A Reduction in the Cooling Impact of the Land-Use Change in the Global Warming Scenarios from the CMIP6-LUMIP Climate Experiments

Abstract:

Land-use change (LU) is a major regional climate forcing that affects carbon-water-energy fluxes and, therefore, near-surface air temperature. Although there are uncertainties in LU impacts in the historical climate, there is a growing consensus towards a cooling influence in the mid-latitudes. However, how a drier and warmer land surface condition in the future climate can change the LU impacts are not investigated well.

We use a comprehensive set of eight coupled climate models from the CMIP6-LUMIP project to assess the changing influence of the LU change. We use two methodologies: (1) direct method – where LU impacts are estimated by subtracting the ‘no-LU’ climate experiment from the control experiment that includes LU, and (2) Kumar et al., 2013 (K13) method where LU impacts are estimated by comparing climate change impacts between LU and no-LU neighboring regions.

First, we compared the LU impacts in the historical climate and between the direct method and K13 methods using the multi-model analysis. In the North America LU change region, the direct method shows a cooling impact of (-0.29°C ±0.16°C). The K13 methods show a smaller cooling impact (-0.12+±0.16°C). In terms of energy balance, the direct method shows a reduction of net shortwave radiation (-0.82±0.91 watts/m²) the K13 method shows a cleaner result of (-1.76±1.17 watts/m²), as expected. We suspect that a more substantial influence of the LU change in the direct method is due to large-scale circulation driven response or due to the internal variability that has been canceled out in the K13 method.

Next, we extend the K13 method to assess the LU impacts in the future climate. Direct methods are not available for the future climate experiment in CMIP6-LUMIP datasets. We find that a cooling impact of LU change disappeared in the future climate (0.01°C ±0.14°C). A similar influence is also found in the reduction of the net shortwave radiation (-0.34+2.72 watts/m²). The issue of the smaller LU impacts in the future climate is being investigated.

Reference

Kumar, S., Dirmeyer, P. A., Merwade, V., DelSole, T., Adams, J. M., & Niyogi, D. (2013). Land use/cover change impacts in CMIP5 climate simulations: A new methodology and 21st century challenges. Journal of Geophysical Research: Atmospheres, 118(12), 6337-6353.