Stuart Jenkins

Abstract:

• Criticism of the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) largely stems from its reliance on downstream offsetting markets that lie outside its direct oversight.
• Geologically Balanced Fuels (GBFs) – kerosene bundled with geological CO2 storage units and accounted for at the point of fuel supply – could strengthen the integrity and effectiveness of CORSIA.
• GBFs can be treated as either a distinct CORSIA Eligible Fuel category, or as an offsetting instrument under the Eligible Emissions Units framework. We argue that the GBF product is most complementary and additional to existing approaches when treated as an eligible fuel.
• GBFs are intended to complement, rather than replace, Sustainable Aviation Fuels and Lower Carbon Aviation Fuels.
• Although the current CORSIA framework does not yet accommodate GBFs, it already contains important institutional building blocks, including recognition of fossil-derived eligible fuels and governance rules for carbon capture and storage projects, including a Reserve Inventory Account.
• Integrating GBFs into CORSIA would require three core amendments: legal recognition of GBFs, a methodology for accounting for GBFs, and GBF-specific sustainability criteria.
• The most important technical reform would fall within the CORSIA Methodology for Calculating Actual Life Cycle Emissions Values, through the addition of a GBF-specific chapter covering applicability and underlying fuel value, allocation of qualifying geological storage to a GBF batch, calculation of the GBF actual value, exclusivity and non-overlap, and Sustainability Certification Scheme verification of the GBF methodology.

Abstract:

Abstract. In a rapidly changing climate, evidence-based decision-making benefits from up-to-date and timely information. We track twelve key sets of indicators of the state of the climate system, closely following Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment report (AR6) methods, to produce our fourth annual publication. One of the indicators, the Earth's energy imbalance (EEI) provides a crucial integrative measure of the overall heating of the planet and the pace of climate change – this has more than doubled since the 1976–1995 period. A newly added indicator of temperature extremes, the number of days experiencing marine heatwaves, has more than tripled between 1991 and 2025. For the 2016–2025 decade average, observed warming relative to 1850–1900 was 1.26 [1.13 to 1.36] °C, of which 1.24 [1.0 to 1.5] °C was human-induced. Human-induced warming reached 1.37 °C relative to 1850–1900 in the year 2025, increasing at a rate of 0.27 [0.2–0.4] °C per decade over 2016–2025. This high rate of warming, which matches the all-time high seen last year in the instrumental record, was caused by a combination of greenhouse gas emissions being at an all-time high of 54.6 ± 5.5 GtCO2e yr−1 over the last decade (2015–2024), as well as reductions in the strength of aerosol cooling. Despite this, there is evidence that CO2 emission growth is slowing. The continuation of these annual updates could track decreases or increases in the rate of human influence and climatic changes presented here, reflecting the outcomes of societal choices during the critical 2020s decade. The data presented herein can provide a useful reference point for the drafting of the IPCC seventh assessment report. In total, we employ analysis from over 40 global datasets (https://doi.org/10.5281/zenodo.20499280, Smith et al., 2026a). Future monitoring of these indicators, such as ocean and satellite measurements of the Earth's energy imbalance, are threatened by geopolitical and public funding decisions. Our ability to consistently track many of the indicators requires the continuity of observation programs and coordination mechanisms, including the Global Climate Observing System (GCOS) program, that enable their effective integration and use.

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