We have published a new study that documents the role of atmospheric aerosols on plant growth and heat and moisture fluxes [and, therefore, on the climate system].
The paper is
Matsui, T., A. Beltrán-Przekurat, D. Niyogi, R. A. Pielke Sr., and M. Coughenour (2008), Aerosol light scattering effect on terrestrial plant productivity and energy fluxes over the eastern United States,J. Geophys. Res., 113, D14S14, doi:10.1029/2007JD009658.
and the abstract reads
“This study reports the first regional-scale assessment of aerosol effects on plant productivity and surface energy fluxes over the eastern United States. Analysis is conducted using an established modeling framework, which is composed of a regional land surface model, regional daily aerosol optical depth (AOD) estimates, and meteorological forcings. The sensitivity experiments were conducted from May to September in 2000 and 2001 over the eastern United States with and without the aerosol light scattering effect.
Results show that the aerosol light scattering effect results in enhanced productivity for high-LAI and optimum temperature environments under cloudless-sky conditions around noon, while it results in least productive for low-LAI, low-temperature environments under cloud-sky conditions in early morning or late afternoon. As a result, domain-averaged plant productivities, measured as net primarily production, are changed by —0.71 g C m-2 (—0.09%) in 2000 and +5.00 g C m-2 (+0.5%) in 2001.
These responses of plant productivity and photosynthesis to the aerosol light scattering effect uniquely modulate the surface flux as follows. The aerosol light scattering effect reduces the surface downwelling solar radiation (14.9 W m-2 in 2000 and 16.0 W m-2 in 2001) and net radiation in vegetation canopy, but simultaneously increases the photosynthesis and stomatal conductance. Consequently, surface latent heat flux (transpiration and evaporation) is reduced by a small amount particularly over the forests, while aerosol loading often results in larger reduction in the sensible heat flux. For the whole domain, latent heat flux is changed by —3.10 W m-2 (—2.1%) in 2000 and —3.12 W m-2 (—2.1%) in 2001, sensible heat flux is changed by —7.57 W m-2 (—12.9%) in 2000 and —8.36 W m-2 (—11.3%) in 2001, and surface skin temperature is changed by —0.25 K (—0.1%) in 2000 and —0.27 K (—0.1%) in 2001.”
One important message from this study is that unless vegetation effects from aerosols are included in climate models, an important real world (and human) climate forcing is ignored. Moreover, in the assessment of “global warming” using land surface temperature data, the neglect of the effect of aerosols on vegetation (and thus the near surface air temperature) necessarily results in errors in the attribution of temperature anomalies to specific climate forcings (such as from the radiative effect of added CO2).