An Earlier Paper on the Role of Regional Land Surface Change and Regional Aerosol Emissions As First-Order Climate Forcings

Professor Dev Niyogi has alerted me to another important paper that was published in a journal that is normally not read by climate scientists. This paper, published in 1994, provides important background information on why land surface processes are so important as climate forcings.

The paper is the “Effects of pre-industrial human activities on climate” (subscription required) which appeared in Chemosphere and was written by Alan Robock and Hans-Friedrich Graf in 1994. The abstract states,

“Pre-industrial human activities which changed the atmospheric greenhouse gas or aerosol loading, or which modified the properties of the earth’s surface, such as albedo, roughness, or vegetation cover, had the potential to modify the regional or even global climate. The primary activities which could have produced these effects were deforestation, burning, and agriculture. These activities were not independent, and often occurred together. Deforestation could have produced warming or cooling at the surface, and different effects on different scales, depending on the fate of the biomass removed and the new use of the land. Burning is much less now than it was in the past in some regions, which would have produced warming as the burning decreased. This may be a partial explanation for the Little Ice Age. While a thorough survey of such pre-industrial human activities is called for, current information indicates that regional climatic effects were large in some regions, such as western North America, and hemispheric or global effects were possible. Once these pre-industrial human climatic forcing factors are better quantified, existing numerical models of the climate can be used to examine the impacts on regional and global scales. ”

The text includes the statements,

“Anthropogenic activities can affect climate in a number of well-known ways (Houghton et al., 1990,1992). These changes can be classified as changes of atmospheric constituents, changes of surface properties, and changes of gradients of properties, which would not directly affect the energy balance, but would change the circulation.”

This statement supports the conclusions in the 2005 National Research Council Report on the need to expand climate assessments to include land use/land cover changes and regional climate change in response to regional climate forcings. The first order climate forcing of

“changes of surface properties, and changes in the gradients of properties, which would not directly affect the energy balance, but would change the circulation”

have also been discussed, repectively, in the 2005 Science article “Land use and climate change” and in the 2006 Geophysical Research Letters paper “Measurement-based estimation of the spatial gradient of aerosol radiative forcing“.

The Robock and Graf paper further state,

“It is known that about 60% of the land surfaces are changed by man; today only about 30% of the land area is wood or forest (Bernhardt and Kortuem, 1976). This implies a tremendous impact on the natural energy exhange conditions….In Europe, for example, huge areas were deforested when the Mediterranean Cultures developed. Since the soil was the completely eroded, any reforestation failed, and the Mediterranean landscape was changed permanently. This must have also had a strong influence on the regional climates.”

While the Robock and Graf studied focused on pre-industrial time, its conclusions are very much applicable to the need to include regional climate forcings of land surface processes and of aerosols, and their effect on atmospheric circulations, as major study topics in assessments such as the new IPCC Report.

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