The Effect of Modeling Strategies on Assessments of Differential Warming Impacts of 0.5°C

Abstract

The differential climatic impact between the 1.5° versus 2°C warmer worlds is a global concern on the post-Paris science agenda. The recently released Intergovernmental Panel on Climate Change (IPCC) special report “Global Warming of 1.5ºC” (hereafter SR15) concludes robustly avoided impacts from climate extremes from the 0.5°C less warming globally, based on multiple lines of evidence. However, the methodological uncertainties, including those arising from modeling strategy (i.e., different pathways to achieve the temperature targets), remain a knowledge gap. Here, by comparing simulations with the common Community Atmosphere Model (CAM5) using different modeling strategies (i.e., fully coupled transient, fully coupled quasi-equilibrium, and atmosphere-only quasi-equilibrium simulations), we explored the uncertainty from modeling strategy in the reduced impacts of climate extremes from the half-degree less warming. While the globally aggregated reduced impacts are reasonably consistent among modeling strategies, substantial quantitative uncertainties of reduced impact exist at regional scales. For temperature extremes, the largest methodological uncertainty exists in northern high-latitudes, especially in Europe and North America, with spread exceeding 0.8°C. For precipitation extremes, large uncertainties (even opposite signs) are expected scatteredly over the globe, particularly in the populous South Asia for wet extremes, and in South-East Asia, West North America, and the Amazon for dry extremes, with spread of ∼5%. The different regional responses among modeling strategies result from multiple physical processes, dominated by the transient versus quasi-equilibrium responses, prescribed sea surface temperatures and aerosol forcing. The comparisons improve the understanding of methodological uncertainties in the projected impacts in light of model setups.