Emergent constraints on climate sensitivity and recent record-breaking warm years

Abstract. The Earth’s climate sensitivity remains a significant source of uncertainty in climate projections. A key metric is the Transient Climate Response (TCR), which incorporates aspects of both the Equilibrium Climate Sensitivity (ECS) and ocean heat uptake, and correlates well with historical global warming simulated by Earth System Models (ESMs). CMIP6 ESMs display a wider range of TCR values compared to earlier phases, with many exceeding the IPCC AR6 very likely (90 % confidence) range of 1.2–2.4 K. These high-sensitivity models also predict that warming will exceed the 2 °C Paris climate agreement limit, even under the relatively low emissions SSP1-2.6 scenario. Record global temperatures in 2023 and 2024 highlight how close the world already is to 1.5 °C of warming, raising doubts about whether the 2 °C limit remains within reach. Here, we use the latest observational data to update emergent constraints on TCR and projected warming. We estimate a TCR of 1.81 K with a very likely range of 1.28 K to 2.33 K, which represents a small increase compared to estimates that use observational data through to 2019. Furthermore, we find that warming projections constrained by data through to 2024 fall within the low to mid-range of CMIP6 ESM projections for both the mid- and late-21st century, indicating that limiting global warming to below 2 °C remains feasible.