Professor Myles Allen CBE FRS

Abstract:

To limit the detrimental impacts of climate change, large-scale and rapid decarbonization is required. China announced their plan to peak carbon emissions before 2030 and to reach carbon neutrality by 2060, which faces many challenges including rising energy consumption and a significant, ongoing expansion of coal-based electricity generation capacity. This study employs mixed methods to explore a portfolio of climate policies related to the transport and energy sectors for two leading Chinese cities: Beijing and Hong Kong. A total of 32 expert interviews were conducted with four stakeholder groups in both cities to canvas opinions on the most important policies for decarbonization. With the aim to understand how local policy measures can be prioritized for disproportionately large emissions reductions, the Sensitive Intervention Points (SIPs) framework was applied to identify city-level policy interventions with the potential for high impact, speed, feasibility, persistence, and low risk, based on these expert interviews and literature review. With all attributes combined, leveraging the global cost declines in renewable energy was identified as a shared accelerated carbon neutrality pathway for both cities, facilitated by policies to promote the import of low-carbon energy and accelerating the electrification of transport. Alignments were found between this final list of SIPs and policies perceived as important by the experts, indicating that SIPs are generally intuitive, with alternative policy prioritizations likely influenced by additional factors such as the national agenda, budgetary constraints, and the availability of co-benefits.

Abstract:

This study aims to improve our understanding of the response of precipitation to forcings by proposing a physically-based equation that resolves simulated precipitation based on the atmospheric energy budget. The equation considers the balance between latent heat release by precipitation and the sum of the slow response by tropospheric temperature changes and the fast response by abrupt radiative forcing (RF) changes. The equation is tuned with three parameters for each climate model and then adequately reproduces time-varying precipitation. By decomposing the equation, we highlight the slow response as the largest contributor to forcing-driven responses and uncertainty sizes in simulations. The second largest one to uncertainty is the fast-RF response from aerosols or greenhouse gases (GHG), depending on the low or highest Coupled Model Intercomparison Projection 6 future scenarios. The likely range of precipitation change at specific warming levels under GHG removal (GGR) and solar radiation management (SRM) mitigation plans is evaluated by a simple model optimizing the relationship between temperature and decomposed contributions from multi-simulations under three scenarios. The results indicate that GGR has more severe effects from aerosols than GHG for a 1.5 K warming, resulting in 0.91%–1.62% increases in precipitation. In contrast, SRM pathways project much drier conditions than GGR results due to the tropospheric cooling and remaining anthropogenic radiative heating. Overall, the proposed physically-based equation, the decomposition analysis, and our simple model provide valuable insights into the uncertainties under different forcings and mitigation pathways, highlighting the importance of slow and fast responses to human-induced forcings in shaping future precipitation changes.