How do we know humans are warming the climate?
Rising temperatures alongside rising emissions alone would not be proof. Three distinct tests matter: Where does the extra CO₂ come from? Does it change the flow of energy? And do observed temperatures fit a world without human influences?
The atmosphere reveals where the CO₂ comes from
Fossil fuels have a different carbon-isotope mix from today's atmosphere. Burning them also consumes oxygen. These clues are examined independently of temperature measurements.
Carbon isotopes
Fossil carbon contains virtually no radioactive ¹⁴C. The measured composition of atmospheric CO₂ is shifting in the expected direction. ¹³C alone would not be conclusive; multiple sources and sinks must be considered.
Oxygen in the air
Atmospheric O₂ is declining relative to N₂. This is consistent with burning carbon-containing materials. Together with isotopes and the carbon budget, it narrows down the source of the extra CO₂.
Conclusion: The increase in atmospheric CO₂ is predominantly due to human activities. A single isotope measurement cannot yield an exact global share.
More greenhouse gases change radiative forcing
NOAA calculates the radiative forcing of long-lived greenhouse gases from measured concentrations relative to 1750. CO₂ accounts for the largest share. Forcing is a driver of the energy budget, not a measured temperature or Earth's entire energy balance.
The values are derived from atmospheric measurements and radiative-transfer formulas. Other influences, such as aerosols and changes in sunlight, are not included here. Data through 2024.
Which influences explain the temperature record?
The IPCC compares observations with climate models that include different drivers. This is a scientific assessment of many model runs, not a single experiment or a directly observed second Earth.
Independent temperature datasets show this.
The sun, volcanoes and internal variability explain individual fluctuations, not the long-term trend.
Only when greenhouse gases and other human influences are included do simulations match observations over the long term.
Origin, effect and temperature record fit together
No single line of evidence has to explain everything. Together they support the IPCC's assessment that human influence has warmed the atmosphere, ocean and land. This does not establish whether climate change caused a particular flood or fire.
What does this not imply?
A single weather event is not proof of climate change. Claims about altered risks or climate change's role in a specific event require dedicated attribution studies. We will cover this distinction in the next step.
Review natural factors again