Quantification of the influence of anthropogenic and natural factors on the record-high temperatures in 2023 and 2024

Abstract. The unexpectedly high global mean surface temperature (GMST) anomalies in 2023 and 2024 greatly exceeded the temperatures observed in the years directly prior. In this paper, we use a multiple linear regression energy balance model to quantify the contributions of several natural and anthropogenic factors to the GMST, including the large reduction of sulfur emissions from the shipping sector since 2020. The model is trained on 170 years of historical climate data, and allows for the attribution of warming to various natural and anthropogenic factors. The influence of anthropogenic activity on the GMST is quantified using a 160,000 member ensemble that considers the uncertainty in the magnitude of aerosol radiative forcing and the strength of climate feedbacks. We find that in response to a rise in global radiative forcing of either 0.1 W m⁻² or 0.15 W m⁻² due to the reduction of sulfur emissions from international shipping, the associated rise in GMST by the end of 2024 is either 0.028 °C [0.025 to 0.031 °C, 5−95 % range] or 0.043 °C [0.038 to 0.046 °C], respectively. We also show that approximately 0.092 °C of the rise in annual mean GMST from 2022 to 2023 can be attributed to a shift from La Niña to El Niño conditions, which is approximately a third of the observed 0.3 °C rise in GMST between these two years. Additional increases in the annual mean GMST in 2023 and 2024 (both relative to 2022) of 0.075 °C [0.036 to 0.096 °C] and 0.053 °C [0.019 to 0.074 °C] are attributed, respectively, to a strong positive Indian Ocean Dipole (IOD) event that began in 2023. Our study is the first to suggest a significant contribution from the IOD to the anomalously high values of GMST observed in 2023 and 2024. Anomalously high Sea Surface Temperatures (SSTs) in the North Atlantic region led to a rise in GMST of 0.070 °C [0.054 to 0.094 °C] and 0.069 °C [0.055 to 0.091 °C] in 2023 and 2024 relative to 2022, respectively. This contribution is almost 90 % lower when the short-term variability component of North Atlantic SSTs is removed, resulting in lower estimates of the GMST anomaly in 2023 and 2024 than observed. These results suggest that short-term variability in the North Atlantic SSTs may have played a significant role in influencing the GMST anomalies in both 2023 and 2024; however, it is unclear whether this variability is internally or externally forced. Increased incoming solar radiation due to the 11-year solar cycle led to an additional rise in GMST of 0.025 °C [−0.009 to 0.051 °C] and 0.029 °C [−0.008 °C to 0.056 °C] in 2023 and 2024 relative to 2022, respectively. While the 2023 and 2024 GMST anomalies can be reconstructed fairly well from a combination of natural and anthropogenic factors, uncertainties remain in the reconstruction, driven primarily by the imprecise knowledge of the radiative forcing of aerosols, and the strength of climate feedbacks.