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ESM2025 Bridging the Gap: Science to Policy Meeting in Brussels

Scientists from four EU-funded climate research projects met with policy officers from the European Commission in Brussels in November 2023. It has been a few years since we’d met face-to-face with EU policy officers, so our efforts to travel to Brussels were really appreciated. Projects 4C (Climate-Carbon Interactions in the Current Century), CONSTRAIN (creating improved climate projections), ESM2025 (building a new generation of Earth system models) and PROVIDE (overshoot scenarios and respective impacts) had worked together to develop an overview of recent research from all of the projects and included contributions from the RESCUE project.

Figure 1: In-person meeting at the European Commission, Brussels.

Holding the meeting in a hybrid format ensured that we engaged a wider audience from several Directorate-Generals, with another 38 people joining the meeting online and enabling some colleagues to present their research online.

Our aim for the meeting was to enable lots of discussion, so each topic had a short introductory presentation followed by plenty of time for discussion. Brief overviews of the topics discussed are presented in the following paragraphs.

The human perturbation on the Global Carbon Cycle – Pierre Friedlingstein talked about investigating the behaviour of the carbon cycle and the climate system. Pierre pointed out that global warming scales almost linearly with cumulative CO2 emissions and about 0.45°C warming occurs for every 1000 billion tonnes CO2 emission. Whilst the estimate of the Global Carbon budget is improved there are still several uncertainties due to large ranges in model-based land flux estimates, divergence between model-based and data- based ocean estimates and oxygen derived budgets suggesting a slightly larger ocean sink / weaker land sink. Climate change is already affecting the sinks, reducing the ocean sink by about 4% and the land sink by about 17% globally.

Piers Forster – focused discussion on near-term warming, the 1.5oC long-term temperature goal, and providing information to support UNFCCC-related discussions.  IPCC doesn’t have an annual update of climate system metrics, so the CONSTRAIN project has initiated the Indicators of Global Climate Change which includes an estimate of current warming and warming rates, which are currently accelerating. Non-CO2 emissions are driving near-term warming alongside CO2. Rapidly reducing methane would help to counter some of the warming from CO2, as well as the effects of cleaning up aerosol pollution such as marine sulphur. Current work is aiming to interpret where the planet is now and what it means for passing 1.5 – e.g. what changes we are likely to see in the next 1, 3, 5, or 10 years (this will be presented at COP28).  There is still a lot of uncertainty, but there is a significant chance of passing this threshold around 2030.

Joeri Rogelj presented the remaining carbon budget for ambitious climate targets. Joeri explained how the estimate for the remaining carbon budget has changed between updates of the IPCC Working Group I report in 2020, the IPCC Climate Change 2022 report, Indicators of Global Climate Change, and the paper by Lamboll et al. (2023) These changes demonstrate how scientific research has progressed and reflect on the continued emissions. The overall conclusion is that the remaining carbon budget to remain below 1.5oC is extremely small.

Carl-Friedrich Schleussner identified that there is a fundamental overconfidence in discussions about how we look into climate overshoot: in when it might happen, in the availability of required carbon dioxide removal technologies and in a favourable response of the Earth system.

If we stop global CO2 emissions into the atmosphere, the ocean continues to take up heat but this uptake slows down and becomes zero over time. The Zero Emissions Commitment (ZEC) quantifies how much warming or cooling is expected if man-made CO2 emissions are stopped. Currently, there is substantial uncertainty in both the sign and magnitude of the Zero Emissions Commitment (ZEC).

To build a more confident assessment of ZEC, a paper by Palazzo Corner et al. (2023), presents an overview of the changes expected in major Earth system processes after net zero and their potential impact on global surface temperature.

Figure 2: An overview of some of the processes that determine the Zero Emissions Commitment (ZEC), including ocean heat uptake, the ocean biogeochemical cycle, the land carbon cycle, and physical climate feedbacks. Frontiers | The Zero Emissions Commitment and climate stabilization (

We still need to understand what happens to climate impact drivers if we overshoot temperature targets and whether climate impact drivers will be reversible. Even if they are reversible will they reverse in the same way? Research has already identified irreversible long-term committed impacts in ocean, cryosphere and biosphere (about 0.4m 2300 sea-level rise commitment per 100 years of overshoot). Another study found that changes in ocean temperature and oxygen drive a centuries-long irreversible loss in the habitable volume of the upper 1000 m of the world ocean (Santana-Falcón et al. (2023)). 

Roland Séférian presented an overview of advances in Earth system modelling. The ESM community has made key insights in improving our understanding of the Earth system and its responses to anthropogenic greenhouse gas emissions, assessing the key properties of future emissions pathways and determining the carbon budget for 1.5°C and 2°C, uncertainty for risk assessment and using observations to deliver constrained projections.

New questions and demands are arising from discussions with stakeholders and in the research community and these are driving model development and research effort, for example:

  • Development of novel narrative and pathways exploring plausible futures and scrutinizing key emerging properties of those pathways
  • Increasing our understanding of anthropogenic warming, including the role of non-CO2 forcing and natural climate variability.
  • Assessing the reliability of climate mitigation solutions, including nature-based options, or non-CO2 mitigation strategies.
  • Improving our understanding and our predictive capability for extreme events to inform adaptation strategies. The latest ESMs use emission of CO2 and allow the model’s carbon cycle to determine what fraction of this remains in the atmosphere. Traditionally, models were forced by atmospheric concentrations of CO2, for the past and into the future. Running ESMs with emissions of CO2 allows for a more complete representation of climate – carbon cycle feedbacks and is the best option available today for accurately linking different carbon emission budgets to different levels of global warming.

Finally, through transdisciplinary working between ESM and IAM modelling groups, the community aims to deliver a new generation of emission scenarios, with greater consistency in the treatment of human land-use across the two modelling platforms.

The project cluster is developing a policy brief to follow-up on our meeting, in early 2024.