Timothy Andrews

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.

Earth's climate stability, characterized by the global radiative feedback parameter , varies decadally due to changing surface temperature patterns. Recent variations in are poorly understood as coordinated model simulations typically end in 2014. We apply a convolutional neural network trained on climate model simulations to observation‐based surface temperature reconstructions to estimate variations in up to 2025. We find that reached a minimum (maximum stability) around the mid 1990s (−3 Wm ⁻² /K), but has since weakened significantly (−2 Wm ⁻² /K). We confirm these results with climate model simulations extended to 2022. The recent weakening is not significantly affected by El Niño Southern Oscillation or Pacific Decadal Oscillation. Attribution reveals that warming in the subtropical Northeast Pacific is an important driver of the recently weakened feedback, confirmed by targeted experiments in E3SMv2. Our approach enables near real‐time monitoring of Earth's climate stability.

Paleoclimates provide examples of past climate change that inform estimates of modern warming from greenhouse-gas emissions, known as Earth’s climate sensitivity. However, differences between past and present climate change must be accounted for when inferring climate sensitivity from paleoclimate evidence. The closest paleoclimate analog to near-term warming from greenhouse-gas emissions is the Pliocene (5.3 to 2.6 Ma), a warm epoch with atmospheric CO ₂ concentrations similar to today. Recent reconstructions indicate the Pliocene was 1 °C warmer than previously thought, implying higher climate sensitivity, which is also supported by recent reconstructions showing more cooling with reduced CO ₂ at the Last Glacial Maximum (LGM; 19 to 23 thousand years ago). However, large-scale patterns of paleoclimate temperature change differ strongly from modern projections. Climate feedbacks and sensitivity depend on temperature patterns, and such “pattern effects” must be accounted for when using paleoclimates to constrain modern climate sensitivity. Here we combine data-assimilation reconstructions with atmospheric general circulation models to show Earth’s climate is more sensitive to Pliocene forcing than modern CO ₂ forcing. Pliocene ice sheets, topography, and vegetation alter patterns of ocean warming and excite destabilizing cloud feedbacks, and LGM feedbacks are similarly amplified by the North American ice sheets. Accounting for paleoclimate pattern effects produces a best estimate (median) for modern climate sensitivity of 2.8 °C and 66% CI of 2.4 to 3.4 °C (90% CI: 2.1 to 4.0 °C), substantially reducing uncertainty in projections of 21st-century warming.

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.

Global warming results from anthropogenic greenhouse gas emissions which upset the delicate balance between the incoming sunlight, and the reflected and emitted radiation from Earth. The imbalance leads to energy accumulation in the atmosphere, oceans and land, and melting of the cryosphere, resulting in increasing temperatures, rising sea levels, and more extreme weather around the globe. Despite the fundamental role of the energy imbalance in regulating the climate system, as known to humanity for more than two centuries, our capacity to observe it is rapidly deteriorating as satellites are being decommissioned.

Between 1985 and 2014 observations show an expansion of Southern Ocean sea‐ice. This phenomena is not simulated in CMIP6 Atmosphere‐Ocean General Circulation Models (AOGCMs). Here we quantify the impact of this discrepancy on radiative feedback and simulated global temperature trends. We find that both satellite reconstructions of the Earth's energy budget and atmosphere‐only GCM simulations forced with observed Sea Surface Temperature (SST) and sea‐ice trends support the hypothesis of a negative surface albedo feedback over the Southern Ocean. In contrast, declining sea‐ice in AOGCMs gives rise to a positive feedback. We estimate that had the AOGCMs simulated the observed Southern Ocean sea‐ice evolution their global feedback parameter would be less positive by 0.07–0.23 (which is 12%–29% of the total difference between AOGCM simulations and simulations forced with observed SST and sea‐ice) and their global temperature trend over this 30 year period reduced by 0.01–0.06 K .

Between 1985 and 2014 observations show an expansion of Southern Ocean sea ice. This phenomena is not simulated in CMIP6 Atmosphere-Ocean General Circulation Model (AOGCM) simulations, nor in a large ensemble of HadGEM3-GC3.1-LL simulations. Here we quantify the impact of this discrepancy on radiative feedback and global temperature trends. We find that both satellite reconstructions of the Earth’s energy budget and atmosphere-only GCM simulations forced with observed SST and sea-ice trends support the idea that the expansion of sea-ice led to a negative surface albedo feedback over the Southern Ocean. In contrast, the decline of sea-ice in AOGCMs gives rise to a positive feedback. We estimate that, had the AOGCMs warmed with the observed Southern Ocean sea-ice evolution, their global feedback parameter would be less positive, by 0.15 to 0.36 Wm−2K−1, and their global temperature trends reduced by 0.072 to 0.02 K per decade over this period.

Since the 5th Assessment Report of the Intergovernmental Panel on Climate Change (AR5) an extended concept of the energetic analysis of climate change including forcings, feedbacks and adjustment processes has become widely adopted. Adjustments are defined as processes that occur in response to the introduction of a climate forcing agent, but that are independent of global‐mean surface temperature changes. Most considered are the adjustments that impact the Earth energy budget and strengthen or weaken the instantaneous radiative forcing due to the forcing agent. Some adjustment mechanisms also impact other aspects of climate not related to the Earth radiation budget. Since AR5 and a following description by Sherwood et al. (2015, https://doi.org/10.1175/bams‐d‐13‐00167.1), much research on adjustments has been performed and is reviewed here. We classify the adjustment mechanisms into six main categories, and discuss methods of quantifying these adjustments in terms of their potentials, shortcomings and practicality. We furthermore describe aspects of adjustments that act beyond the energetic framework, and we propose new ideas to observe adjustments or to make use of observations to constrain their representation in models. Altogether, the problem of adjustments is now on a robust scientific footing, and better quantification and observational constraint is possible. This allows for improvements in understanding and quantifying climate change.