Local sources of global climate forcing from different categories of land use activities

Abstract

Abstract. Identifying and quantifying the sources of climate impacts from land use and land cover change (LULCC) is necessary to optimize policies regarding LULCC for climate change mitigation. These climate impacts are typically defined relative to emissions of CO 2 , or sometimes emissions of other long-lived greenhouse gases. Here we use previously published estimates of the radiative forcing (RF) of LULCC that include the short-lived forcing agents O 3 and aerosols, in addition to long-lived greenhouse gases and land albedo change, for six projections of LULCC as a metric for quantifying climate impacts. The LULCC RF is attributed to three categories of LULCC activities: direct modifications to land cover, agriculture, and wildfire response, and sources of the forcing are ascribed to individual grid points for each sector. Results for the year 2010 show substantial positive forcings from the direct modifications and agriculture sectors, particularly from south and southeast Asia, and a smaller magnitude negative forcing response from wildfires. The spatial distribution of future sources of LULCC RF is highly scenario-dependent, but we show that future forest area change can be used as a predictor of the future RF from direct modification activities, especially in the tropics, suggesting that deforestation-prevention policies that value land based on its C-content may be particularly effective at mitigating climate forcing originating in the tropics from this sector. However, the response of wildfire RF to tropical land cover changes is not as easily scalable and yet imposes a non-trivial feedback onto the total LULCC RF.

Trends and Variability of Global Fire Emissions Due To Historical Anthropogenic Activities

“…Fires reduce the amount of carbon available to be released to the atmosphere or into litter and product pools following deforestation or vegetation harvesting. This effect was also shown by Ward and Mahowald (), which attributed about half of this carbon flux reduction to lower LULCC carbon emissions and half to LULCC‐driven reductions in fire activity.…”

“…This could improve the comparison of our results with the historical emissions inventory produced by van Marle et al (). Most global fire models are structured such that LULCC can decrease fires on natural lands indirectly by reducing the average aboveground biomass of the grid cell (Ward & Mahowald, ). Here because of the tiling framework of LM3, natural lands' aboveground biomass is not reduced by adjacent land cover changes within a grid box, thereby removing this potential artifact of averaging biomass for fire modeling on the coarse model resolution.…”

Denitrification, leaching, and river nitrogen export in the Community Earth System Model

“…An additional consideration with respect to the partitioning between leaching/runoff and denitrification is that a large fraction of the net total N input to CLM‐CN is lost through pyrodenitrification, i.e., biomass burning of organic N resulting in pyrolytic conversion to N 2 . This is especially true in the No Crop model, in which cropped regions, where CLM does not permit agricultural fires, are replaced by frequent‐burning natural vegetation [ Ward and Mahowald , ]. Pyrodenitrification is a term that is not explicitly considered in many global and regional N budgets in the literature, but removes about 20–35% of annual N inputs in various configurations of CLM (Table ).…”

Impact on short-lived climate forcers (SLCFs) from a realistic land-use change scenario via changes in biogenic emissions

“…Land-use change can dramatically alter fire activity, with associated changes in emissions of trace gases and aerosol 97 , which we do not account for here. We also do not yet consider changes to agricultural emissions that accompany LUC which may be important for nitrate aerosol formation and subsequent radiative impact 26 .…”