New Paper “Biogeophysical Versus Biogeochemical Climate Response To Historical Anthropogenic Land Cover Change” By Pongratz Et Al 2010

There is a new paper which adds to the literature of the role of land surface processes within the climate system. It is

Pongratz, J., C. H. Reick, T. Raddatz, and M. Claussen (2010), Biogeophysical versus biogeochemical climate response to historical anthropogenic land cover change, Geophys. Res. Lett., 37, L08702, doi:10.1029/2010GL043010.

The abstract reads

“Anthropogenic land cover change (ALCC) is one of the few climate forcings with still unknown sign of their climate response. Major uncertainty results from the often counteracting temperature responses to biogeochemical as compared to biogeophysical effects. Here, we separate the strength of these two effects for ALCC during the last millennium. We add unprecedented detail by (i) using a coupled atmosphere/ocean general circulation model (GCM), and (ii) applying a high‐detail reconstruction of historical ALCC. We find that biogeophysical effects have a slight cooling influence on global mean temperature (−0.03 K in the 20th century), while biogeochemical effects lead to strong warming (0.16–0.18 K). During the industrial era, both effects cause significant changes in certain regions; only few regions, however, experience biogeophysical cooling strong enough to dominate the overall temperature response. This study therefore suggests that the climate response to historical ALCC, both globally and in most regions, is dominated by the rise in CO2 caused by ALCC emissions.”

This is an interesting new paper as it adds to our understanding of the role of landscape processes on the climate system. However, it also includes an unnecessarily narrow perspective on biogeochemical processes as well as the continued focus on global average radiative forcing as the primary climate metric.

The article writes, for example,

“Probably the most important biogeochemical mechanism of ALCC for global climate is the influence on the carbon cycle, and the associated impact on the global atmospheric CO2 concentration.”

This is certainly one of the important effects of anthropogenic land cover change, but other papers show that, at least on the regional spatial scales, th effect of  anthropogenic land cover change is to also result in large changes in the surface heat and moisture fluxes, which feeds upscale to alter regional climate patterns such as the monsoon, as shown in, for example, in 

Kumiko Takata, Kazuyuki Saitoa and Tetsuzo Yasunari, 2009: Changes in the Asian monsoon climate during 1700–1850 induced by preindustrial cultivation PNAS published online June 1, 2009, doi:10.1073/pnas.0807346106

which is not cited in the Pongratz et al 2010 paper.

Other neglected studies which show a significant regional  and global climate effects from the effects of landscape change on the climate patterns include

Chase, T.N., R.A. Pielke, Sr., T.G.F. Kittel, M. Zhao, A.J. Pitman, S.W. Running, and R.R. Nemani, 2001: The relative climatic effects of landcover change and elevated carbon dioxide combined with aerosols: A comparison of model results and observations. J. Geophys. Res., Atmospheres, 106, 31,685 -31,691.

Eastman, J.L., M.B. Coughenour, and R.A. Pielke, 2001: The effects of CO2 and landscape change using a coupled plant and meteorological model. Global Change Biology, 7, 797-815.

Nair, U.S., D.K. Ray, J. Wang, S.A Christopher, T. Lyons, R.M. Welch, and R.A. Pielke Sr., 2007: Observational estimates of radiative forcing due to land use change in southwest Australia. J. Geophys. Res., 112, D09117, doi:10.1029/2006JD007505.

Narisma, G.T., A.J. Pitman, J. Eastman, I.G. Watterson, R. Pielke Sr., and A. Beltran-Przekurat, 2003: The role of biospheric feedbacks in the simulation of the impact of historical land cover change on the Australian January climate. Geophys. Res. Letts., 30(22), 2168, doi:10.1029/2003GL018261.

Thus, while the Pongratz et al 2010 paper adds important new insight into the role of landscape processes within the climate system, it needs to broaden out to consider the relative role of biophysical  and biogeochemical effects of  anthropogenic land cover change in altering regional atmospheric and oceanic circulation patterns.

Such statements in the Pongratz et al 2010 paper that there is a

“…weak global biogeophysical response…”

ignores the that heterogenous diabatic heating can alter atmospheric and oceanic circulations far from the location of landscape change; e.g. see

What is the Importance to Climate of Heterogeneous Spatial Trends in Tropospheric Temperatures?

The metric to assess this effect on circulation patterns could build on that proposed in our paper

Matsui, T., and R.A. Pielke Sr., 2006: Measurement-based estimation of the spatial gradient of aerosol radiative forcing. Geophys. Res. Letts., 33, L11813, doi:10.1029/2006GL025974

where we applied a new metric with respect to the diabatic heating changes from the human input of aerosols. This same metric can be applied for the diabatic heating changes from biophysical and biogeochemical effects using the model results of Pongratz et al (2010).

The 2007 IPCC failed to adequately consider anthropogenic land cover change in their assessment of how humans can alter the climate system on the regional and global scales. This serious oversight needs to be remedied in  the next assessment.

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