he Paris Agreement sets the framework for international climate action. Within that context, countries are aiming to hold warming well below 2 °C and pursue limiting it to 1.5 °C. How such global temperature outcomes can be achieved has been explored widely in the scientific literature [1][2][3][4] and assessed by the IPCC, for example, in its Fifth Assessment Report (AR5; ref. 5 ) and its Special Report on Global Warming of 1.5 °C (SR1.5; ref. 6 ). Studies explore aspects of the timing and costs of emissions reductions and the contribution of different sectors 3,7,8 . However, there has been critique that, with the exception of a few notable studies [9][10][11][12] , the scenarios in the literature first exceed the prescribed temperature limits in the hope of recovering from this overshoot later through net-negative emissions [13][14][15][16] . Some pioneering studies [10][11][12] have explored implications of limiting overshoot through, for example, zero emissions goals, or have looked into the role of bioenergy with carbon capture and storage (BECCS) in reaching different temperature targets 9 . All these studies have relied on one or two models and/or a limited set of temperature targets.We bring together nine international modelling teams and conduct a comprehensive modelling intercomparison project (MIP) on this topic. Specifically, we explore mitigation pathways for reaching different temperature change targets with limited overshoot. We do this by adopting the scenario design from ref. 11 and contrast scenarios with a fixed remaining carbon budget until the time when net-zero CO 2 emissions (net-zero budget scenarios) are reached with scenarios that use an end-of-century budget design. The latter carbon budget for the full century permits the budget to be temporarily overspent, as long as net-negative CO 2 emissions (NNCE)

Closing the emissions gap between Nationally Determined Contributions (NDCs) and the global emissions levels needed to achieve the Paris Agreement’s climate goals will require a comprehensive package of policy measures. National and sectoral policies can help fill the gap, but success stories in one country cannot be automatically replicated in other countries. They need to be adapted to the local context. Here, we develop a new Bridge scenario based on nationally relevant, short-term measures informed by interactions with country experts. These good practice policies are rolled out globally between now and 2030 and combined with carbon pricing thereafter. We implement this scenario with an ensemble of global integrated assessment models. We show that the Bridge scenario closes two-thirds of the emissions gap between NDC and 2 °C scenarios by 2030 and enables a pathway in line with the 2 °C goal when combined with the necessary long-term changes, i.e. more comprehensive pricing measures after 2030. The Bridge scenario leads to a scale-up of renewable energy (reaching 52%–88% of global electricity supply by 2050), electrification of end-uses, efficiency improvements in energy demand sectors, and enhanced afforestation and reforestation. Our analysis suggests that early action via good-practice policies is less costly than a delay in global climate cooperation.

Global emissions scenarios play a critical role in the assessment of strategies to mitigate climate change and their related societal transformations. The current generation of scenarios, however, are criticized because they rely heavily on net negative CO2 emissions (NNCE) that result from allowing temperature limits to be temporarily exceeded. In this study we present a new set of emissions scenarios that exclude NNCE. We show that such scenarios require a more rapid near-term transformation with significant long-term gains for the economy (even without considering the benefits of avoided climate impacts). Scenarios that avoid temperature overshoot and NNCE are thus not only economically more attractive over the long term, they also involve lower climate risks. Our study further identifies possible alternative configurations of net-zero CO2 emissions systems and the distinct roles of different sectors and regions in order to balance emissions sources and sinks.

Closing the remaining emissions gap between Nationally Determined Contributions (NDCs) and the global emissions levels needed to achieve the Paris Agreement’s climate goals will likely require a comprehensive package of policy measures. National and sectoral policies can help fill the gap, but success stories in one country cannot be automatically replicated in other countries, but need to be adapted to the local context. Here, we develop a new Bridge scenario based on nationally relevant measures informed by interactions with country experts. We implement this scenario with an ensemble of global integrated assessment models (IAMs). We show that a global roll-out of these good practice policies closes the emissions gap between current NDCs and a cost-optimal well below 2 °C scenario by two thirds by 2030 and more than fully by 2050, while being less disruptive than a scenario that delays cost-optimal mitigation to 2030. The Bridge scenario leads to a scale-up of renewable energy (reaching 50%-85% of global electricity supply by 2050), electrification of end-uses, efficiency improvements in energy demand sectors, and enhanced afforestation and reforestation. Our analysis suggests that early action via good-practice policies is less costly than a delay in global climate cooperation.