The past three years have been the world’s hottest on record by far, with 2025 almost tied with 2023 for second place. With that energy came extreme weather, from flash flooding to powerful hurricanes and severe droughts. Yet, by most indicators, the planet should have been cooler in 2025 than it was.

So, what happened, and what does that say about the year ahead?

As an earth and environmental scientist, I study influences that affect global temperatures year to year, such as El Niño, wildfires and solar cycles. Some make Earth hotter. Some make it cooler. And one particularly unhealthy influence has been quietly hiding a large amount of global warming – until now.

Factors that made 2025 cooler than 2024

The Earth’s climate is the result of many factors that change from year to year. Some that helped make 2025 cooler than 2024 include:

La Niña’s arrival: La Niña is part of the El Niño-Southern Oscillation, a natural climate pattern that fluctuates between warm El Niño conditions and cooler La Niña conditions. During El Niño, the Pacific Ocean heats up along the equator, influencing the atmosphere in ways that can cause intense storms, droughts and heat waves around the planet. La Niña does the opposite; it’s like putting an ice pack on the atmosphere.

Both 2023 and 2024 were El Niño years, but in 2025 conditions shifted to neutral and then to La Niña starting in September.

The solar cycle: The Sun reached its solar maximum near the end of 2024, the peak of its energy output in an approximately 11-year cycle, and began declining in 2025. So, while the sun’s output was still stronger than average in 2025, it was less than in 2024.

Fewer wildfires: Despite some destructive blazes, the world also saw fewer wildfires during 2025 than 2024, which put less carbon dioxide – a planet-warming greenhouse gas – into the atmosphere.

Despite those points, 2025 still ended up as the third-hottest year in over 175 years of record-keeping and likely one of the warmest in at least several thousand years. It was nearly as warm as 2023, at 2.6 degrees Fahrenheit (1.47 Celsius) above the 1850-1900 average, according to the European Union’s Copernicus Climate Change Service. It also had the second-highest average land temperature recorded, up 3.6 F (2 C) compared to preindustrial years, with more than 10% of the land experiencing record-high temperatures.

Factors that made 2025 warmer than expected

Several other factors made 2025 warmer than expected, and some are likely to continue to increase in 2026. They include:

Greenhouse gas emissions: The big driver of global warming is excess greenhouse gas emissions, largely from burning fossil fuels, and 2025 had plenty.

Greenhouse gases trap heat near Earth’s surface like a blanket, raising the temperature. They also linger in the atmosphere for years to centuries, meaning gases released today will continue to warm the planet well into the future. The levels of carbon dioxide, methane and nitrous oxide in the atmosphere all increased in 2025.

Rising energy demand drove an increase in fossil fuel use. About 80% of the increasing electric power demand came from emerging economies, largely for rising air conditioning demands as the world gets hotter. In the U.S., the rapid growth of data centers for AI and cryptocurrency mining helped boost U.S. carbon dioxide emissions by 2.4%.

Earth’s energy imbalance: Other sources can disrupt the natural balance between the amount of sunlight that reaches Earth and the lesser amount radiated back to space. A recent study found that Earth’s energy uptake surged and temperatures rose quickly when a rare three-year La Niña in 2020-2022 shifted to El Niño in 2023-2024.

Declining polar ice, which efficiently reflects sunlight back into space, also affects the energy balance. As sea ice declines, it leaves dark ocean water that absorbs most of the sunlight that reaches it. In a spiraling feedback, warmer water melts sea ice, allowing more sunlight into the ocean, warming it faster; 2025 had the lowest winter peak of Arctic sea ice on record and the third-lowest minimum extent of Antarctic ice.

Air pollution: Sulfate aerosol pollution from coal combustion and burning heavy fuel oil in shipping has also been affecting Earth’s energy balance. It has been masking the full effects of human-caused greenhouse gases for years by reflecting sunlight back into space, creating a cooling effect. But sulfate aerosol pollution is also a serious health hazard, blamed for about 8 million human deaths per year from lung diseases.

Recent reductions in sulfate pollution – now 40% less than 20 years ago – have meant about a 0.2 F (0.13 C) increase in global temperatures. Much of the reduction was from China’s efforts to reduce its notoriously bad air pollution in recent years and international shipping rules in effect since 2020 that have reduced sulfur emissions from large ships by 85%.

Taking all factors together, humans are now warming the planet at a faster rate than at any point in human history: at about 0.5 F (0.27 C) per decade. That extra heat can fuel extreme weather, including flash floods, heat waves, extended droughts, wildfires and coastal flooding, affecting human lives and economies.

Predictions for 2026

Most climate models predict 2026 will be about as hot as 2025, depending on whether a Pacific El Niño develops, which forecasters give about a 60% chance of happening. The planet is already starting the year out warm, even if it doesn’t feel like that everywhere. While January was very cold in parts of the U.S., globally, Earth saw its fifth-warmest January on record, and much of the western U.S. saw one of its warmest winters on record.

Solar output will continue to decrease slowly in 2026. However, the International Monetary Fund projects strong global economic growth at about 3.3%, suggesting electricity demand will also continue to grow. The International Energy Agency expects global electricity demand to increase by 3.6% per year through at least 2030.

Even though global renewable energy use is growing quickly, it isn’t growing fast enough to meet rising demand, meaning more fossil fuel use in the coming years. More fossil fuels burned means more emissions and more warming, while the ability of the ocean and land to absorb carbon dioxide continues to decrease. As a result, the atmosphere and oceans heat up, increasing the risks of passing tipping points – glaciers disappear, Atlantic Ocean circulation shuts down, permafrost thaws, coral reefs die.

If greenhouse gas emissions continue at a high rate, humanity may look back at 2025 as one the coolest years globally in the rest of our lives.

Michael Wysession does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.