Piers Forster

The Earth’s energy imbalance is the net radiative flux at the top-of-atmosphere. Climate model simulations suggest that the observed positive imbalance trend in the previous two decades is inconsistent with internal variability alone and caused by anthropogenic forcing and the resulting climate system response. Here, we investigate anthropogenic contributions to the imbalance trend using climate models forced with observed sea-surface temperatures. We find that the effective radiative forcing due to anthropogenic aerosol emission reductions has led to a 0.2 ± 0.1 W m−2 decade−1 strengthening of the 2001–2019 imbalance trend. The multi-model ensemble reproduces the observed imbalance trend of 0.47 ± 0.17 W m−2 decade−1 but with 10-40% underestimation. With most future scenarios showing further rapid reductions of aerosol emissions due to air quality legislation, such emission reductions …

The climate science community aims to improve our understanding of climate change due to anthropogenic influences on atmospheric composition and the Earth's surface. Yet not all climate interactions are fully understood and diversity in climate model experiments persists as assessed in the latest Intergovernmental Panel on Climate Change (IPCC) assessment report. This article synthesizes current challenges and emphasizes opportunities for advancing our understanding of climate change and model diversity. The perspective of this article is based on expert views from three multi-model intercomparison projects (MIPs) – the Precipitation Driver Response MIP (PDRMIP), the Aerosol and Chemistry MIP (AerChemMIP), and the Radiative Forcing MIP (RFMIP). While there are many shared interests and specialisms across the MIPs, they have their own scientific foci and specific approaches. The partial overlap between the MIPs proved useful for advancing the understanding of the perturbation-response paradigm through multi-model ensembles of Earth System Models of varying complexity. It specifically facilitated contributions to the research field through sharing knowledge on best practices for the design of model diagnostics and experimental strategies across MIP boundaries, e.g., for estimating effective radiative forcing. We discuss the challenges of gaining insights from highly complex models that have specific biases and provide guidance from our lessons learned. Promising ideas to overcome some long-standing challenges in the near future are kilometer-scale experiments to better simulate circulation-dependent processes where it …

The observed rate of global warming since the 1970s has been proposed as a strong constraint on equilibrium climate sensitivity (ECS) and transient climate response (TCR)—key metrics of the global climate response to greenhouse-gas forcing. Using CMIP5/6 models, we show that the inter-model relationship between warming and these climate sensitivity metrics (the basis for the constraint) arises from a similarity in transient and equilibrium warming patterns within the models, producing an effective climate sensitivity (EffCS) governing recent warming that is comparable to the value of ECS governing long-term warming under CO forcing. However, CMIP5/6 historical simulations do not reproduce observed warming patterns. When driven by observed patterns, even high ECS models produce low EffCS values consistent with the observed global warming rate. The inability of CMIP5/6 models to reproduce …

With the human induced increase in global temperatures continuing, the question if and how we might exceed the 1.5 C warming level enshrined in the long-term temperature goal of the Paris Agreement has received increased public and scientific interest. Identifying the level of human induced warming at any given year is subject to a range of uncertainty including from short-term natural variability. A single year, or even several consecutive years, above 1.5 C thus does not imply that the human induced warming level is reached but does provide an early warning of the risk of crossing that threshold.Here we find that under an emission pathway following current policies, a single year above 1.5 C might imply that a crossing of the global warming threshold could materialise within 11 years thereafter (66% or likely range). For a three (5) year consecutive average, this time window decreases to 5 (2) years. If 1.5 C is …

To manage Earth in the Anthropocene, new tools, new institutions, and new forms of international cooperation will be required. Earth Virtualization Engines are proposed as international federation of centers of excellence to empower all people to respond to the immense and urgent challenges posed by climate change.

Our study looks at the precipitation responses to two possible future emission‐mitigation pathways, pushed by the Coronavirus Disease 2019 pandemic (COVID‐19) and achieving carbon neutrality in the mid‐21st century. We find that a simultaneous‐reduction in well‐mixed greenhouse gases (WMGHGs) and anthropogenic aerosol emissions results in an enhanced interhemispheric precipitation contrast in the 2040s by amplifying the interhemispheric thermal contrast and strengthening the meridional overturning circulation in the tropics. Reduced aerosol emissions induce generally‐increased precipitation in the Northern Hemisphere (NH) and an amplified intertropical rainfall contrast, while reduced WMGHG emissions dominate decrease in precipitation in the areas away from aerosol emission sources. Further, the above precipitation contrast will be enhanced under stronger emission‐mitigation pathways …

The global aviation fleet modifies cloudiness through contrail formation and their subsequent competition with natural cirrus for ambient water vapor, along with enhanced ice-nuclei concentrations from aircraft soot emissions. Contrails form in the upper troposphere at temperatures below 233 K and pressures below 300 hPa, when plume gases from jet engines, having appreciable water vapor content, saturate with respect to liquid water (Schmidt-Appleman Criterion, SAC). Realistic assessments of the aviation-induced modifications to global cloud cover demand improved representation of contrails and their interaction with background cloudiness in climate models. We have implemented a process-based parametrization of contrail cirrus, that applies to both young (≤ 5 h) and aged contrails, in the UK Met Office Unified Model, version 12.0. Contrail cirrus is introduced as a new prognostic cloud class, forming in …

One of the successes of COP26 (the 26th Conference of the Parties) was the prominence of climate science and its implications. Science was written into the Glasgow Climate Pact, recognizing'the importance of the best available science for effective climate action and policy making'. This paper discusses the reasons for COP26's success and reflects on subsequent events at COP27. The continued importance of science in global climate negotiations throughout this critical decade for climate is clear.

Participants of the Berlin Summit on Earth Virtualization Engines (EVEs) discussed ideas and concepts to improve our ability to cope with climate change. EVEs aim to provide interactive and accessible climate simulations and data for a wide range of users. They combine high-resolution physics-based models with machine learning techniques to improve the fidelity, efficiency, and interpretability of climate projections. At its core, EVEs offer a federated data layer that enables simple and fast access to exabyte-sized climate data through simple interfaces. In this article, we summarize the technical challenges and opportunities for developing EVEs, and argue that they are essential for addressing the consequences of climate change.

The Earth's energy imbalance (EEI) is the difference in the net solar radiative flux and outgoing longwave radiative flux at the top-of-atmosphere. It has been shown that the positive EEI trend in the previous two decades is unexplained by internal variability and caused by anthropogenic forcing and response, such as that resulting from anthropogenic CO2 emissions. In this work we apply two state-of-the-art global climate models, the CESM2 and ICON-HAM, forced with observed (evolving) sea-surface temperature fields for the period 2000-2019 and with multiple ensemble members, to explore causes for the positive trend in EEI. Both models are able to reproduce the observed EEI trend from the CERES satellite product relatively well. Sensitivity simulations with aerosol emissions kept constant at year 2000 values indicate a relatively strong influence of recent aerosol emission reductions on the EEI trend …

Intergovernmental Panel on Climate Change (IPCC) assessments are the trusted source of scientific evidence for climate negotiations taking place under the United Nations Framework Convention on Climate Change (UNFCCC), including the first global stocktake under the Paris Agreement that will conclude at COP28 in December 2023. Evidence-based decision-making needs to be informed by up-to-date and timely information on key indicators of the state of the climate system and of the human influence on the global climate system. However, successive IPCC reports are published at intervals of 5–10 years, creating potential for an information gap between report cycles. We follow methods as close as possible to those used in the IPCC Sixth Assessment Report (AR6) Working Group One (WGI) report. We compile monitoring datasets to produce estimates for key climate indicators related to forcing of the climate system: emissions of greenhouse gases and short-lived climate forcers, greenhouse gas concentrations, radiative forcing, surface temperature changes, the Earth's energy imbalance, warming attributed to human activities, the remaining carbon budget, and estimates of global temperature extremes. The purpose of this effort, grounded in an open data, open science approach, is to make annually updated reliable global climate indicators available in the public domain (https://doi.org/10.5281/zenodo.8000192, Smith et al., 2023a). As they are traceable to IPCC report methods, they can be trusted by all parties involved in UNFCCC negotiations and help convey wider understanding of the latest knowledge of the climate system and its …

Effective radiative forcing (ERF) measures the change in net radiative flux at the top-of-atmosphere (TOA) following a perturbation to the Earth’s energy balance by drivers such as greenhouse gases (GHGs), aerosols or changes in land-surface properties. These drivers alter the balance of radiative fluxes in the climate system and impose an instantaneous radiative forcing (IRF) at the TOA that subsequently initiates rapid adjustments in tropospheric and stratospheric temperature, water vapour, surface albedo and clouds, which further alter the initial TOA radiative imbalance. ERF is defined as the sum of both IRF and the total radiative effect of rapid adjustments associated with a climate driver (Forster et al. 2021).Quantifying ERF is an important component of climate science, principally because changes in the Earth’s energy balance result in changes in surface temperature. Calculating ERF therefore allows us to estimate the amount of surface warming or cooling associated with a forcing mechanism and compare the strength of different drivers. It also allows us to attribute historical trends in surface temperature to a particular

Understanding the health co-benefits of climate change mitigation and how they manifest across different socioeconomic groups is crucial to justify and prioritise future decarbonisation pathways to achieve net zero. In this work, we quantify future worldwide air quality and public health co-benefits of decarbonisation to limit end-of-century warming to either 2ºC (scenario SSP1-2.6) or 1.5 ºC (scenario SSP1-1.9), relative to the middle-of-the-road pathway with a medium long-term radiative forcing target of 4.5 W m-2 (scenario SSP2-4.5). We use simulated ambient fine particulate matter (PM2. 5) concentrations for the period 2015-2100 from the Coupled Model Intercomparison Project (CMIP6) experiments. We estimate the mortality burden attributable to exposure to ambient PM2. 5 using population attributable fractions of relative risk, incorporating projected changes in population demographics and per-capita GDP …

We consider the science, policy, and implementation (science-policy-society interface) issues around the agrifood system and the UK's transition to net zero. We conclude that agrifood policy should become more targeted, and the marriage of top-down and bottom-up approaches is key to co-create a pathway that is plausible for each stakeholder.

In every IPCC Assessment cycle, a multitude of scenarios are assessed, with different scope and emphasis throughout the various Working Group and Special Reports and their respective chapters. Within the reports, the ambition is to integrate knowledge on possible climate futures across the Working Groups and scientific research domains based on a small set of ‘framing pathways’, such as the so-called RCP pathways from the Fifth IPCC Assessment report (AR5) and the SSP-RCP scenarios in the Sixth Assessment Report (AR6). This perspective, initiated by discussions at the IPCC Bangkok workshop in April 2023 on the “Use of Scenarios in AR6 and Subsequent Assessments”, is intended to serve as one of the community contributions to highlight needs for the next generation of framing pathways that is being advanced under the CMIP umbrella for use in the IPCC AR7. Here we suggest a number of policy research objectives that such a set of framing pathways should ideally fulfil, including mitigation needs for meeting the Paris Agreement objectives, the risks associated with carbon removal strategies, the consequences of delay in enacting that mitigation, guidance for adaptation needs, loss and damage, and for achieving mitigation in the wider context of Societal Development goals. Based on this context we suggest that the next generation of climate scenarios for Earth System Models should evolve towards ‘Representative Emission Pathways’ (REPs) and suggest key categories for such pathways. These ‘framing pathways’ should address the most critical mitigation policy and adaptation needs over the next 5–10 years. In our view the …

The Working Group I (WGI) contribution to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6) assesses the physical science basis of climate change. As part of that contribution, this Technical Summary (TS) is designed to bridge between the comprehensive assessment of the WGI Chapters and its Summary for Policymakers (SPM). It is primarily built from the Executive Summaries of the individual chapters and Atlas and provides a synthesis of key findings based on multiple lines of evidence (eg, analyses of observations, models, paleoclimate information and understanding of physical, chemical and biological processes and components of the climate system). All the findings and figures here are supported by and traceable to the underlying chapters, with relevant chapter sections indicated in curly brackets.

Condensation trails (contrails) are aircraft-induced line-shaped high clouds, which may persist and grow to form contrail cirrus in ice supersaturated regions. Contrail cirrus is the largest known component of aviation radiative forcing, with a large uncertainty associated with troposphere water budgets and contrail radiative properties. In addition, due to the limited number of climate models able to simulate contrail cirrus, the uncertainty in the global contrail cirrus radiative forcing cannot be estimated statistically. The aim of this work is to implement a contrail cirrus parameterisation in the UK Earth System Model, therefore providing a new independent estimate of the contrail cirrus radiative forcing to be used in future assessments of aviation climate impacts. The new diagnostic scheme is based on the processes governing contrail formation (Schmidt-Appleman Criteria) and persistence (ice supersaturation). Persistent …

The regional climate impacts of hypothetical future emissions scenarios can be estimated by combining Earth system model simulations with a linear pattern scaling model such as MESMER (Modular Earth System Model Emulator with spatially Resolved output), which uses estimated patterns of the local response per degree of global temperature change. Here we use the mean trend component of MESMER to emulate the regional pattern of the surface temperature response based on historical single-forcer and future Shared Socioeconomic Pathway (SSP) CMIP6 (Coupled Model Intercomparison Project Phase 6) simulations. Errors in the emulations for selected target scenarios (SSP1–1.9, SSP1–2.6, SSP2–4.5, SSP3–7.0, and SSP5–8.5) are decomposed into two components, namely (1) the differences in scaling patterns between scenarios as a consequence of varying combinations of external forcings and (2) the intrinsic time series differences between the local and global responses in the target scenario. The time series error is relatively small for high-emissions scenarios, contributing around 20 % of the total error, but is similar in magnitude to the pattern error for lower-emissions scenarios. This irreducible time series error limits the efficacy of linear pattern scaling for emulating strong mitigation pathways and reduces the dependence on the predictor pattern used. The results help guide the choice of predictor scenarios for simple climate models and where to target for the introduction of other dependent variables beyond global surface temperature into pattern scaling models.

Understanding the costs and benefits of climate change mitigation and adaptation options is crucial to justify and prioritize future decarbonization pathways to achieve net zero. Here, we quantified the co‐benefits of decarbonization for air quality and public health under scenarios that aim to limit end‐of‐century warming to 2°C and 1.5°C. We estimated the mortality burden attributable to ambient PM2.5 exposure using population attributable fractions of relative risk, incorporating projected changes in population demographics. We found that implementation of decarbonization scenarios could produce substantial global reductions in population exposure to PM2.5 pollution and associated premature mortality, with maximum health benefits achieved in Asia around mid‐century. The stringent 1.5ºC‐compliant decarbonization scenario (SSP1‐1.9) could reduce the PM2.5‐attributable mortality burden by 29% in 2050 …

2 The three Special Reports are: Global Warming of 1.5 C: An IPCC Special Report on the impacts of global warming of 1.5 C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty (SR1. 5); Climate Change and Land: An IPCC Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems (SRCCL); IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC).