29 April 2025 by David Spratt, first published at Pearls and Irritations
The record-breaking warming years of 2023 (1.5°C) and 2024 (1.6°C) were above expectations and shocked scientists. Their responses and the subsequent research are a good example of how quickly the physical reality is changing, driving new and contested understandings.
In late 2023, as global and ocean temperatures soared, the most upfront assessment came from Zeke Hausfather: “Staggering. Unnerving. Mind-boggling. Absolutely gobsmackingly bananas.” It was a widely-shared view, with responses such as “unprecedented” and “frightening”. “We’re not as aware of what’s coming as we thought we were,” said Sarah Perkins-Kirkpatrick of the University of NSW…
…Hansen’s views are sometimes portrayed as outliers because he defies group-think, yet he has an impeccable record. For a long time, Hansen has been saying that the impact of sulphate aerosols — which are a byproduct of burning fossil fuels, cause acid rain, and have a strong but short-term cooling effect by reducing incoming radiation — is much greater than generally stated. A higher aerosol forcing implies higher climate sensitivity, which is a measure of the Earth’s temperature response to increase greenhouse gases, and a higher forcing means more future warming than in conventional climate modelling.
Whilst the orthodox estimates for the aerosol impacts are around 0.5°C of cooling, Hansen and his colleagues say it is likely above 1°C. More on Hansen’s analysis may be found in the 2023 paper “Global warming in the pipeline”, which former UK Chief Scientist Sir David King says is “one of the most important published on the state of the climate crisis in years”.
In Hansen’s view, the efforts to clean up maritime shipping emissions by mandating emissions with much lower sulphur content resulted in the “Faustian bargain” being exposed: as the sulphate cooling impact has reduced — particularly in the North Atlantic which is the world’s busiest shipping route — greater warming has been revealed.
Then, in late 2024, researchers announced that the unexpected jump in warming could be primarily explained by “a record-low planetary albedo” (less reflection of incoming solar radiation) that is “quite obviously linked to cloud changes and, in particular, low cloud changes” in the northern mid-latitudes and tropics, in continuation of a multi-annual trend, due to “internal variability, reduced aerosol concentrations, or a possibly emerging low-cloud feedback”…
…We have reached what Hansen has called an “acid” test of his interpretation, which 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”, due to both the reduction of sulfate aerosols over the ocean remaining in place, and to high climate sensitivity which implies that the warming from recently added forcings is still growing significantly…
…The evidence so far in 2025 leans Hansen’s way. January was 1.75°C, February was 1.59°C and March 1.6°C, an average of 1.65°C for the first quarter of the year. Even though the El Nino was fading by April 2024, the 12-month period of April 2024 – March 2025 was 1.59°C above the pre-industrial level…
…The “Pipeline” paper warns that “we are in the early phase of a climate emergency” and that acceleration in warming is “dangerous in a climate system that is already far out of equilibrium. Reversing the trend is essential – we must cool the planet – for the sake of preserving shorelines and saving the world’s coastal cities” (emphasis added).
Read the full article here.
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