Transparency crucial to Paris climate scenarios-Response

Response

King et al. raise important issues, several of which pertain to the broader policy discourse surrounding international climate negotiations and countries’ climate pledges rather than the modeling conducted in our Policy Forum. We agree with King et al. that the updated Paris Agreement pledges could paint an overly optimistic picture of the future, especially if their success depends on postponing deeper reductions until after 2030. To illustrate a less optimistic future, our Policy Forum includes scenarios showing what would happen if countries continued to implement current policies alone. The “Current policy” scenarios in the Policy Forum result in less than a 10% chance to limit global warming to below 2°C this century, whereas the “Updated pledges” scenarios result in at least a 33% chance of achieving the same temperature goal. Additional policy measures could help bridge emission gaps between current policies, updated pledges, and the global emission levels needed to cost-effectively achieve the Paris Agreement’s climate goals (1).

We also agree with King et al. that greater near-term ambition is necessary, particularly to set up the technological, political, and financial infrastructures required to achieve the long-term strategies and net-zero pledges (2). Delaying emission reductions would substantially reduce the likelihood of achieving the Paris temperature goal (3) and could prove to be very expensive (4). However, we disagree that the updated 2021 pledges result in negligible global emission reductions. Our Policy Forum suggests that the updated pledges alone—if successfully implemented—could result in a reduction of 2030 global greenhouse gas emissions of about 6.6 GtCO₂ (12%) compared with the 2015 pledges. The United Nations Environment Programme suggests reductions of 4 GtCO₂ (7.5%) (5).

The “Updated pledges-Continued ambition” scenario in our Policy Forum assumes that countries with net-zero pledges achieve those pledges in the specified target years. Achieving net-zero pledges will require large-scale deployment of negative emissions technologies (6, 7). Our study assumes the availability of bioenergy with carbon capture and storage (BECCS) and direct air capture (DAC) technologies in addition to terrestrial sinks and makes conservative assumptions about their deployment compared to the literature (7). Our assumptions about BECCS and DAC are transparently documented in several publications [e.g., (8, 9)]. Moreover, the entire model code and data files used in our study are publicly available (10). Future research should explore the implications of alternative assumptions about the type and scale of negative emissions technologies.

Our Policy Forum addressed uncertainties in the climate system, but we only focused on one source of uncertainty in emissions trajectories: stringency of climate policy. The minimum decarbonization rate assumption (2% per year) in our “Continued ambition” scenarios is on the conservative side of historical trends [Fig S2 in (11)] and conservative compared to developed countries with decarbonization policies (tables S14-S17 in our Policy Forum). By contrast, our “Increased Ambition” scenarios assume a higher minimum decarbonization rate (5%) which is on the higher end of historically observed rates [Fig S2 in (11)]. Many other uncertainties—including and beyond those highlighted by King et al.—could affect future emissions trajectories (12). Future research should also explore the implications of these variables.