Study examines whether the recent year of global temperatures 1.5°C above pre-industrial levels signals an earlier exceedance of the Paris Agreement goals.

• The study examines whether 12 consecutive months with average global surface temperatures 1.5°C above pre-industrial levels means exceeding the Paris Agreement goal of limiting long-term warming below this threshold.
• The analysis is based on climate model projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6), which are calibrated based on consistency with the assessed distribution of Earth's climate sensitivityThe models are weighted to reduce bias, especially due to the overrepresentation of "hot models".
• The findings suggest that 12 consecutive months of 1.5°C warming typically occur after the Paris Agreement threshold of 1.5°C is reached in archived simulationsThe median time remaining until the long-term exceedance in the CMIP6 calibrated set is -33 months. This suggests that with a similar chain of anomalies, the Paris Agreement threshold has probably already been exceeded.
• The study considers the impact of El Niño on the time course of short-term and long-term 1.5°C exceedances using a large set of simulations from the Community Earth System Model v.2 (CESM2). Taking into account strong El Niño conditions reduces the simulated probability that a short-term exceedance will occur only after the long-term threshold of the Paris Agreement has been reached.
• An important factor is whether the processes associated with 12 consecutive months at or above 1.5°C in the models are the same as in the real world. If not, then the real-world exceedance would not necessarily be analogous to the exceedance in the models. Potential candidates to explain the difference between observations and models include the impact of the Tonga volcano eruption in 2022 and the introduction of shipping regulations in 2020.
• The analysis shows that even when taking into account recent El Niño conditions, warming of 1.5°C occurs over 12 consecutive months, typically after the Paris Agreement threshold of 1.5°C is reached in archived simulations.
• The document emphasizes the importance of faster incorporation of current or updated scenarios into operational modeling.

Spring

Glossary of Key Terms

• Paris Agreement: A global agreement on climate change that aims to limit global warming to well below 2°C, while pursuing 1.5°C above pre-industrial levels.
• Global Average Surface Temperature (GSAT): The average temperature of the Earth's surface, used as a key indicator of global warming.
• Pre-industrial levels: Temperature levels before the start of large-scale industrial development (it is assumed that this is the period 1850-1900).
• 20-year moving average: An average of values over a 20-year period, which is gradually shifted forward and is used to smooth out short-term fluctuations and determine long-term trends in temperatures.
• CMIP6 (Coupled Model Intercomparison Project Phase 6): A framework for coordinated climate modeling that provides projections of future climate based on different emissions scenarios.
• SSP (Shared Socioeconomic Pathways): Scenarios that describe possible future developments in society, demography, and economics, used in climate models to examine various climate impacts.
• Model calibration: A technique used to adjust climate models by weighting them based on their consistency with the observed distribution of Earth's climate sensitivity, with the aim of reducing bias caused by the overrepresentation of "hot models".
• Climate sensitivity: The rate of global temperature warming due to a doubling of CO2 concentration in the atmosphere.
• El Niño/Southern Oscillation (ENSO): Natural climate variation in the Pacific Ocean that can affect global temperatures.
• Oceanic Niño Index (ONI): A standardized index for monitoring sea surface temperatures in the Niño 3.4 region of the Pacific Ocean, used to identify and measure El Niño and La Niña events.
• ERA5: Global reanalysis of data from the Copernicus Climate Change Service (C3S), providing a comprehensive record of atmospheric, land and ocean climate variables.
• CESM2 (Community Earth System Model v.2): A climate model used to simulate the Earth's climate system.
• Radiative forces: Factors that affect the balance of incoming and outgoing energy of the Earth, such as volcanic eruptions and changes in albedo (the reflectivity of the Earth's surface).
• Probability of exceedance: The probability that a certain threshold will be reached or exceeded, in this case the 1.5°C global warming threshold.