What causes global warming? The main cause is greenhouse gases, which trap the sun’s heat as they build up in the atmosphere. Carbon dioxide accounts for 66% of the warming effect (radiative forcing) of long-lived greenhouse gases, followed by methane at 16% and nitrous oxide at 6% (NOAA, 2024). In 2024 the planet recorded its warmest year since measurements began (about 1.55 °C above the pre-industrial era), and the monthly average CO2 concentration at Mauna Loa exceeded 430 ppm for the first time in May 2025.
- Carbon dioxide (CO2)
- Methane
- Nitrous oxide
- Fluorinated gases
- Water vapor
Of these, the most harmful is CO2, which accounted for 66% of the radiative forcing of long-lived greenhouse gases in 2024, followed by methane (16%) and nitrous oxide (6%), according to NOAA’s Annual Greenhouse Gas Index. The IPCC Sixth Assessment Report (AR6, 2021) confirms that CO2 is the main driver of radiative forcing among greenhouse gases, followed by methane.
The latest data confirm this trend: in May 2026 the average concentration of CO2 in the atmosphere reached a record 432.34 ppm, according to NOAA’s Mauna Loa Observatory, far above the roughly 280 ppm of the pre-industrial era. Driven by these record greenhouse gas levels, with an added boost from El Niño, 2024 was the warmest year on record, with an average temperature of around 1.55 °C above the pre-industrial level, and likely the first calendar year to exceed the 1.5 °C threshold, according to the World Meteorological Organization (WMO, 2025).
These gases cause global warming, but which specific activities produce them and therefore drive climate change? The answer is that basically all human activity produces these gases, but certain industries and activities account for the vast majority of them, and it is to these that most climate change is due. Some of them are:
- Burning coal, oil and gas in factories, businesses, homes, transportation, etc.
- Cutting down forests and rainforests: felling trees disrupts climate regulation, because trees absorb CO2, and when they are cut down the carbon they had stored is released.
- The use of nitrogen fertilizers on crops around the world.
- Fluorinated gases, which are very harmful and far more powerful than CO2: their effect on global warming is estimated to be up to 23,000 times greater.
- Water vapor, the most abundant greenhouse gas, is not a direct cause of climate change: as the planet warms, the amount of water vapor in the atmosphere increases, which amplifies the warming further.
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Last updated: September 26, 2026
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