Near-Term Inevitability, Long-Term Choice: First Exceedances of Global Warming Thresholds with CMIP6

The year 2024 marked the first time that the Earth’s average temperature exceeded 1.5°C, sparking critical questions about the accuracy of climate model projections and the implications for long-term climate goals. Here, we systemat- ically quantify the timing and likelihood of first exceedance for global warming threshold from 1.5°C to 3°C across a large CMIP6 ensemble and a range of emission scenarios. We find that the CMIP6-projected 1.5°C crossing aligns with observations, particularly in models with higher near-future warming trends, and is independent of emission scenarios. While a pre-mid-century 2°C crossing is difficult to avoid even under stringent mitigation, the influence of emission reduc- tions becomes increasingly pronounced at higher warming levels, delaying a 3°C world by up to 20 years. Our findings underscore that while near-term warm- ing is likely inevitable, immediate and substantial mitigation is essential to buy invaluable time for adaptation and to prevent a future with dangerous warming levels.