Figure 1. Global Surface Temperature Relative to 1880-1920¹


The Acid Test: Global Temperature in 2025

James Hansen and Pushker Kharecha 20 February 2025

The unprecedented leap of global temperature in 2023 and early 2024 exceeded 0.4°C (Fig. 1). We and coauthors² interpret that uniquely large warming as being due about equally to a moderate El Nino and reduction of ship aerosols, with a smaller contribution from the present solar maximum (our entire paper, including Abstract & Supplementary Material is available in a single compressed PDF here). An “acid” test of our interpretation will be provided by the 2025 global temperature: unlike the 1997-98 and 2015-16 El Ninos, which were followed by global cooling of more than 0.3°C and 0.2°C, respectively, we expect global temperature in 2025 to remain near or above the 1.5°C level. Indeed, the 2025 might even set a new record despite the present weak La Nina. There are two independent reasons. First, the “new” climate forcing due to reduction of sulfate aerosols over the ocean remains in place, and, second, high climate sensitivity (~4.5°C for doubled CO₂ ) implies that the warming from recently added forcings is still growing significantly.

The impact of high climate sensitivity warrants clarification. High climate sensitivity implies a large contribution from amplifying feedbacks: water vapor, surface albedo (sea ice/snow) and clouds. The feedbacks do not come into play immediately in response to a climate forcing, but rather in response to the global warming caused by the forcing. That warming takes time, and it takes longer for higher sensitivity.³ Thus, response to a forcing in the first few years depends little on climate sensitivity, as shown by the response functions for three climate sensitivities (Fig. 2); early response is due mainly to the forcing itself, not feedbacks. But as temperature change grows, feedbacks come into play and are the main cause of the continued, growing, response.a The relevant point here is that feedbacks stretch out the response time, so, within a decade or two, higher climate sensitivity yields a significantly greater response. If climate sensitivity is 3°C or less, the rapid, early, response to the ship aerosol forcing introduced in 2020 is complete in 2025, but if climate sensitivity is high, there is still substantial “juice” in the aerosol forcing change, which can thus offset tropical cooling.ᵇ

Figure 2. Global Temperature Response to 2×CO₂


Why are we confident that climate sensitivity is high? We have shown that in three independent ways: (1) climate sensitivity 4.8°C ± 0.6°C (1𝜎 ) based on comparison of glacial and interglacial climate states,4 (2) sensitivity of 4.5°C ± 0.5°C (1𝜎 ) based on temperature from 1750 through 2024,² (3) the large “darkening” (reduced albedo) of Earth between 2000 and 2024, which implies a strong cloud feedback (Fig. 3) – and strong cloud feedback implies high climate sensitivity.²

Figure 3. Contributions to Reduced Earth Albedo


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