TL;DR
Copernicus data show 2025 will finish as the second-warmest year on record, following 2024, and the last three years are the hottest ever measured. Scientists are investigating multiple drivers — a 2022 volcanic eruption, increased solar output, a strong El Niño and falling sulfur dioxide pollution — but no single factor fully explains the recent jump in temperatures.
What happened
According to the Copernicus Climate Change Service, 2025 is on track to be the second-warmest year in the instrumental record, behind 2024. The agency reports that each of the last three years has exceeded 1.5°C above preindustrial averages, temporarily putting the planet past that international threshold. Observed warming in the recent period has outpaced many climate model projections, prompting researchers to examine several possible contributors. In a Carbon Brief analysis, climate scientist Zeke Hausfather reviewed four candidates: the 2022 Hunga Tonga–Hunga Ha‘apai eruption, which injected water vapor into the upper atmosphere; a modest rise in solar output; the emergence of a powerful El Niño in late 2023; and a substantial decline in sulfur dioxide emissions, including large drops from coal-fired sources over the past two decades and a 2020 international rule that reduced sulfur emissions from shipping. Studies differ on how much each factor contributed, and some work suggests shipping reductions alone could explain much of the recent warming.
Why it matters
- Each of the past three years exceeding 1.5°C above preindustrial levels signals a temporary breach of an important international warming limit.
- Observed warming that outstrips model predictions raises questions about model sensitivity and near-term climate variability.
- Reductions in sunlight-blocking pollutants such as sulfur dioxide can have near-term warming effects, complicating mitigation trade-offs.
- Uncertainty over whether recent warming is transient or indicates an acceleration of long-term warming affects planning for adaptation and emissions policy.
Key facts
- Copernicus Climate Change Service indicates 2025 will be the second-hottest year on record, after 2024.
- The last three years are the warmest measured in the instrumental record and each exceeded 1.5°C above preindustrial averages.
- Recent observed warming has been larger than many climate models projected.
- Zeke Hausfather analyzed four potential drivers of the surge: the 2022 Hunga Tonga–Hunga Ha‘apai eruption, increased solar output, a strong El Niño in late 2023, and a drop in sulfur dioxide emissions.
- The Hunga Tonga–Hunga Ha‘apai eruption injected large amounts of water vapor into the upper atmosphere, which can trap heat.
- Solar output rose modestly on a timeline that aligns with recent warming, but these factors together explain less than half of the temperature jump in Hausfather’s assessment.
- A strong El Niño likely contributed to exceptional warmth in 2024 but does not account for the early-2023 temperature increase.
- Sulfur dioxide emissions have declined roughly 40% over the past 18 years, and a 2020 international rule significantly reduced shipping sulfur emissions.
- Research on the impact of reduced shipping pollution varies: most studies find modest temperature effects, though one study by James Hansen concluded the drop in shipping pollution could explain most of the recent warming.
What to watch next
- Whether temperatures in coming years return below the 1.5°C threshold or remain elevated — Hausfather highlights this as an open question.
- Peer-reviewed assessments quantifying the combined contribution of volcanoes, solar variability, El Niño and aerosol declines to the recent warming surge.
- Ongoing monitoring of sulfur dioxide emissions from power generation and shipping, and any policy changes that could alter aerosol concentrations.
- not confirmed in the source
Quick glossary
- Preindustrial baseline: A reference period, often mid- to late-19th century, used to measure how much global temperatures have risen due to human activities.
- El Niño: A natural climate pattern featuring unusually warm ocean temperatures in the eastern tropical Pacific that can raise global average temperatures and alter weather patterns.
- Sulfur dioxide (SO2): A pollutant emitted by burning fossil fuels that forms aerosols which reflect sunlight and can have a cooling effect on climate.
- Climate models: Computer simulations that use physics and observations to project how the climate system responds to greenhouse gases, aerosols and other forcings.
Reader FAQ
Was 2025 the hottest year on record?
No. The source reports 2025 will conclude as the second-hottest year, with 2024 remaining the warmest.
Have we exceeded 1.5°C of warming?
According to Copernicus data cited in the source, each of the last three years measured more than 1.5°C above preindustrial levels, temporarily breaching that benchmark.
What caused the sudden warming surge?
Hausfather’s analysis examined four potential contributors — the 2022 Hunga Tonga eruption, a rise in solar output, the late-2023 El Niño and declines in sulfur dioxide emissions — but no single factor fully explains the surge.
Is the recent warming permanent or temporary?
Not confirmed in the source.
E360 DIGEST DECEMBER 30, 2025 2025 Was Another Exceptionally Hot Year PEXELS This year will conclude as the second hottest on record, surpassed only by 2024. It continues a recent…
Sources
- 2025 Was Another Exceptionally Hot Year
- 2025 was one of three hottest years on record, scientists say
- 2025 set to be second or third warmest year on record …
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