Cost and attainability of meeting stringent climate targets without overshoot

Abstract

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)

An emission pathway classification reflecting the Paris Agreement climate objectives

“…In addition, some models also represent CDR via direct air capture and enhanced weathering (ref. 31 discusses this in further detail). Not all scenarios report carbon sequestration from land use, so we follow the approach adopted by ref.…”

“…The set of emission pathways in the AR6 WGIII database represents an ensemble of opportunity that has not been designed with a specific focus on net zero GHGs (Crit III) and therefore does not allow for a systematic analysis of these interdependencies. To the contrary, the net zero carbon budget design approach pursued in a large number of scenarios in the database from the ENGAGE project and others, may lead to less scenarios reaching net zero GHGs compared to an end-of-century budget approach 31 . Other scenarios may deploy large amounts of CDR beyond net zero GHGs by design 26 .…”

Paris Agreement requires substantial, broad, and sustained policy efforts beyond COVID-19 public stimulus packages

“…Two more recent datasets from NGFS and ENGAGE based on IAMs that include recent trends and understanding (Creutzig et al 2017 ; Riahi et al 2021 ) yield comparably higher estimates of the additional total energy investments for 2020–2024 (335 and 227 billion US$/year; green and black horizontal bars in Fig. 2b , respectively).…”

The IPCC Sixth Assessment Report WGIII climate assessment of mitigation pathways: from emissions to global temperatures

“…To further analyse the characteristics of scenario categories beyond the analysis in AR6 we use the concept of overshoot degree years (ODY), which is similar to what was shown as "overshoot severity" in Table 2.SM.12 in SR1.5 (Forster et al, 2018), and was included in the metadata of the SR1.5 scenario database (Rogelj et al, 2018;Huppmann et al, 2018b) as "exceedance severity". Inspired by Geden and Löschel, (2017) and recent scenario studies investigating temperature overshoot (Drouet et al, 2021;Riahi et al, 2021;Johansson, 2021;Tachiiri et al, 2019).…”

“…The climate-assessment workflow is available as an installable open-source Python package with an MIT license (Kikstra et al). The code utilises functions of existing scientific software packages including 'pyam' (Huppmann et al, 2021) and has been parallelised to enable doing runs of many scenarios. It could be used as a community tool for scenario assessment that enables both easier access to well-calibrated climate emulators and the possibility to assess a wider range of scenarios due to the possibility of infilling emissions trajectories.…”