UC Berkeley

Proposed strategies for managing terrestrial carbon in order to mitigate anthropogenic climate change largely ignore the direct effects of land use change on climate via biophysical processes that alter surface energy and water budgets. Subsequent influences on temperature, hydrology, and atmospheric circulation at regional and global scales could potentially help or hinder climate stabilization efforts. However, due to geographic variability, incomplete understanding of the relevant physical processes, and differences in the spatial scale of biophysical influences compared to greenhouse gases, the best strategies for addressing biophysical aspects of land use change are not clear. To provide insight into this problem, I explore the theoretical implications of various metrics for characterizing the full climate effects of land use change. Furthermore, using a state-of-the-art earth system model coupled to an integrated assessment model that generates scenarios of future anthropogenic climate forcing, I address policy-relevant questions regarding the physical climate system response to land use change.

I demonstrate that the biophysical effects of land use change can be large and vary significantly among policies. Thus ignoring these effects in greenhouse gas policies can lead to unintended consequences. Different hypothetical strategies for meeting an identical global greenhouse gas concentration target - either one with modest afforestation or one with large-scale deforestation for biofuel production - yield different global and regional patterns of climate change, particularly when Boreal forests are converted to agriculture. Additional simulations illuminate the forcing and feedbacks processes that drive regional differences between these scenarios.

Many policies and programs rely on a measure of net CO2 emissions to rank the climate damages of different activities or to generate credits within a trading scheme designed to minimize negative climate outcomes. While it is relatively straightforward to include well-mixed non-CO2 greenhouse gases in such schema using metrics based on radiative forcing, the climate system response to biophysical forcing differs from that of greenhouse gases. I examine the size and nature of this theoretical difference by modeling the equilibrium climate response to equivalent levels of radiative forcing from both land use change and elevated atmospheric CO2 concentrations, drawing conclusions about the climate consequences of including biophysical forcing in carbon markets using this metric.