The Interlinked Role Of Changes In Radiative Forcings And Hydrology In The Climate System by Dev Niyogi

In 2004 we published a study entitled Direct observations of the Effects of Aerosol loading on Net Ecosystem CO2 Exchanges over Different Landscapes, in Geophysical Research Letters, 31, L20506, doi:10.1029/2004GL020915. The study used observations over different landscapes to show that changes in aerosol levels alter the quantity and quality of radiation (i.e., increased diffuse radiation) reaching the surface. This in turn affects the vegetation response for transpiration and photosynthesis, which can affect the water cycle and the terrestrial carbon cycle (since transpiration is an efficient means of water loss from the earth’s surface; and photosynthesis is a major component of the terrestrial carbon cycle).

We recently expanded this analysis to investigate the effect of radiative changes on the land surface response and the feedback on the regional hydrology. The study appears in the journal ‘Natural Hazards’ (Monsoon special issue) and is entitled Potential Impacts of Aerosol-Land-Atmosphere Interaction on the Indian Monsoonal Rainfall Characteristics. The abstract reads

“Aerosols can affect the cloud – radiation feedback and the precipitation over the Indian monsoon region. In this paper, we propose that another pathway by which aerosols can modulate the multi-scale aspect of Indian monsoons is by altering the land-atmosphere interactions. The nonlinear feedbacks due to aerosol/diffuse radiation on coupled interactions over the Indian monsoon region are studied by: (i) reviewing recent field measurements and modeling studies, (ii) analyzing the MODIS and AERONET aerosol optical depth datasets, and (iii) diagnosing the results from sensitivity experiments using a mesoscale modeling system. Study results suggest that, the large magnitude of aerosol loading and its impact on land-atmosphere interactions can significantly influence the mesoscale monsoonal characteristics in the Indo-Ganges Basin.â€?

There is growing appreciation that aerosols and associated changes in radiative forcing can alter the climate system by interacting with the land surface. The recent issue of the journal Geophysical Research Letters, has two important papers related to this broader perspective.

The first paper is by Alan Robock and Haibin Li titled ‘Solar dimming and CO2 effects on soil moisture trends’. The abstract reads

“Summer soil moisture increased significantly from 1958 to the mid 1990s in Ukraine and Russia. This trend cannot be explained by changes in precipitation and temperature alone. To investigate the possible contribution from solar dimming and upward CO2 trends, we conducted experiments with a sophisticated land surface model. We demonstrate, by imposing a downward trend in incoming shortwave radiation forcing to mimic the observed dimming, that the observed soil moisture pattern can be well reproduced. On the other hand, the effects of upward CO2 trends were relatively small for the study period. Our results suggest tropospheric air pollution plays an important role in land water storage at the regional scale, and needs to be addressed accurately to study the effects of global warming on water resources.â€?

The second paper is by Wilfried Brutsaert titled ‘Indications of increasing land surface evaporation during the second half of the 20th century’. The abstract reads

“It is generally agreed that the evaporation from pans has been decreasing for the past half century over many regions of the Earth. However, the significance of this negative trend, as regards terrestrial evaporation, is still somewhat controversial, and its implications for the global hydrologic cycle remain unclear. The controversy stems from the alternative views that these evaporative changes resulted, either from global radiative dimming, or from the complementary relationship between pan and terrestrial evaporation. Actually, these factors are not mutually exclusive but act concurrently. It is shown quantitatively that, if the presently available data records are taken at face value, despite global dimming, the observed decreases in pan evaporation are generally evidence of increased terrestrial evaporation in those regions. This is consistent with independent hydrologic budget calculations for several large river basins in the USA, and likely further evidence of an accelerating hydrologic cycle in many areas.â€?

These recent studies provide additional evidence regarding the significance of land surface response in the climate system.

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