Professor Myles Allen CBE FRS

Abstract:

Greenhouse gas emission metrics are widely used for comparing climate impacts of different gases and for guiding mitigation policy. Conventional metrics such as GWP100 perform well for representing the warming effects of long-lived gases which behave like CO₂ but poorly for short-lived climate pollutants (SLCPs). Methane (CH4) is the most important SLCP and has been the main focus of alternative metrics. GWP* was developed to more accurately capture impact on global warming, particularly from stable and declining CH4 emissions which are not well served by GWP100. This means that GWP* better connects emissions pathways to long-term temperature targets (Cain et al., 2022). Previous studies optimised GWP* for CH4 for a limited range of scenarios up to 2100. However, future mitigation pathways involve a wider range of gases and transition speeds, overshoot behaviour, and long-term stabilization beyond this period. In addition, highly radiatively efficient fluorinated gases are increasingly important in mitigation strategies yet have not been demonstrated with the GWP* framework. In this study, we systematically test the performance of GWP* across an expanded set of emissions scenarios, including rapid mitigation, delayed action, and prolonged temperature overshoot pathways, and extend the analysis to multi-century time horizons with an optimisation of the flow term of GWP* (Mastropierro et al., 2025). We further develop and evaluate a generalized formulation of GWP* for fluorinated gases with diverse atmospheric lifetimes. The outcomes examine the performance of GWP* under realistic transition pathways and its representation of temperature responses for fluorinated gases. This work supports the development of more physically consistent multi-gas emission metrics for climate targets, carbon budgeting, and policy design, as it is a simple tool to calculate how much global warming is added or avoided by increasing or cutting SLCPs such as F-gases.Cain, M., Jenkins, S., Allen, M.R., Lynch, J., Frame, D.J., Macey, A.H., Peters, G.P. Methane and the Paris Agreement temperature goals. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 380 (2022). https://doi.org/10.1098/rsta.2020.0456Mastropierro, M., Tanaka, K., Melnikova, I. et al. Testing GWP* to quantify non-CO2contributions in the carbon budget framework in overshoot scenarios. npj Clim Atmos Sci 8, 101 (2025). https://doi.org/10.1038/s41612-025-00980-7

Abstract:

Integrating physical, socio-economic, and technological perspectives is indispensable for addressing climate mitigation challenges. While directly coupling state-of-the-art Earth System Models (ESMs) and Integrated Assessment Models (IAMs) offers a way to explore feedbacks between these domains, doing so with full-complexity models remains computationally prohibitive. This is particularly true for cost-effective intertemporal optimization IAMs due to fundamental operational differences: while ESMs perform forward simulations, such IAMs optimize over time. Consequently, direct coupling would require numerous computationally intensive iterations to converge, a complication further compounded by the stochastic nature of ESMs.To overcome the barriers to coupling ESMs and IAMs, we employ their reduced-complexity representations (i.e., emulators). We couple an IAM emulator representing 9 distinct IAMs (Xiong et al. 2025) with an ESM emulator, FaIR, representing 66 ESM configurations (Smith et al. 2024a). Using this coupled ESM-IAM emulator framework in an optimization setting, we calculate cost-effective pathways that achieve the temperature targets of the Paris Agreement with and without overshoot.Our preliminary results indicate that the uncertainty ranges for such pathways are significantly larger than previously estimated. Our results also have implications for target setting; we show how pathways differ when IAMs optimize directly for a temperature target – a capability IAMs traditionally lack. Instead, IAMs typically rely on temperature proxies, such as carbon budgets (or their corresponding carbon price pathways), which do not necessarily provide an accurate representation of the temperature target. Furthermore, this study offers advanced insights into the dynamics of climate-economy interactions, providing a roadmap for future efforts to couple full-complexity models. ReferencesXiong, W., Tanaka, K., Ciais, P., Johansson, D. J. A., & Lehtveer, M. (2025). emIAM v1.0: an emulator for integrated assessment models using marginal abatement cost curves. Geosci. Model Dev., 18(5), 1575-1612. doi:10.5194/gmd-18-1575-2025Smith, C., Cummins, D. P., Fredriksen, H. B., Nicholls, Z., Meinshausen, M., Allen, M., . . . Partanen, A. I. (2024). fair-calibrate v1.4.1: calibration, constraining, and validation of the FaIR simple climate model for reliable future climate projections. Geosci. Model Dev., 17(23), 8569-8592. doi:10.5194/gmd-17-8569-2024

Abstract:

2025 has been the third warmest year on the observational record globally, while the past decade has been the warmest on record. Accelerating climate change is altering the hydrological cycle, with associated extreme events - including droughts, floods, and compound multivariate extremes - occurring with increasing frequency and intensity in many regions of the world.  The 2025 October–November-December (OND), short rains (Vuli/Deyr/Hageya) season largely disappointed across East Africa, with major drought hotspots involving highly vulnerable regions in Somalia, eastern Kenya, and southern Ethiopia. Many areas received well under 60% of climatological precipitation level, with patches even below 30%, and some locations ranking among the driest in the satellite-era record (roughly since 1979). This substantial rainfall deficit, amplified by unusually high near-surface air temperature, rapidly degraded rangelands and diminished water availability, driving livestock stress and low crop yields. Additionally, in Somalia, the OND drought contributed to population displacements and heightened need for humanitarian aid.  We analyse the dynamic and thermodynamic drivers of the 2025 OND, Vuli/Deyr/Hageya drought in East Africa. Using a multi-method attribution framework, we assess the influence of anthropogenic climate change on the development of these high-impact drought conditions, with the strongest signal magnitude in the eastern Horn of Africa. We combine multiple observational and reanalysis datasets with large ensembles of bias-corrected CMIP6 historical simulations and future projections under SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios. This enables us to evaluate the roles of climate-change indices and internal climate variability modes – most importantly ENSO and IOD - in shaping this extreme hydrological event on subseasonal to seasonal timescales.  Additionally, we use the Met Office HadGEM3-A attribution system to quantify the extent to which anthropogenic forcing has altered the probability and intensity of such class of meteorological drought in the region. We consider both unconditional and circulation-conditioned attribution perspectives. Preliminary results indicate a significant contribution of climate change to the likelihood and intensity of the 2025 Vuli/Deyr/Hageya drought over regions in Ethiopia, Kenya, and Somalia that would greatly benefit from adaptation measures.

Abstract:

With the 1.5 °C target rapidly approaching, net-zero emissions plans routinely call for the rapid expansion of offsets based on carbon removals. This is particularly true in sectors such as aviation, where decarbonisation options are limited or progressing slowly. To comply with like-for-like offsetting requirements, such sectors will likely rely on durable CO₂ offsets, which may be delivered through bioenergy production or direct air capture combined with geological CO₂ storage. If geological CO2 removals are to ensure broad societal buy-in and market integration, they must be underpinned by robust Measurement, Reporting and Verification (MRV) and certification systems. These systems must also be recognised by a diverse set of policies, standards, and voluntary schemes.This study investigates what certificates for geological CO₂ removal would need to look like to play a meaningful role in the decarbonisation of the aviation sector. Aviation provides a relevant focus because it is a strong use-case for high-durability offsets since direct decarbonisation options such as sustainable and low-carbon aviation fuels have high energy demands and feedstock limitations. Moreover, these options partially overlap with components of geological CO2 removals and similarly rely on extensive certification under aviation climate policy frameworks.  To address this question, we conduct a comprehensive empirical assessment that: identifies the most common MRV and certification criteria embedded in leading policies and standards; and evaluates the certification requirements for geological CO₂ removal. Based on this mapping, we identify priority design features that geological CO₂ offsets would need to satisfy to achieve policy recognition and market uptake in aviation.To do so, we analyse a corpus of 10 policies and over 45 supporting documents, including carbon-crediting and voluntary carbon market (VCM) frameworks, as well as aviation-related policies and fuel mandates. These control the design of certification programmes and how geological CO2 removals may integrate in the aviation sector. We combine natural-language processing with an LLM-as-a-judge approach to assess the presence and strength of certification criteria across policies, followed by manual expert coding of a subset to identify differences in requirements. These 38 criteria span governance, adaptability, quantification, counterfactuals, MRV, permanence, accounting integrity, and sustainability safeguards.We find that criteria most consistently present across both policy families relate to MRV (reporting, verification, recordkeeping), quantification (system boundaries, demonstrable climate benefits), counterfactuals (baselines and leakage), and accounting (registries and tracking) and governance (compliance). Fuel policies place stronger emphasis on quantification, boundaries, reporting, and compliance, but tend to be less specific with respect to governance, permanence, long-term accounting, and social safeguards. When manually assessing certification requirements, we find recent certification standards (Paris Agreement 6.4, ICVCM, CRCF) to perform best as they have extensive and specific requirements. Fuel policies, on the other hand, are more explicit in their treatment of lifecycle quantification for narrowly defined pathways.

Abstract:

Net-zero targets are widely adopted by companies and countries worldwide. To achieve these goals, more companies are investing in diverse carbon removal portfolios. This study develops a new risk management framework that combines forestry, biochar, and geological storage offsets into portfolios that could stabilize global temperatures over multi-century time periods. We find that if a carbon storage portfolio reaches an equilibrium state of CO2 stored, it can be leveraged to stabilize global temperatures by increasing the size of the portfolio relative to the amount of removal claimed. For moderate-risk primarily forestry portfolios retaining 0.75–0.55 tCO2 of the 1 tCO2 stored, an additional 0.30–0.80 tCO2 removal is needed to offset re-releases over 1,000 years. High-risk portfolios retaining only 0.10 tCO2 require over 9 tCO2 additional removal. Portfolios that are predicted to re-release almost all CO2 cannot be leveraged and are ineffective at meeting temperature stabilization goals. These findings have implications for policy and corporate climate action.