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Re: Global Dimming

Started by rpasken, 08/06/2008 09:17PM
Posted 08/06/2008 09:17PM Opening Post
Nothing about assessing the future is easy grin If it were then we would all be stock market millionaires 8)

Nothing about aerosols is easy. From a meteorologist/climatologist point of view aerosols are any solid particles or liquid droplets that are temporarily suspended within the atmosphere. Naturally occurring examples are sea spray or sulfate droplets, along with soil particles (dust) eroded by the wind. Some aerosols, like sulfates and nitrates are reflective ("light") and hence increase the planetary albedo lowering temperatures, but they also absorb a little and cause a local warming. Soots ("Dark") aerosols absorb more and darken the earth relative to what it was and so end up warming the planet. Starting in the late 1970's, satellite instruments have detected aerosols routinely with nearly global coverage. The original instrumentation couldn't distinguish between dust and sulfate aerosols. Recent instruments, like the Moderate Resolution Imaging Spectroradiometer (MODIS) measure radiation at multiple wavelengths. This allows particle size to be distinguished with greater confidence, which can be used with some assumptions to infer the aerosol species. The instrumentation aboard MODIS allow the amount of forcing by aerosols by type (anthropogenic vs natural) to be determined. I use MODIS data to determine the amount of Saharan Air Layer (SAL) dust that is being incorporated into hurricanes. In the case of hurricane Helene when the natural aerosols determined when the Helene's character changed. It appears the SAL plays a role in how and where hurricanes develop. The current generation of satellite instrumentation is at the heart of recent attempts to reduce the large uncertainty of direct radiative forcing by aerosols. Bellouin et al. (2005; Nature) arrive at a top of the atmosphere forcing of -0.8 ± 0.1 W/m2. While near the center of the range published by the IPCC, this estimate is noteworthy for its comparatively small uncertainty. Yet on the same day, Chung et al. (2005; Journal of Geophysical Research) estimating based upon similarly extensive calculations that the forcing by aerosols at the top of the atmosphere is -0.35 ± 0.25 W/m2. The original version of the PBS/NOVA film shown in the UK by BBC focused mainly on the observational recognition of global dimming. An important aspect of the original film that did not receive much attention in the film is the oft-claimed lack of global dimming in climate models. This led some to assume that climate modelers were ignoring air pollution other than greenhouse gases emissions from fossil fuel burning, which very far from the truth. Another implication was that climate models are not capable of adequately simulating the transfer of sunlight through the atmosphere and the role of clouds, sunlight extinction of aerosols and aerosol effects on clouds etc, and therefore model projections should not be trusted. Of course none of the above is true, as demonstrated by the papers by Chung et. al. (2005) and Bellouin et al.

I took the attached picture during NAMMA RF#9 you can see the SAL as the brown layer above the cumulus deck

Attached Image:

rpasken's attachment for post 43177
Posted 08/07/2008 06:23PM #1
Exemplary comentary that lends credibility to the discussion and comprehension of those factors affecting climate change. We are indebted to those who diligently seek the truth. My position in the past has been one of disbelief primarily because of the monumental task of accurately building and competently running a global model. With so many variables to describe with accuracy, the modelers have a daunting task. It would be valuable to learn of those offshoots that have been realized through such efforts.

I Yam What I Yam!