Why 2025 Was So Hot, Even Without El Niño

The year 2025 should have been a slightly cooler one. Instead, it ended up as the third hottest year ever recorded. This week, the main climate-monitoring groups in Europe and America released their yearly reports, and the message was the same in each one: the Earth is warming faster than before.

All eleven of the past eleven years are the warmest since records began, and the top three spots all belong to the most recent three. The hottest of all was 2024, which lined up with a strong El Niño—a Pacific Ocean pattern that pushes temperatures upward—and with the brightest point of the 11-year solar cycle. In 2025 that El Niño tailed off and was replaced by its opposite, La Niña, which usually cools the planet down.

By every reasonable expectation, 2025 should have been clearly cooler than 2024. It was—but only slightly. Compared to other La Niña years, it was extraordinary. The last La Niña year, 2022, was 1.15°C warmer than the world's pre-industrial average. Last year was 1.44°C warmer, according to the World Meteorological Organisation. The average warming over the past three years has sat between 1.48°C and 1.5°C—almost touching the limit set in the Paris agreement.

Many scientists are still cautious about announcing that warming is speeding up. They remember the early 2000s, when global temperatures stayed lower than the climate models predicted. That period was nicknamed the climate hiatus, and at the time, some people argued that climate change had stopped. In reality, several natural cycles had simply combined to cool things briefly before warming continued.

Today, however, several signs point to a real acceleration. The first is the most obvious one: greenhouse-gas emissions, especially carbon dioxide, are still rising rather than falling. The second is more surprising. A different kind of pollution—tiny sulphate particles released by ships and coal-fired power plants—is now decreasing because of stricter rules on air quality. Sulphates are dangerous for human health, so removing them is good news. But they also reflect sunlight back into space, so cleaner air actually allows more heat to reach the Earth's surface.

There is also a growing debate about how strongly the climate reacts to extra greenhouse gases. A new study from the University of Exeter suggests this reaction may be at the higher end of what scientists thought possible. If the study is correct, global temperatures could pass 2°C by the middle of this century. That matters because climate models show the danger of tipping points—sudden, hard-to-reverse changes—rises sharply once warming goes beyond 1.5°C.

The extremes of 2025 made the year stand out further. In February, sea ice at both poles dropped to its lowest level since satellite measurements began in the late 1970s, and Antarctica had its hottest year on record. In Europe, hot and windy conditions spread wildfires across Spain and Portugal in late summer. These fires released nearly 14 million tonnes of carbon—more than any year before—and added black soot that absorbs even more sunlight.

If the trend of the past 30 years continues, the world could cross the 1.5°C limit before the end of this decade. Carlo Buontempo, who runs Europe's Copernicus Climate Change Service, says the question is no longer whether we will pass 1.5°C, but how we will manage life beyond it. Forecasters already expect El Niño to return later this year, so 2026 is likely to add yet another hot year to the list.