Post <scp>COP26</scp>: does the 1.5°C climate target remain alive?

One of the COP26 aims was to keep 1.5°C within reach by asking countries come forward with ambitious year 2030 emission reductions targets further pursue necessary action meet Paris climate targets. We assess mean global temperature rise given updated pledges in context future pathways considered international scientific community. Overall, we find current are not consistent a likely meeting this century without overshoot. Meeting goal 2100 post overshoot remains feasible, but urgent is required ensure met and policies place for very deep rapid that 2030. International negotiations agreed 2015 as part Agreement (COP21) limit increase well below 2°C while pursuing efforts even relative pre-industrial. Achieving requires emissions first peak then decline zero over coming decades. However, fossil carbon continue yet remaining total envelope – budget fast running out (Friedlingstein et al., 2022). All all, implies on pathway ambition. express 26th annual United Nations Framework Convention Climate Change (UNFCCC), held Glasgow, known COP26, move target (UK Government, 2021). The arising ‘Glasgow Pact’ reconfirmed commitment emphasised ‘urgent need Parties their collectively reduce through accelerated action’ (UNFCCC, 2022) left open question whether commitments made would be enough reach. There no definition ambition, two commonly assumed; involves reaching directly reducing net staying an which temperatures do exceed 1.5°C, second ‘overshoot’ temporarily return level 2100. In latter case, can later exceeding dependent negative century. Either these seen have different implications decades possible impacts. Stabilising all active forcers controlled. Gases long atmospheric lifetimes, such CO2, decrease zero. Other shorter-lived forcers, methane, managed minimum requirement. To particular CO2 achieved or envelope. Intergovernmental Panel (IPCC) assessed 500 GtCO2 (IPCC, 2021) from 2020 50% chance 1.5°C. Net and/or dioxide removal (CDR) technologies. Exceeding means ‘net negative’ offset budget. primary mechanism agreeing UNFCCC process conditional unconditional nationally determined contributions (NDCs) individual blocs make levels saw increased ambition amongst several corresponding tightening NDCs, additional longer-term including achieving net-zero methane reductions. themselves binding unless they incorporated into domestic law, so represent indication rather than commitment. crucial if limited IPCC Special Report Global Warming (SR15; IPCC, 2018) found half scenarios avoiding range 25–30 GtCO2e 2018). Current greenhouse gas (GHG) reached 51.2 2018 (Crippa implying halve track Assessments country-level NDC Action Tracker (CAT) 45–49 2030, meaning there substantial ‘emission gap’ between (UNEP, warming been organisations Energy Association (IEA, 2021a,b), Resource (Meinshausen 2021), Environment Programme CAT (2021). These ‘bottom up’ approaches involve addition national combined assumptions longer term implications. four estimates use differing methods probability thresholds, agree outcome 1.8°C should NDCs announcements met. Just considering 2.4°C (1.9–3°C) large share experienced after period. Post therefore important determining outcomes (Rogelj 2016). challenges approach building country upwards assuming appropriate mitigation actions stalls, continues accelerates. Common assume constant according other pledges. This (Höhne however made, choice (Gütschow Our approach, presented here, differs taking ‘top-down’ view literature compatible take full database available SR15 (Huppmann 2019), consisting 400 pathways, difference characteristics inconsistent NDCs. report included assessment existing model-based were close target. compiled plausible community at time when being updated. sample how technology policy may evolve. Alternative remain possible, instance significant ratcheting importantly outside evolution policy. 2019) used conjunction simple observationally constrained emulator complexity Earth system models (ESMs). using computationally expensive ESM allows us explore much broader probabilistic framework. relationship (and scenarios) response. model FaIR (finite amplitude impulse response model, Smith one AR6 WGI help interpret models. AR6, cycle properties perturbed uncertain ranges run many thousands versions produce climates. simulated climates lines evidence, analyses observations, models, paleoclimate information understanding physical, chemical biological processes components constrain ensemble distribution enable projections. simulate threshold exceeded. scenarios, include Committee inform advice UK's Sixth Carbon Budget (CCC, 2020). scenario represents delivering (CCC PA), requiring scaling up across economy next decade. other, 2020) meets previously pledged decarbonisation slower rate aligned (Gohar 415 explored its consistency policies. simulations, show simulations version 6 Model Assessment Greenhouse Gas-Induced MAGICC6 emulator, (consisting 411 scenarios). identical setup AR5 WGIII (Clarke 2014), latest constraint AR6. For typical (AR5) derived will differ (AR6). due structure application constraints. representation databases. missing gases infilled method Bernie Lowe (2014). CCC Silicone (Lamboll 2020), open-source Python package, infers anthropogenic unmodelled species based reported emissions. taken anomalies 2011–2020 period, observed pre-industrial period added on. methodology more recent (AR6 1995–2014). each likelihood ‘peak’ sampling cumulative 2237 members. divided classes A lower class, where 45 higher class 174 above threshold. those greater thresholds reports summation any (<20% emissions) form totals additions aviation, shipping land-use change.1 new shown Figure 1 relates outcomes. Of 134 100 MAGICC high certainty approximately middle pre Stated Policies Scenario (STEPS) IEA (53 GtCO2e, IEA, 2021c). and, pledge become policy, clearly shows overshooting reduced rapidly few years. An conclusion analysis go far terms lowering specified agreement consider chances weaker dependency action. less rigorous opens fulfilment. Using classified emission, select least (415 MAGICC), just 18 (less 5%) (Figure 1, red) 9 (2%) blue) during twenty-first 1a). require back (117, 28% FaIR, 90, 22% MAGICC) 1b). PA has 56%; however, similar both grey shaded region cyan line respectively), gives 100% 2 (a) (b) series dividing low categories. overshoot, 96 fall category (grey plume) 21 (red plume, 2). goes challenge options makers avoid Pathways tend cuts early reductions, average earlier combines overall warmer end graphically 3. associated magnitude duration some cases near excess multiple clear differences 3b) having none 3a). Most 1.5 1.7°C Sustained impact risk (Schleussner Furthermore, year-on-year higher, delayed offsetting case here database. suggests lead exceedance 75% likelihood. Remaining present UN Emissions Gap imply pathway. comparatively feasible non-overshoot feasibility arises flexibility offered achievement date shifted dependence near-term removals. Delayed increasing reliance deeper turn, transition risks energy systems decarbonisation, demand, (Akimoto 2018); hazards impacts temporary (SR15). Although our alternative possible. because exceeded 420 2022 38.0 GtCO2. Should stabilise slowly (as implied commitments) small exists extremely scope scenario-based assessments largely rely CDR technologies, must residual bring again larger CDR. technologies deployed scales considerable uncertainty regarding taken. confidence CDR, bioenergy capture storage (BECCS) afforestation, could land, energy, water nutrients scale 2018, Direct air environmental BECCS currently (Fuss diverse portfolio mitigate economic costs (Napp 2019). amount investment research development robust comprehensive 2019; Strefler 2021; van der Wijst impacts, extremes already caused widespread adverse beyond natural variability At substantially compared levels, cannot eliminated entirely. severity Already warming, deterioration land ocean ecosystems driven temperature. led migration loss cases, effects fully reverse. level, length point unexpected abrupt change. irreversible changes forms tipping system. example, permafrost thaw Arctic retreat glaciers mountain possibly decadal timescale. Palter al. (2018) geographic patterns sea sensitive leading long-lasting sea-level (Tokarska Zickfeld, 2015). Pattyn collapse Greenland West Antarctic ice sheets increases temperature, linked rise. millennial timescales, sheet stability around 1.5–2°C highlighting minimise worst highly extent immediate termination path inertia (Eom does say inevitable, possibility discovery novel sources major shift consumption thinking. play role committing As participants monitor progress towards collective goals 5-yearly peer reviewed called Stocktake. specifically calls prior following requirement successive progression gone before Stocktake, ratchet stocktake completed COP28 2023 provides opportunity gap. results CAT, UNEP CR 2030's limiting alive under now Keeping urgent, Not only put getting 2030s Stocktake alive, demand. Multi-decadal strongly emissions, link partly systems. Early technological reliance.