There is a new paper that examines, using a modeling sensitivity study, the relative roles of added CO2 and landscape change. The paper is
Li, Z., and N. Mölders, 2008: Interaction of impacts of doubling CO2 and changing regional land-cover on evaporation, precipitation, and runoff at global and regional scales.Int. J. Climatol. 28: 1653-1679
The abstract reads
“The Community Climate System Model version 2.0.1 is running for 40 years under 355 ppm CO2conditions, without and with natural and anthropogenic land-cover changes that are assumed in the inner core of four hydrothermally different, but similar-sized (3.27 x 106 km2) regions (Yukon, Ob, St. Lawrence, Colorado, and lands adjacent to them). A further set of simulations assumes 710 ppm CO2 conditions without and with these land-cover changes. Impacts of (1) doubled CO2 , (2) changed land-cover, and (3) the interaction between doubled CO2 and changed land-cover on the four regional water cycles are elucidated using analysis of variance plus multiple testing. For the Yukon, Ob, and St. Lawrence regions, doubling CO2significantly increases precipitation, evapotranspiration, and residence time nearly year-round; the opposite is true for precipitation and evapotranspiration in Colorado. In general, doubling CO2slows down water cycles regardless of land-cover changes. Since land-cover changes occur locally, they more strongly affect regional than global water cycling. Sometimes land-cover changes alone reduce regional-scale precipitation and evapotranspiration. Water-cycle changes of comparable absolute magnitude can occur in response to either changed land-cover or doubled CO2. Significant interactions between the two treatments indicate that local land-cover changes, even if they have little impact under reference climate conditions, may have substantial regional impact in a warmer climate. Increased residence time after doubling CO2 indicates a generally increased influence of upwind regions on downwind regions. If land-cover changes occur concurrently with CO2 changes, they will have farther-reaching impact than under reference CO2 conditions. Thus, due to atmospheric transport the interaction between impact of land-cover changes and CO2 doubling on water-cycle-relevant quantities may occur even in regions with unchanged land-cover. A sensitivity study for tripled CO2 showed similar results, but more pronounced slowed-down regional water cycles and interaction of the two treatments.”
The paper includes the text in the introduction that
“……changes in globally averaged evapotranspiration, precipitation, and residence time are not equally distributed, but rather vary on regional scales (e.g. Douville et al., 2002; Bosilovich et al., 2005). Therefore, there is an urgent need to explore regional water-cycle complexity and response to global changes, and also to compare regional responses to each other.
“Recent investigations using regional weather forecast or climate models showed that mid-latitude and high latitude land-cover changes may appreciably alter near surface energy budgets and cloud and precipitation distribution in the region of, and adjacent to land-cover changes (e.g. Chase et al., 1999; Bounoua et al., 2000; Mölders and Olson, 2004). General Circulation Model (GCM) studies show that regional land-cover changes, such as the Amazon deforestation, can alter global and/or regional water cycles at mid-/high latitudes by modifying large-scale atmospheric circulation (e.g. Sud et al., 1996; Zhang et al., 1996; Chen et al., 2001; Avissar and Werth, 2005).”
with an extract of the conclusions stating that
“Evapotranspiration is more sensitive to land-cover changes and interactions between CO2 concentration and land-cover changes than precipitation, because land-cover changes directly influence the evapotranspiration by altering surface wind speed, canopy interception-storage capacity, or transpiration. Nevertheless, despite the complexities of these interactions and the difficulties posed by the great variability that exists between regions in response to changing climate and landscape, the likely future increases in atmospheric CO2 concentration and the vital importance of the global water cycle and regional water cycles to life on earth and within a region makes it clear that elucidating the characteristics of regional water cycles and accurately projecting their response to possible future climate and land-cover changes is a task of vital importance for the atmospheric science community.”
This is yet another peer reviewed paper that clearly documents that the role of humans within the climate system is more than just the radiative effect of added CO2.