Narrowed uncertainty in future global temperature and remaining carbon budget

<h2>Summary</h2> Reducing uncertainty in future temperature projections is crucial for understanding climate impacts of anthropogenic CO₂ emissions and for guiding effective climate policy. This remains a complex challenge, as uncertainty in temperature projections arises from both climate and carbon-cycle feedbacks. Here, we introduce an observational constraint that integrates these feedbacks into emissions-driven simulations, reducing uncertainties in projected global temperature and effective transient climate response to cumulative CO₂ emissions (eTCRE). Our approach lowers the projected mid-21st century eTCRE from 2.2°C (1.3°C–3.1°C) to 1.9°C (1.3°C–2.5°C) per 1,000 GtC. It also reduces projected end-of-century global temperature from 4.6°C (2.8–6.4°C) to 4.2°C (2.9°C–5.4°C) and refines estimates of the remaining carbon budget since 2020 for limiting warming to 2°C from 352.2 (2.1–702.3) to 458.9 (251.4–666.3) GtC under high-emission representative concentration pathway (RCP)8.5/shared socioeconomic pathway (SSP)5–8.5 scenarios. These refined projections provide critical insights for policymakers, enabling more effective mitigation strategies.