How do we know humans are the cause?
The conclusion that humans are the main cause is not a guess. The extra carbon in the air can be traced, and the way the planet is warming matches greenhouse gases rather than natural causes.
The carbon carries a fossil signature
Carbon comes in slightly different forms, or isotopes. Carbon from ancient fossil fuels has a distinct mix. As CO2 has risen, the isotope balance in the atmosphere has shifted in exactly the way expected if the extra carbon comes from coal, oil and gas, not from volcanoes or the oceans.
Oxygen is falling as fuels burn
Burning fossil fuels uses up oxygen. Precise measurements show atmospheric oxygen declining slightly, in step with the CO2 rise. Natural CO2 release from the oceans would not do this. Combustion does.
The upper atmosphere is cooling
If the Sun were driving the warming, the whole atmosphere would warm. Instead, the lower atmosphere is warming while the upper atmosphere, the stratosphere, is cooling. That split is the specific signature of greenhouse gases trapping heat lower down.
The timing and pattern fit
Warming has been strongest at night, in winter and over recent decades, tracking the rise in greenhouse gases. Natural drivers, checked one by one, do not reproduce this timing, size and pattern. Human emissions do.
Frequently asked
Its chemical fingerprint matches fossil carbon, oxygen is falling as fuels burn, and the rise tracks human emissions rather than natural sources.
No. If the Sun were the cause, the whole atmosphere would warm. Instead the upper atmosphere is cooling while the surface warms, which is the fingerprint of greenhouse gases.