Lee de Mora

When anthropogenic CO ₂ emissions cease, global temperature does not stabilize immediately. The resulting global temperature change, the zero emissions commitment, is central to carbon budget assessments and understanding reversibility of global warming, yet its dependence on prior warming levels remains unclear. Here we show, using zero-emission simulations with the UK Earth System Model (UKESM1.2) across global warming levels of 1.5–5 K, that the zero emissions commitment stays near zero up to 2 K but becomes positive at higher warming levels, reaching 0.4–0.6 K after ~ 150 years. This dependence indicates that stabilizing or reversing warming becomes progressively more difficult as peak temperature rises. We identify the mechanism behind this behaviour: stronger warming from higher radiative disequilibrium and weakening carbon sinks, driven by saturated land carbon uptake and the reduced radiative impact of CO ₂ drawdown at higher background CO ₂ concentrations. Higher peak warming therefore commits the planet to additional warming, limiting temperature-overshoot reversibility.

The North Atlantic phytoplankton bloom (NAB) forms the base of the food web upon which marine ecosystem services throughout the northern North Atlantic rely. The NAB's nutrient supply is predominantly driven by deep winter mixing, which is most pronounced in the subpolar gyre where it has been projected to abruptly decline within the next few decades. Here we use idealized experiments with an Earth system model to investigate how mixed layer depth, surface nutrients and chlorophyll concentrations over the gyre may respond to positive carbon emissions followed by both zero and negative emissions mitigation efforts initiated at various Global Warming Levels (GWLs). Results show a decline in all three variables during positive emissions, with the warmer the GWL the less likely the prospect of recovery back to pre‐industrial conditions. In the scenario where carbon emissions are held at zero once reaching a GWL of 1.5°C, the NAB started to recover toward preindustrial conditions. However, zero emissions after 2.5°C and warmer showed a continued decline in the NAB. These results suggest a tipping threshold may lie between 1.5 and 2.5°C GWL, beyond which only negative emissions can achieve recovery of the NAB. Even in the lowest warming scenario examined, 1.5°C, recovery of the NAB did not occur within 5 centuries under zero emissions, nor did it occur within one century under negative emissions.

When anthropogenic CO2 emissions cease, global temperature does not stabilise immediately. The resulting global temperature change, the Zero Emissions Commitment (ZEC), is central to carbon budget assessments and understanding reversibility of global warming, yet its dependence on prior warming levels remains unclear. Here we show, using zero-emission simulations with the UK Earth System Model (UKESM1.2) across global warming levels of 1.5–6K, that ZEC stays near-zero up to 2K, but becomes positive at higher warming levels, reaching 0.2–0.6K after ~150 years. This dependence indicates that stabilising or reversing warming becomes progressively more difficult as peak temperature rises. We identify the mechanism behind this behaviour: stronger warming from higher radiative disequilibrium and weakening carbon sinks, driven by saturated land carbon uptake and the reduced radiative impact of further CO2 drawdown. Higher peak warming therefore commits the planet to additional warming, limiting temperature-overshoot reversibility.

The North Atlantic phytoplankton bloom (NAB) forms the base of the food web upon which marine ecosystem services throughout the northern North Atlantic rely. The NAB’s nutrient supply is predominantly driven by deep winter mixing, which is most pronounced in the sub-polar gyre (SPG) where it has been predicted to collapse within the next few decades. Here we use idealized experiments with an Earth system model to investigate how mixed layer depth, surface nutrients and chlorophyll concentrations over the SPG may respond to positive carbon emissions followed by both zero and negative emissions mitigation efforts initiated at various global warming levels (GWLs). Results show a decline in all three variables during positive emissions, with the warmer the GWL the less likely the prospect of recovery back to pre-industrial conditions. In the scenario where carbon emissions are held at zero once reaching a GWL of 1.5°C, the NAB starts to recover and is extrapolated to return to preindustrial levels only after several centuries to millennia. However, zero emissions after 2.5°C and warmer shows a continued decline in the NAB. These results suggest a tipping threshold may lie between 1.5-2.5°C GWL, beyond which only negative emissions can achieve recovery of the NAB. Even in the lowest warming scenario examined, 1.5°C, recovery of the NAB under negative emissions does not occur within one century.

To address shortcomings of global climate models (CMIP6) and project the impacts of climate change in the Atlantic Ocean with focus on regional scales, we downscale globally a 4-member ensemble of future climate projections (2 emission scenarios and 2 global climate models) with a NEMO-ERSEM coupled hydrodynamic-ecosystem ocean model. In our global ocean downscaling of climate projections, the resolution for the ocean is increased to ¼ of a degree and the river runoffs are represented more realistically in an attempt to better resolve the shelf break and the exchange of water between the shelf break and the deep ocean, as well as the influence of this exchange on both large and regional scales. Here, using the projections from our global ocean downscaling we investigate the cross shelf-open ocean volume, heat and salt transports along the Atlantic Ocean margins and how they may change in the near future (up to year 2070). We also present preliminary analysis for the role of changes in the large-scale circulation patterns versus changes in the regional water content/properties (e.g., heat and salt) on driving changes in the shelf-open ocean exchange, with a particular interest in the North Atlantic western boundary region and the Gulf Stream.

The Zero Emissions Commitment (ZEC) describes the climate response following the cessation of emissions and is critical for understanding long-term climate projections and remaining carbon budgets. Using simulations from the UK Earth System Model (UKESM), we explore ZEC behaviour across stabilized warming levels (WLs) following the protocol developed for TIPMIP. UKESM simulations reveal a strong dependence of ZEC on WL: while ZEC is near zero for WL

The latest generation CMIP6-class Earth system models (ESMs) are a great tool for projecting climate variability on multi-centennial and global scales as they are designed to explicitly represent the process-coupling amongst the different Earth system components (atmosphere, ocean, land, cryosphere, biosphere) and prioritise system robustness such as minimisation of drift. However, CMIP6-ESMs do not accurately represent the fine-scale circulation and water-masses in ocean margins and shelf seas since by design: (i) their resolution is too coarse and so they only implicitly include regional-scale processes or even exclude these processes (particularly shelf-seas related processes); and (ii) their initialisation from a steady state leads to their divergence from reality and present-day conditions. To address these shortcomings and project the impacts of climate change in the Atlantic Ocean with focus on regional scales, we downscale globally an ensemble of future ocean projections with a NEMO-ERSEM coupled hydrodynamic-ecosystem model. Here, we discuss the design-methodology for our global ocean downscaling experiment: (i) selection of future scenarios, (ii) initialisation from “real” ocean conditions, (iii) selection of the CMIP6-ESMs atmospheric conditions to force our model based on their realism and uncertainty span, and (iv) treatment of the river runoffs as to impose both a realistic rivers state and a future trend consistent with CMIP6-ESMs. Comparisons of our global ocean downscaling simulations to CMIP6-ESMs during the historical period demonstrate their added value in terms of representation of physical ocean conditions and circulation in the Atlantic Ocean. We also present preliminary analysis in terms of future trends in temperature, salinity and circulation patterns in the Atlantic Ocean, with focus on regional features like changes in the Gulf Stream and trends in coastal regions.

This is the first projection of marine circulation and biogeochemistry for the Ascension Island Marine Protected Area (AIMPA). Marine Protected Areas are a key management tool used to safeguard biodiversity, but their efficacy is increasingly threatened by climate change. To assess an MPA's vulnerability to climate change and predict biological responses, we must first project how the local marine environment will change. We present the projections of an ensemble from the Sixth Coupled Model Intercomparision Project. Relative to the recent past (2000–2010), the multi‐model means of the mid‐century (2040–2050) project that the AIMPA will become warmer (+0.9 to +1.2°C), more saline (+0.01 to +0.10), with a shallower mixed layer depth (−1.3 to −0.8 m), a weaker Atlantic Equatorial Undercurrent (AEU) (−1.5 to −0.4 Sv), more acidic (−0.10 to −0.07), with lower surface nutrient concentrations (−0.023 to −0.0141 mmol N m⁻³ and −0.013 to −0.009 mmol P m⁻³), less chlorophyll (−6 to −3 µg m⁻³) and less primary production (−0.31 to −0.20 mol m⁻² yr⁻¹). These changes are often more extreme in the scenarios with higher greenhouse gases emissions and more significant climate change. Using the multi‐model mean for two scenarios in the years 2090–2100, we assessed how five key ecosystem services in both the shallow subtidal and the pelagic zone were likely to be impacted by climate change. Both low and high emission scenarios project significant changes to the AIMPA, and it is likely that the provision of several ecosystem services will be negatively impacted.

We show that the distribution of anthropogenic carbon between the atmosphere, land surface, and ocean differs with the choice of projection scenario even for identical changes in mean global surface temperature. Warming thresholds occur later in lower-CO₂-emissions scenarios and with less carbon in the three main reservoirs than in higher-CO₂-emissions scenarios. At 2 ^∘C of warming, the mean carbon allocation differs by up to 63 PgC between scenarios, which is equivalent to approximately 6 years of the current global total emissions. At the same warming level, higher-CO₂-concentration scenarios have a lower combined ocean and land carbon allocation fraction of the total carbon compared to lower-CO₂-concentration scenarios. The warming response to CO₂, quantified as the equilibrium climate sensitivity, ECS, directly impacts the global warming level exceedance year and hence the carbon allocation. Low-ECS models have more carbon than high-ECS models at a given warming level because the warming threshold occurs later, allowing more emissions to accumulate. These results are important for carbon budgets and mitigation strategies as they impact how much carbon the ocean and land surface could absorb at a given warming level. Carbon budgeting will be key to reducing the impacts of anthropogenic climate change, and these findings could have critical consequences for policies aimed at reaching net zero.

The anthropogenic carbon distribution between the atmosphere, land surface and ocean varies significantly with the choice of scenario for identical changes in mean global surface temperature. Moving to a lower CO₂ emissions scenario means that warming levels occur later, and with significantly less carbon in the three main carbon reservoirs. After 2 °C of warming, the multi-model mean ocean allocation can be up to 3 % different between scenarios, or 36 Pg in total with an even larger difference in some single model means. For the UKESM1 model, the difference between the minimum and maximum atmospheric fraction at the 2 °C Global Warming Level (GWL) is 3.6 %. This is equivalent to 50 Pg of additional carbon in the atmosphere, or the equivalent of five years of our current global total emissions. In the lower CO₂ concentration scenarios, SSP1-1.9 and SSP1-2.6, the ocean fraction grows over time while the the land surface fraction remains constant. In the higher CO₂ concentration scenarios, SSP2-4.5, SSP3-7.0 and SSP5-8.5, the ocean fraction remains constant over time while the the land surface fraction decreases over time. Higher equilibrium climate sensitivity (ECS) models reach the GWLs sooner, and with lower atmospheric CO₂ than lower sensitivity models. However, the choice of scenario has a much larger impact on the percentage carbon allocation at a given warming level than the individual model's ECS.