Quantifying the impact of alternate climate response uncertainty distributions under the global mitigation scenarios in the UK’s Committee on Climate Change Sixth Carbon budget.

Abstract We examine two scenarios of global greenhouse gas emissions developed to inform the Committee on Climate Change’s advice on the level of the UK’s Sixth Carbon Budget - focusing on the uncertainty in the global temperature outcomes associated with new estimates for the Earth’s equilibrium climate sensitivity. The first scenario is a rapid global decarbonisation scenario and the second follows an interpretation of pre-2021 current national commitments to reduce emissions prior to 2030. Updated estimates of equilibrium climate sensitivity have little impact on the median global temperature response, resulting in peak temperature values between 1.7°C to 1.9°C for the scenario aligned with the Paris Agreement, compared to estimated peak warming based on the assessment of IPCC Special Report on Global Warming of 1.5°C. Under all climate sensitivity distributions considered the scenario has a greater than 66% likelihood of limiting peak global mean temperatures to under 2°C throughout the century. The choice of equilibrium sensitivity distribution has greater impact at higher percentiles with differences of up to 0.5°C and 0.8°C in the 95th percentile outcome for the Paris Agreement aligned and current policy scenarios respectively. These differences can bring forward the timing of reaching dangerous levels of global warming by up to a decade.