Earth’s climate has always changed. Ice ages came and went. Volcanoes cooled the planet for a year or two. The sun flickered. Oceans moved heat around for decades at a time.
Those are old forces. They are still here.
The question now is different. What warmed the world since the late 1800s — those old cycles, or the gases people added to the air?
The gas
Carbon dioxide and methane trap heat. That is measured in laboratories and in the air.
Before coal and oil were burned at scale, carbon dioxide in the atmosphere was about 280 parts per million. In 2025 it was about 426 parts per million. The rise in carbon dioxide comes mainly from fossil fuels, cement production, and land-use change, including deforestation.
Each new slice of carbon dioxide adds a little less heat than the slice before it. That is a real property of the gas. It does not mean the pile stops mattering. The pile is still growing.
Two thermometers
Agencies do not all use the same starting line.
NASA’s record begins in 1880 and is often compared with the years 1951 to 1980. In January 2026, NASA said 2024 was the hottest year in that record. 2025 was cooler than 2024. NASA put 2025 about 1.19°C above the 1951–1980 average, and said 2025 was effectively tied with 2023. Other groups that use much of the same raw data called 2025 the third-warmest year.
A 2026 climate-indicator update compared 2025 with 1850–1900. On that older baseline, 2025 was about 1.39°C warmer. The same update judged human-caused warming in 2025 at about 1.37°C. In other words, the long rise matches greenhouse gases from human activity. A hot or cool Pacific year can nudge one calendar year. It does not build the century-scale slope.
If two headlines are a few tenths of a degree apart, look at the starting year. NASA often starts later than 1850. That alone can change the number.
What nature still does
A strong El Niño year runs hot. A La Niña year runs cooler. A large eruption throws particles high in the air, blocks sunlight, and knocks temperature down until the dust falls. Those swings are real. They explain noisy years. They do not line up with the warming from the late 19th century to now.
The sun’s output has not climbed in step with that rise.
The old climate is not a loophole
Climate shifted before factories. Changes in Earth’s orbit helped start and end ice ages. Some regions were warm during the mid-Holocene, thousands of years ago. A warm spell in one region thousands of years ago is not the same as today’s worldwide thermometer record — or the extra carbon dioxide measured in the air.
Water
Global sea level is about 20 centimetres higher than in 1900. It is now rising by a few millimetres a year. That figure comes from tide gauges and satellites, not from a model of the year 2100.
Ice melting and warmer water taking up more space both add to that rise. How much a given coast floods also depends on storms, sinking land, and what got built there. The global number is still going up.
The answer to the headline
Natural cycles still matter. El Niño can make one year hotter. A big volcano can make a year cooler. Those swings still show up in the record.
They do not explain the climb from the late 1800s to now. Over that stretch, carbon dioxide went from about 280 parts per million to about 426. NASA’s hottest year is 2024. 2025 was still near the top of the chart.
So the honest split is simple. Old cycles run the noisy years. Greenhouse gases from human activity built the long rise. That is the answer to the title. What governments do next is a separate debate.
SOURCES: NASA Goddard Institute for Space Studies, global temperature release, January 14, 2026; Indicators of Global Climate Change update, 2026; IPCC assessments of warming since 1850–1900.
