Originally published June 30, 2010. Updated September 28, 2026.
Yes, human activity is the main driver of climate change, and 2024 was the hottest year ever measured at 1.29°C above the 20th-century average. The Intergovernmental Panel on Climate Change calls the human influence behind that heat “unequivocal”. 2025 came in close behind, ranking as the third-warmest year since NOAA’s global temperature record began in 1850.
Scientists don’t stop at watching that number climb. They can point to a specific gas building up in the atmosphere and a chemical signature in the air that burning fossil fuels leave behind.
The scientific consensus is about as strong as it gets
The Intergovernmental Panel on Climate Change isn’t one scientist’s opinion. It’s built from the peer-reviewed climate literature, written by researchers from dozens of countries and checked line by line by governments before publication. Its 2021 assessment states plainly that human influence has “warmed the atmosphere, ocean and land,” and that the scale of change now underway is “unprecedented over many centuries to many thousands of years.”
Neither line hedges with words like probably or likely. If someone tells you scientists are still split on the basic cause, that split doesn’t show up in the report itself, which anyone can read in full. The rest of the evidence sits underneath that conclusion, and it starts with how much the planet has already warmed.
How much the planet has warmed so far
Every major temperature tracker agrees on the direction, even when the exact numbers differ by dataset. NOAA measured 2024 at 1.29°C above its 20th-century average, its warmest year on record. NASA’s independent analysis, which uses a 1951 to 1980 baseline instead, put the same year at 1.28°C above that average.
Copernicus, which measures against the pre-industrial period, recorded 1.60°C for 2024. That made it the first calendar year on record to cross 1.5°C above pre-industrial levels. The baselines differ, but the direction doesn’t.
2025 cooled slightly but stayed extreme. NOAA ranked it the third-warmest year in its historical temperature record, at 1.17°C above the 20th-century average. However cold your own winter felt, the year started and ended with a cooling La Niña and still placed among the three warmest years on record, according to the World Meteorological Organization’s consolidated analysis of eight independent datasets. Copernicus’s own tally ranked 2025 third-warmest as well, 0.01°C behind 2023 and 0.13°C behind 2024.
Put the last three years together and the 2023 through 2025 average comes to 1.48°C above pre-industrial levels, under the Paris Agreement’s 1.5°C line. The ten warmest years ever measured have all happened since 2015.
The major trackers compare like this for the same two years:
| Tracker | Year | Anomaly | Baseline |
|---|---|---|---|
| NOAA | 2024 | 1.29°C | 20th-century average |
| NOAA | 2025 | 1.17°C | 20th-century average |
| NASA GISS | 2024 | 1.28°C | 1951 to 1980 average |
| Copernicus | 2024 | 1.60°C | Pre-industrial (1850 to 1900) |
| Copernicus | 2025 | 1.47°C | Pre-industrial (1850 to 1900) |
| WMO (8-dataset average) | 2025 | 1.44°C | Pre-industrial (1850 to 1900) |
Source: NOAA NCEI, NASA GISS, Copernicus/C3S 2024, Copernicus/C3S 2025, WMO. All read September 2026.
Carbon dioxide is at the highest level in recorded history
Before industrialization took hold, the atmosphere held roughly 280 parts per million of carbon dioxide. By full-year 2024, that had climbed to 422.8 parts per million, after the largest annual jump ever recorded, up 3.75 ppm in a single year. The most recent reading from NOAA’s Mauna Loa Observatory, taken in August 2026, put CO2 at 427.55 parts per million, up from 425.48 ppm the same month a year earlier.
That rate of increase is 100 to 200 times faster than anything that happened naturally at the end of the last ice age. If you’re wondering where that extra carbon comes from, the buildup traces back to burning fossil fuels, the same gasoline and natural gas that heat homes and fill car tanks.
The fingerprints that rule out the sun and natural cycles
If you’ve heard the argument that this is only the sun or a natural cycle, the physical evidence says otherwise. Coal and oil formed from plants and plankton that died hundreds of millions of years ago, long enough for their radioactive carbon to decay away completely, so the carbon dioxide from a tailpipe or a smokestack carries none of it. Fossil fuel CO2 has no measurable carbon-14, and the ratio of carbon-13 to carbon-12 has been falling since large-scale fossil fuel burning began, a signature ancient, buried carbon leaves behind.
The atmosphere is also cooling exactly where greenhouse-driven warming predicts it should. A team led by Benjamin Santer, publishing in PNAS in 2023, found that greenhouse gases cooled the mid to upper stratosphere by 1.8 to 2.2°C between 1986 and 2022. Natural swings in the sun and volcanoes could account for no more than 0.15°C over the same stretch, a gap the study calls 12 to 15 times larger than anything nature could produce.
Who has emitted the most carbon, historically
Add up all the carbon dioxide humanity has put into the atmosphere from fossil fuels and industry since 1750, and the running total is lopsided. The United States accounts for 24% of that cumulative total, more than any other country, so if you live there, you live in the country most responsible for it. China follows at 15%.
Together, the next seven historical emitters on the list start with Russia and end with Ukraine.
| Country | Share of cumulative CO2 since 1750 |
|---|---|
| Russia | 6.7% |
| Germany | 5.2% |
| United Kingdom | 4.4% |
| Japan | 3.8% |
| France | 2.2% |
| Canada | 1.9% |
| Ukraine | 1.7% |
Source: Our World in Data. Read September 2026.
Add up all nine of these countries and they account for about 65% of everything humanity has put into the atmosphere, using the same historical emissions data. China’s own historical share is smaller than the United States’, but it’s now the world’s largest annual emitter, a reminder that the cumulative ranking above and the current pace of emissions are two different measures.
Is this storm caused by climate change
When an unusually intense heat wave hits close to home, or a dust storm makes national news, you’ll often hear people ask whether climate change caused it. That’s not a fair yes-or-no question for a single event. Weather is what happens on a given day.
Climate is the pattern measured over decades, which is what the temperature and carbon dioxide records above track. Blaming climate change for one storm, or pointing to one cold snap as proof it isn’t happening, both miss the same distinction. Whatever the weather looks like outside your window this week, the ten warmest years ever measured have all happened since 2015, and no single week changes that.
What a rival theory would have to explain
A rival explanation has to account for the warming and the rising carbon dioxide levels every major tracker measures, and it has to explain away the isotopic and stratospheric fingerprints that point specifically at burning fossil fuels. Nothing on the table does both.
You don’t have to take anyone’s word for the trend either. NOAA’s Mauna Loa Observatory posts a new carbon dioxide reading every month, and the comparison above, 427.55 ppm in August 2026 against 425.48 ppm the year before, is the same one anyone can run themselves.
