H. D. Matthews Æ A. J. Weaver Æ K. J. Meissner N. P. Gillett Æ M. Eby Natural and anthropogenic climate change: incorporating historical land cover change, vegetation dynamics and the global carbon cycle Received: 25 June 2003 / Accepted: 19 December 2003 / Published online: 18 March 2004 Ó Springer-Verlag 2004 Abstract This study explores natural and anthropogenic influences on the climate system, with an emphasis on the biogeophysical and biogeochemical effects of his- torical land cover change. The biogeophysical effect of land cover change is first subjected to a detailed sensi- tivity analysis in the context of the UVic Earth System Climate Model, a global climate model of intermediate complexity. Results show a global cooling in the range of –0.06 to –0.22 °C, though this effect is not found to be detectable in observed temperature trends. We then in- clude the effects of natural forcings (volcanic aerosols, solar insolation variability and orbital changes) and other anthropogenic forcings (greenhouse gases and sulfate aerosols). Transient model runs from the year 1700 to 2000 are presented for each forcing individually as well as for combinations of forcings. We find that the UVic Model reproduces well the global temperature data when all forcings are included. These transient experiments are repeated using a dynamic vegetation model coupled interactively to the UVic Model. We find that dynamic vegetation acts as a positive feedback in the climate system for both the all-forcings and land cover change only model runs. Finally, the biogeo- chemical effect of land cover change is explored using a dynamically coupled inorganic ocean and terrestrial carbon cycle model. The carbon emissions from land cover change are found to enhance global temperatures by an amount that exceeds the biogeophysical cooling. The net effect of historical land cover change over this period is to increase global temperature by 0.15 °C. 1 Introduction Changes in global climate in the latter half of the twentieth century represent a distinct anomaly in the historical climate record. The global temperature in- crease of 0.6 ± 0.2 °C since 1900 has occurred at a rate unprecedented in the last 1000 years (Houghton et al. 2001). The current global temperature is unmatched in the Holocene (the last 10,000 years), and indeed has not occurred since the peak of the last interglacial, some 126,000 years ago (Petit et al. 1999). In the last several decades, the field of climate science has been motivated largely by the challenge to under- stand the changes that we are currently observing in the climate system. In the last decade, the global scientific community, as represented by the Intergovernmental Panel on Climate Change (IPCC), has sent a clear message that human activities are in large part respon- sible for the climate changes we are currently experi- encing. Key among the identified causes of this global warming are elevated levels of greenhouse gases: levels that are unprecedented in the last 420,000 years (Houghton et al. 2001, Petit et al. 1999). Numerous studies have identified processes that can act to force changes in global climate. Anthropogenic influences, of which greenhouse gases are known to be the most significant, also include emissions of sulfate aerosols and human land cover change change, as well as numerous smaller forcings such as stratospheric ozone depletion, black and organic carbon aerosols and jet contrails. Natural climate forcing processes include solar variability due to sunspot and other solar cycles, long-term changes in solar orbital parameters, and intermittent volcanic eruptions (Hansen et al. 1998, Houghton et al. 2001). Many of these processes and their effects on global climate are very well understood, and are not discussed in detail here (see e.g. Ramaswamy et al. 2001). One of the less well understood anthropogenic influences on climate is that H. D. Matthews (&) Æ A. J. Weaver Æ K. J. Meissner N. P. Gillett Æ M. Eby School of Earth and Ocean Sciences, University of Victoria, PO Box 3055, Victoria, BC, V8W 3P6, Canada E-mail: damon@ocean.seos.uvic.ca Climate Dynamics (2004) 22: 461–479 DOI 10.1007/s00382-004-0392-2