Sunday, August 3, 2014

New paper finds marine clouds cause a negative-feedback cooling effect on climate

A new paper published in Nature Geoscience finds global marine clouds exert a negative feedback cooling effect on Earth's surface temperatures. However, all current IPCC models adopt net positive feedbacks for water vapor and clouds, and this false assumption may account for a large portion of the exaggerated warming models project (in addition to the climate being less sensitive to CO2 than the IPCC assumes).

According to the authors, 
"a 6% increase in the albedo of global marine stratiform clouds could offset the warming that would result from a doubling of atmospheric CO2 concentrations" i.e. just clouds over the oceans alone, not considering clouds over land.
and as previously noted by Dr. Roy Spencer,
"The most obvious way for warming to be caused naturally is for small, natural fluctuations in the circulation patterns of the atmosphere and ocean to result in a 1% or 2% decrease in global cloud cover. Clouds are the Earth’s sunshade, and if cloud cover changes for any reason, you have global warming — or global cooling."
According to the paper,  
"The estimated intrinsic [marine cloud] forcing is −0.49 ± 0.33 Wm−2, and for the extrinsic (cloud-cover effect) forcing, we estimate −0.46 ± 0.31 Wm−2" "We have also estimated the long-wave component of aerosol–cloud radiative forcing (Supplementary Information); the estimated long-wave TOA [Top Of Atmosphere] intrinsic and extrinsic forcings are −0.01 and 0.09 W m−2, respectively."
Adding the 4 forcings obtains a net shortwave and longwave marine cloud forcing of negative 0.87 Wm-2, higher than many climate sensitivity estimates for a doubling of CO2 levels. 

Global mean intrinsic aerosol-cloud radiative forcing by global marine clouds is estimated to be negative 0.49 Wm-2

Using equations (1) and (2), we estimate the aerosol–cloud
 radiative   forcing   for   global   single-layer   marine   warm   clouds
 between   60◦ S   and   60◦ N.   The   estimated   intrinsic   forcing   is
 −0.49 ± 0.33 W m−2,   and   for   the   extrinsic   (cloud-cover effect)
 forcing, we estimate −0.46 ± 0.31 W m−2, a similar magnitude
 to the estimated intrinsic forcing (Supplementary Fig. 2). This
 corroborates the finding in ref. 21 that the combined Twomey
 and   LWP   effects   (that   is,   intrinsic   forcing)   are   comparable   in
 magnitude to the cloud-cover effect. It should be noted that the
 observed positive correlation between  cloud  cover and aerosol
 level may be subject to other processes or artefacts 22,23, including
 cloud contamination of satellite-retrieved AOD, co-variation of
 cloud fraction and relative humidity/wind speed, cloud processing
 of aerosols, and so on. Owing to uncertainties that cannot be
 quantified directly, the magnitude of the extrinsic forcing may
 be biased. We have also estimated the long-wave component of
 aerosol–cloud radiative forcing (Supplementary Information); the
 estimated long-wave Top Of Atmosphere intrinsic and extrinsic 
forcings are −0.01  and 0.09 W m−2, respectively.

Satellite-based estimate of global aerosol–cloud radiative forcing by marine warm clouds

Nature Geoscience
Published online
Changes in aerosol concentrations affect cloud albedo and Earth’s radiative balance1. Aerosol radiative forcing from pre-industrial time to the present due to the effect of atmospheric aerosol levels on the micro- and macrophysics of clouds bears the largest uncertainty among external influences on climate change1. Of all cloud forms, low-level marine clouds exert the largest impact on the planet’s albedo2. For example, a 6% increase in the albedo of global marine stratiform clouds could offset the warming that would result from a doubling of atmospheric CO2 concentrations3. Marine warm cloud properties are thought to depend on aerosol levels and large-scale dynamic or thermodynamic states456. Here we present a comprehensive analysis of multiple measurements from the A-Train constellation of Earth-observing satellites, to quantify the radiative forcing exerted by aerosols interacting with marine clouds. Specifically, we analyse observations of co-located aerosols and clouds over the world’s oceans for the period August 2006–April 2011, comprising over 7.3 million CloudSat single-layer marine warm cloud pixels. We find that thermodynamic conditions—that is, tropospheric stability and humidity in the free troposphere—and the state of precipitation act together to govern the cloud liquid water responses to the presence of aerosols and the strength of aerosol–cloud radiative forcing.

1 comment:

  1. Hansen said all feedbacks are positive. The science is settled. Or maybe not .......