How do we know humans are warming the planet?

We know humans are warming the planet because the evidence points to one suspect and rules out the others. Burning coal, oil and gas releases carbon dioxide, and the air now holds roughly 50 percent more of it than before the industrial era. Those gases absorb some of the heat Earth gives off and keep it in the lower atmosphere. Earth's average surface temperature has risen about 1 °C since the late 19th century, and it keeps climbing by around 0.2 °C per decade.

The clever part is the fingerprint test. Every possible cause leaves its own pattern. If a brighter Sun were to blame, the whole atmosphere would warm. Instead, the air near the ground is warming while the upper atmosphere is cooling, exactly what you expect when greenhouse gases trap heat low down. And since 1880 the Sun's energy reaching Earth hasn't trended upward at all.

The idea isn't new. In 1856 the American scientist Eunice Newton Foote filled glass cylinders with different gases, left them in the sun, and found that carbon dioxide warmed the most. A century later, in 1958, Charles David Keeling began measuring carbon dioxide on top of Mauna Loa in Hawaii. His record, the Keeling Curve, rose from 313 parts per million in 1958 to 406 in 2018, while air bubbles trapped in ancient ice show it had hovered between 275 and 285 for thousands of years.

Today the scientific consensus is nearly unanimous, and in 2021 the UN's climate panel, the IPCC, called human responsibility "unequivocal".