Observational Estimates of Radiative Forcing due to Land Use Change in Southwest Australia

We have a new paper accepted for publication in the Journal of Geophysical Research. It is by U.S.Nair, U.S., D.K. Ray, J. Wang, S.A Christopher, T. Lyons, R.M. Welch, and R.A. Pielke Sr. and is entitled “Observational estimates of radiative forcing due to land use change in southwest Australia”.

In addition to the very important regional scale issues of climate change that this paper illustrates, the study also shows the need to investigate regional scale albedo changes due to land cover change, as necessary to scale up to the global scale in order to more accurately estimate the global average radiative forcing of this particular climate forcing.

The abstract of the paper reads,

“Radiative forcing associated with land use change is largely derived from Global Circulation Models (GCM), and the accuracy of these estimates depends on the robustness of the vegetation characterization used in the GCMs. In this study, we use observations from the Clouds and Earth’s Radiant Energy System (CERES) instrument onboard the Terra satellite to report top-of –the-atmosphere radiative forcing values associated with clearing of native vegetation for agricultural purposes in southwest Australia. Over agricultural areas, observations show consistently higher shortwave fluxes at the top-of-the atmosphere (TOA) compared to native vegetation, especially during the time period between harvest and planting. Estimates using CERES observations show that, over a specific area originally covered by native vegetation, replacement of half the area by croplands results in a diurnally averaged shortwave radiative forcing of approximately -7 Wm-2. GCM-derived estimates for areas with 30% or more croplands range from -1 to -2 Wm-2 compared to observational estimate of -4.2 Wm-2, thus significantly underestimating radiative forcing due to land use change by a factor of 2 or more. Two potential reasons for this underestimation are incorrect specification of the multi-year land use change scenario and the inaccurate prescription of
seasonal cycles of crops in GCM’s.”

One excerpt from the paper reads,

“Betts (2001) and Hansen et al. (1997) both report that the radiative forcing due to land use change is most significant in the mid-latitude agricultural areas that experience snowfall, which accentuates the albedo effect associated conversion of forests to farm lands. The present analysis suggests that shortwave radiative forcing associated with agricultural land use may be significant even in areas without frequent snow cover.”

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