Over the last decades, climate science has evolved rapidly across multiple expert domains. Our best tools to capture state-of-the-art knowledge in an internally self-consistent modelling framework are the increasingly complex fully coupled Earth System Models (ESMs). However, computational limitations and the structural rigidity of ESMs mean that the full range of uncertainties across multiple domains are difficult to capture with ESMs alone. The tools of choice are instead more computationally efficient reduced complexity models (RCMs), which are structurally flexible and can span the response dynamics across a range of domain-specific models and ESM experiments. Here we present Phase 2 of the Reduced Complexity Model Intercomparison Project (RCMIP Phase 2), the first comprehensive intercomparison of RCMs that are probabilistically calibrated with key benchmark ranges from specialised research communities. Unsurprisingly, but crucially, we find that models which have been constrained to reflect the key benchmarks better reflect the key benchmarks. Under the low-emissions SSP1-1.9 scenario, across the RCMs, median peak warming projections range from 1.3 to 1.7{degree sign}C (relative to 1850-1900, using an observationally-based historical warming estimate of 0.8{degree sign}C between 1850-1900 and 1995-2014). Further developing methodologies to constrain these projection uncertainties seems paramount given the international community’s goal to contain warming to below 1.5{degree sign}C above pre-industrial in the long-term. Our findings suggest that users of RCMs should carefully evaluate their RCM, specifically its skill against key benchmarks and consider the need to include projections benchmarks either from ESM results or other assessments to reduce divergence in future projections.

This is a reply to a comment on the original research study with the title: ‘Unintentional unfairness when applying new greenhouse gas emissions metrics at country level’. This reply responds to some of the claims made in the comment and provides a scientific rebuttal.

Since its adoption, the Paris Agreement sets and defines the global climate ambition. The overall scope of this ambition is expressed in its long-term temperature goal in Article 2 as well as the ‘net zero’ mitigation goal in Article 4. To provide guidance to climate policy, the scientific community has explored the characteristics of greenhouse gas (GHG) emission reduction pathways that can meet the Paris Agreement goals. However, when categorizing and presenting such pathways including in reports of the Intergovernmental Panel on Climate Change (IPCC), the focus has been put on the temperature outcome and not on the emission reduction criteria set out in Article 4.1. Here we propose a pathway classification approach that aims to comprehensively reflect all climate criteria set out in the Paris Agreement. We show how such an approach allows for an internally consistent interpretation of the Paris Agreement in terms of emission reduction pathways. For pathways that simultaneously are very likely to hold warming to below 2°C, pursue efforts to limit warming to 1.5°C and achieve the provisions outlined in Article 4.1, we report 2030 global Kyoto-GHG emissions of between 20-26 Gt CO2eq (interquartile range), net zero CO2 emissions around 2050 and net zero GHG emissions around 2060. We further illustrate how prevalent pathway classifications focusing, for example, on the temperature outcome in 2100 result in additional criteria being applied that are not rooted in the Paris Agreement. We outline the consequences of such approaches including for the deployment of carbon dioxide removal (CDR) in such pathways. We find that across pathways classified as ‘no or low overshoot’ pathways in previous IPCC reports, such non-Paris related, additional criteria for end-of-century outcomes may lead to about 20% higher CDR deployment compared to purely achieving the Paris Agreement objectives in mitigation pathways.

Several objectives of the PROVIDE project depend on a set of scenarios that can be modelled through either a ‘classical’ forward-looking approach or by a novel approach that ‘reverses the impact chain’. These scenarios are also key elements for the integration of PROVIDE findings in the outward-looking stakeholder Dashboard of the project. Here we describe the set of scenarios that has been developed and will be used within PROVIDE. In total, PROVIDE explores three complementary approaches: 1) 10 distinct tier 1 scenarios extending until 2100, mostly based on the existing literature, used for short-term assessments of impacts 2) 15 distinct tier 1 scenarios extending until 2300, based on different extensions of the 10 literature scenarios, used for assessing longer-run impacts and the geophysical impact of significant temperature overshoot 3) ~1350 distinct tier 2 scenarios, exploring several dimensions of emissions space systematically, such as CO2 net zero date and relative methane intensity. This is used to explore which scenarios are compatible with given climate outcomes. These scenarios can be used to reverse the traditional impact chain, going from acceptable climate risks to descriptions of acceptable emissions.

The IPCC’s scientific assessment of the timing of net-zero emissions and 2030 emission reduction targets consistent with limiting warming to 1.5C or 2C rests on large scenario databases. Updates to this assessment, such as between the IPCC’s Special Report on Global Warming of 1.5C (SR1.5) of warming and the Sixth Assessment Report (AR6), are the result of intertwined, sometimes opaque, factors. Here we isolate one factor: the Earth System Model emulators used to estimate the global warming implications of scenarios. We show that warming projections using AR6-calibrated emulators are consistent, to within around 0.1C, with projections made by the emulators used in SR1.5. The consistency is due to two almost compensating changes: the increase in assessed historical warming between the IPCC’s Fifth Assessment Report (AR5) and AR6, and a reduction in projected warming due to improved agreement between the emulators’ response to emissions and the underlying assessment.

The pandemic in 2020 caused an abrupt change in the emission of anthropogenic aerosols and their precursors. We provide the first estimate of the associated change in the aerosol radiative forcing at the top of the atmosphere and the surface. To this end, we perform new simulations with the contemporary Earth system model EC-Earth3 participating in CMIP6, and created new data on the anthropogenic aerosol optical properties and an associated effect on clouds for the implemented aerosol parameterization, MACv2-SP. Our results highlight the small impact of the pandemic on the global aerosol radiative forcing in 2020 compared to the baseline of the order of +0.04Wm-2, which is small compared to the natural year-to-year variability in the radiation budget. Natural variability also limits the ability to detect a meaningful regional difference in the anthropogenic aerosol radiative effects. We identify the best chances to find a significant change in radiation at the surface during cloud-free conditions for regions that were strongly polluted in the past years. The new post-pandemic recovery scenarios indicate a spread in the aerosol forcing of -0.68 to -0.38Wm-2 for 2050, which translates to a difference of +0.05 to -0.25Wm-2 compared to the baseline. This spread falls within the present-day uncertainty in aerosol radiative forcing and the CMIP6 spread in aerosol forcing at the end of the 21st century. We release the new MACv2-SP data for studies on the climate response to the pandemic and the recovery scenarios.

Financial institutions’ investment and lending portfolios could be affected by both physical climate risks stemming from impacts related to increasing temperatures, and from transition climate risks stemming from the economic consequences of the shift to a low-carbon economy. Here we present a consistent framework to explore near term (to 2030) transition risks and longer term (to 2050) physical risks, globally and in specific regions, for a range of plausible greenhouse gas emissions and associated temperature pathways, spanning 1.5-4oC levels of long-term warming. We draw on a technology-rich, regionally disaggregated Integrated Assessment Model representing energy system, agricultural and land-based greenhouse gas emissions, a reduced complexity climate model to simulate probabilistic global temperature changes over the 21st century, and a suite of impacts models to estimate regional climate-related physical hazards and impacts deriving from the temperature change pathways and their underlying socio-economics. We consider 11 scenarios to explore the dependence of risks on both temperature pathways, as well as socio-economic, technology and policy choices. This builds and expands on existing exercises such as the Network for Greening the Financial System (NGFS). By 2050, physical risks deriving from major heatwaves, agricultural drought, heat stress and crop duration reductions depend greatly on the temperature pathway. By 2030, transition risks most sensitive to temperature pathways stem from economy-wide mitigation costs, carbon price increases, fossil fuel demand reductions and potential stranding of carbon-intensive assets such as coal-fired power stations. The more stringent the mitigation action, the higher the abatement costs and sector-specific transition risks. However, such scenarios result in lower physical climate hazards throughout the century. Our study also explores multiple 2 deg C pathways which demonstrate that scenarios with similar longer-term physical risks could have very different near-term transition risks depending on technological, policy and socio-economic factors. As such, “a single scenario will not answer all questions”.

Science has been written into the Glasgow Climate Pact. Four factors helped bring science to the fore: 1) the August 2021 publication of the Intergovernmental Panel on Climate Change (IPCC) Working Group One report; 2) the focus on science within the COP26 programme; 3) targeted synthetic independent advice from a range of national bodies; 4) main streaming of climate science stories by the media. We should capitalize on this success to make our science even more useful going forward.