I have been asked two very interesting and important questions by Bill DiPuccio which I paraphrase and summarize below:
1. What is the area of land that is directly modified by human landscape management? While it is clear that local and regional effects on climate will result, are hemispheric and global atmospheric circulations signficantly affected?
2. Is anthropogenic forcing “driving” (controlling) climate change or is it “modulating” (modifying) natural variability?
In answer to the first question, in my paper
Pielke Sr., R.A., 2002: Overlooked issues in the U.S. National Climate and IPCC assessments. Climatic Change, 52, 1-11
“Actual landscape change [of the Earth’s land surface] has been estimated to be as high as 45% by Vitousek et al. (1997).”
Figure 3 in this paper shows how land use change has accelerated during the last 100 years. Another paper which investigates this issue is
Nemani, R.R., S.W. Running, R.A. Pielke, and T.N. Chase, 1996: Global vegetation cover changes from coarse resolution satellite data. J. Geophys. Res., 101, 7157-7162.
There have been model studies which show that these changes significantly alter hemispheric and global circulation patterns, e.g. see
Feddema et al. 2005: The importance of land-cover change in simulating future climates., 310, 1674-1678
Chase, T.N., R.A. Pielke, T.G.F. Kittel, R.R. Nemani, and S.W. Running, 2000: Simulated impacts of historical land cover changes on global climate in northern winter. Climate Dynamics, 16, 93-105.
Pielke Sr., R.A., G. Marland, R.A. Betts, T.N. Chase, J.L. Eastman, J.O. Niles, D. Niyogi, and S. Running, 2002: The influence of land-use change and landscape dynamics on the climate system- relevance to climate change policy beyond the radiative effect of greenhouse gases. Phil. Trans. A. Special Theme Issue, 360, 1705-1719
but this conclusion still remains debated; see
Pitman, A.J., N. de Noblet-Ducoudré, F.T. Cruz, E.L. Davin, G.B. Bonan, V. Brovkin, M. Claussen, C. Delire, L. Ganzeveld, V. Gayler, B.J.J.M. van den Hurk, P.J. Lawrence, M.K. van der Molen, C. Müller, C.H. Reick, S.I. Seneviratne, B. J. Strengers, and A. Voldoire, 2009: Uncertainties in climate responses to past land cover change: first results from the LUCID intercomparison study, Geophys. Res. Lett., doi:10.1029/2009GL039076, in press.
With respect to the second question, this is very effectively summarized by Bill. He writes that the answer to the second question
” ….has to do with the degree of dominance. Is anthropogenic forcing “driving” (controlling) climate change or is it “modulating” (modifying) natural variability? This is a fuzzy question and probably creates a false dilemma because it depends on the temporal and spatial scale. But a nuanced answer is important not only to skeptics who emphasize natural variability, but also to policy makers who think that since humans control climate change, we can use geoengineering to “save the planet.”
My view is that humans modulate natural variability. This perspective is captured in the hypothesis in our EOS article
Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell, W. Rossow, J. Schaake, J. Smith, S. Sorooshian, and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union
where we wrote
“Although the natural causes of climate variations and changes are undoubtedly important, the human influences
are significant and involve a diverse range of first- order climate forcings, including, but not limited to, the human input of carbon dioxide (CO2)”
“In addition to greenhouse gas emissions, other first- order human climate forcings are important to understanding the future behavior of Earth’s climate. These forcings are spatially heterogeneous and include the effect of aerosols on clouds and associated precipitation [e.g., Rosenfeld et al., 2008], the influence of aerosol deposition (e.g., black carbon (soot) [Flanner et al. 2007] and reactive nitrogen [Galloway et al., 2004]), and the role of changes in land use/land cover [e.g., Takata et al., 2009]. Among their effects is their role in altering atmospheric and ocean circulation features away from what they would be in the natural climate system [NRC, 2005].”
Since the climate system is affected by a diverse range of human climate forcings as well as natural variability and change, we concluded in our EOS article
“We therefore propose that one should not rely solely on prediction as the primary policy approach to assess the potential impact of future regional and global climate variability and change. Instead, we suggest that integrated assessments within the framework of vulnerability, with an emphasis on risk assessment and disaster prevention, offer a complementary approach [Kabat et al., 2004]. This should be conducted in parallel with attempts to improve skill in predicting regional and global climate on multidecadal time scales. This leads to a practical and sensible way forward that will permit a more effective climate policy by focusing on the assessment of adaptation and mitigation strategies that can reduce the vulnerability of all of our important societal and environmental resources (involving water, food, energy, and human and ecosystem health) to both natural and human- caused climate variability and change”
“We recommend that the next assessment phase of the IPCC (and other such assessments) broaden its perspective to include all of the human climate forcings. It should also adopt a complementary and precautionary resource- based assessment of the vulnerability of critical resources (those affecting water, food, energy, and human and ecosystem health) to environmental variability and change of all types. This should include, but not be limited to, the effects due to all of the natural and human caused climate variations and changes.”
Thank you Bill for these two excellent questions!