Abstract
Abstract. Pressure on land resources is expected to increase as global population continues to climb and the world becomes more affluent, swelling the demand for food. Changing climate may exert additional pressures on natural lands as present-day productive regions may shift, or soil quality may degrade, and the recent rise in demand for biofuels increases competition with edible crops for arable land. Given these projected trends there is a need to understand the global climate impacts of land use and land cover change (LULCC). Here we quantify the climate impacts of global LULCC in terms of modifications to the balance between incoming and outgoing radiation at the top of the atmosphere (radiative forcing, RF) that are caused by changes in long-lived and short-lived greenhouse gas concentrations, aerosol effects, and land surface albedo. We attribute historical changes in terrestrial carbon storage, global fire emissions, secondary organic aerosol emissions, and surface albedo to LULCC using simulations with the Community Land Model version 3.5. These LULCC emissions are combined with estimates of agricultural emissions of important trace gases and mineral dust in two sets of Community Atmosphere Model simulations to calculate the RF of changes in atmospheric chemistry and aerosol concentrations attributed to LULCC. With all forcing agents considered together, we show that 40% (±16%) of the present-day anthropogenic RF can be attributed to LULCC. Changes in the emission of non-CO2 greenhouse gases and aerosols from LULCC enhance the total LULCC RF by a factor of 2 to 3 with respect to the LULCC RF from CO2 alone. This enhancement factor also applies to projected LULCC RF, which we compute for four future scenarios associated with the Representative Concentration Pathways. We attribute total RFs between 0.9 and 1.9 W m−2 to LULCC for the year 2100 (relative to a pre-industrial state). To place an upper bound on the potential of LULCC to alter the global radiation budget, we include a fifth scenario in which all arable land is cultivated by 2100. This theoretical extreme case leads to a LULCC RF of 3.9 W m−2 (±0.9 W m−2), suggesting that not only energy policy but also land policy is necessary to minimize future increases in RF and associated climate changes.
Highlights
More than half of the Earth’s land surface has been affected by land use and land cover change (LULCC) activities over the last 300 years, largely from the expansion of agriculture (Hurtt et al, 2011), leading to numerous climate impacts (Foley et al, 2005)
Our approach results in a year 2010 CO2 concentration of 399 ppm (285 ppm preindustrial, 86 ppm fossil fuels, 28 ppm LULCC), which overshoots the observed change in CO2 over the same period by about 10 % but is within the range of values from the Coupled Model Intercomparison Project phase 5 (CMIP5) fully coupled climate model experiment: 368 to 403 ppm in 2005 (Friedlingstein et al, 2013)
Anthropogenic land use and changes to land cover have long been recognized as important contributors to global climate forcing (Feddema et al, 2005), and yet most studies on this topic focus on either land use (e.g., Unger et al, 2010) or land cover change (e.g., Davin et al, 2007; Pongratz et al, 2009), but not both
Summary
More than half of the Earth’s land surface has been affected by land use and land cover change (LULCC) activities over the last 300 years, largely from the expansion of agriculture (Hurtt et al, 2011), leading to numerous climate impacts (Foley et al, 2005). Unger et al (2010) partition sources of global, anthropogenic RF into economic sectors, including agriculture They consider non-CO2 greenhouse gas and aerosol forcing agents but only for present-day land use emissions and they do not include land cover change. More extreme scenario in which all arable and pasturable land is converted to agricultural land, either for crops or pasture, by the year 2100 This scenario, hereafter referred to as the theoretical extreme case (TEC), was not developed within an integrated modeling framework, and its likelihood of occurrence given economical and additional environmental constraints is difficult to judge. The range in outcomes for the RF attributable to LULCC based on these five projections strengthens our understanding of the role that LULCC decision making will play in future climate
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