clouds act as a negative feedback

A recent paper published in the Journal of Climate examined satellite records over the West African Sahel to determine that clouds caused a yearly average decrease in shortwave radiation [solar radiation] at Earth's surface of -83 W m-2 [due to blocking incoming solar radiation]. The paper also finds clouds clouds caused a yearly average increase in longwave radiation [radiation from the 'greenhouse' gas water vapor] at Earth's surface of +37 W m-2. Thus, the average net radiative effect at the surface would be -83 + 37 = -46 W m-2. This would indicate that clouds exert net negative feedback to cool the climate. Climate models, however, assume clouds act as a net positive feedback to warm the Earth. 

The paper also finds cloud effects are very poorly represented in climate models, noting, "Both [global climate models tested] underestimated the surface [longwave] and [shortwave cloud radiative forcing]  and predicted near zero [shortwave cloud radiative effect] when the measured values were substantially larger (70 Wm−2 maximum)."

This paper and many others [including one also published recently in the same journal] show that cloud effects are very poorly represented in climate models and are of the opposite sign to observations.

The Radiation Budget of the West African Sahel and its Controls: A Perspective from Observations and Global Climate Models

Mark A. MillerVirendra P. Ghate, and Robert K. Zahn
Department of Environmental Sciences, Rutgers University, NJ USA
Continuous measurements of the shortwave (SW), longwave (LW), and net cross-atmosphere radiation flux divergence over the West African Sahel were made during the year 2006 using the Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) and the Geostationary Earth Radiation Budget (GERB) satellite. Accompanying AMF measurements enabled calculations of the LW, SW, and net Top-of-Atmosphere (TOA) and surface Cloud Radiative Forcing (CRF), which quantifies the radiative effects of cloud cover on the column boundaries. Calculations of the LW, SW, and net Cloud Radiative Effect (CRE), which is the difference between the TOA and surface radiative flux divergences in all-sky and clear-sky conditions, quantify the radiative effects on the column itself. These measurements were compared to predictions in four Global Climate Models (GCMs) used in the Intergovernmental Panel for Climate Change fourth Assessment report (IPCC-AR4). Reproducing the SW column radiative flux divergence was problematic in the GCMs and SW discrepancies translated into the net radiative flux divergence. Computing cloud-related quantities from the measurements produced yearly averages of the SW TOA CRF, surface CRF, and CRE of ~ −19 Wm−2, −83 Wm−2, and 47 Wm−2, respectively, and yearly averages of the LW TOA CRF, surface CRF, and CRE of ~ 39 Wm−2, 37 Wm−2, and 2 Wm−2. These quantities were analyzed in two GCMs and compensating errors in the SW and LW clear-sky, cross-atmosphere radiative flux divergence conspired to produce reasonable predictions of the net clear-sky divergence. Both GCMs underestimated the surface LW and SW CRF and predicted near zero SW CRE when the measured values were substantially larger (70 Wm−2 maximum).


  1. 'New paper shows clouds act as a negative feedback'

    'over the West African Sahel' so that is a region then! Not exactly global then, which is the overall problem!

    1. This is only one of many papers showing clouds act as a negative feedback.

  2. The effect of clouds is always to cool the climate


  4. Good work Michael, keep 'em coming.