Evaluating the 1.5 °C or 2 °C climate thresholds: when could these thresholds be surpassed?

Abstract The 2024 global mean temperature exceeded the values for all previous years in the instrumental record. Four of five commonly used datasets also exceeded the 1.5 °C warming level for that 12 month period relative to the 19th-century temperature record. The 2025 global temperature was lower in all datasets and below 1.5 °C, due to the transition from El Niño to La Niña. In this study, we ask: when will the 31 year centered global mean surface temperature anomaly relative to 1850–1900 first reach 1.5 and 2.0 °C? Examining available analyses of the temperature records, near-term future global temperatures are evaluated using an observation-based approach, extrapolating historical trends using both 20 year and 30 year linear and a 55 year quadratic method, and a LOESS-based nonlinear sensitivity test. These analyses provide a unique look at the temperature record, indicating that global temperature has not yet exceeded the 1.5 °C threshold for climate time scales but that this will likely occur sooner than prior estimates, perhaps as early as 2026 and likely between 2028 and 2032. Estimated exceedance of the 2.0 °C threshold exhibits a larger range, from the early 2040s to the mid 2050s. This is primarily dependent on whether the recent apparent acceleration of warming persists or is a transient phenomenon. The quadratic method is preferred for projecting temperature because it incorporates both steady quasi-linear multi-decadal trends and the recent apparent accelerated warming through the additional degree of freedom. The LOESS sensitivity analysis yields threshold crossing years very similar to those from 55 year quadratic method. The simplicity and robustness of the method used to project exceedances promote effective communication to diverse audiences.