based on: Iyer, G. et al. Nature Climate Change (2022).

The policy problem

A total of 151 countries submitted updated or new climate pledges to the 26th United Nations Conference of Parties (COP26) held in Glasgow in November 2021, outlining their plans to cut GHG emissions. Countries also communicated long-term strategies that outline emission reduction strategies to the mid-century and net-zero emissions targets. Although these updated and new pledges suggest higher ambition as compared with the 2015 Paris pledges, limiting global warming below 1.5 °C this century — the goal of the Paris Agreement — will require countries to further ratchet or increase ambition. Recognizing the need for more ambition, nations adopted the Glasgow Climate Pact, which calls for countries “to revisit and strengthen the…targets…to align with the Paris Agreement temperature goal by the end of 2022”. This study explores the long-term temperature outcomes of ratcheting or increasing ambition, as well as what this ratcheting implies for sectoral and regional emissions.

The findings

We find that ratcheting near-term ambition to 2030 will be crucial to limiting peak temperature changes this century (Fig. 1). If ratcheting is delayed, it would result in higher temperature overshooting — that is, an exceedance of global mean temperature change above the intended threshold before returning to below the intended level — over many decades, with the potential for adverse and irreversible consequences for human and natural systems. Our results also suggest that ratcheting near-term ambition could facilitate faster transitions to net-zero emissions systems — especially in major economies — resulting in faster reductions in emissions from all sectors of the economy for both CO2 and non-CO2 emissions. Although these transitions can be accomplished with limited availability of nascent technologies such as CO2 removal, further research is required to better understand the role of such technologies in high-ambition emissions pathways.

Fig. 1: Temperature change outcomes of the emissions pathways explored in Iyer et al. (2022).
figure 1

Inner bars show temperature change in 2100, and outer bars show peak temperature change. The emissions pathways vary across assumptions about ambition level in 2030 (NDC, NDC+, NDC++), post-2030 minimum decarbonization rate (2%, 5%, 8%), and timing of net-zero for countries with net-zero pledges (as stated, 5 or 10 years prior). See the main paper for detailed descriptions about these assumptions. The lighter shades within each colour group correspond to different assumptions about the post-2030 minimum decarbonization rate. While the first bar within each colour and shading group corresponds to the central case in which countries with net-zero pledges are assumed to achieve their pledges in the target year as stated, the second and third bars assume that countries advance the accomplishment of their pledges by 5 and 10 years, respectively. Decarb., decarbonization rate; NDC, nationally determined contribution.

The study

We explored a suite of high-ambition emissions pathways in which countries ratchet and achieve ambition through a combination of increasing near-term ambition to 2030, accelerating post-2030 decarbonization, and advancing the dates for national net-zero pledges. We developed the pathways using the Global Change Analysis Model (GCAM), which includes representations of the economy, energy, agriculture and land use, water, and climate systems and their interactions in 32 geopolitical regions (including many individual countries) across the globe. GCAM tracks emissions of GHGs on the basis of the evolution of these systems over the century. These detailed representations allowed us to model countries’ emission reduction pledges and the sectoral and regional implications of ratcheting those pledges. We used a simple climate model (Hector) to compute the temperature change implications of the emissions pathways. Our study also includes sensitivity analyses on the availability of CO2 removal technologies and the underlying assumptions and methodology used to construct our emissions pathways.