Scientists first speculated more than a century ago that carbon dioxide in the atmosphere regulates Earth’s surface temperature. The idea has since been proven correct.
Humans began adding carbon dioxide to Earth’s atmosphere when we learned to light fires and began clearing forests. Starting in the late 18th century English inventors started a period of rapid mechanical innovation that led to the Industrial Revolution. They improved steam engines, refined mining methods and built factories for mass production. To fuel their new machines, they mined coal and, later, pumped oil. As studies of glacier ice later proved, the amount of carbon dioxide in the atmosphere took off by the late 19th century. It’s been growing exponentially ever since.
Carbon dioxide (and, to a lesser extent, other gases, such as nitrous oxide and methane) makes the planet hotter by insulating it, much the way a blanket keeps a sleeper warm in bed. Radiation from the sun passes directly through the atmosphere and strikes Earth’s surface. Some of the incident energy is absorbed and some is reflected back to space. The absorbed energy heats the Earth's surface and then is reradiated by the warmed surface as longer wavelength infrared energy, radiation in a different part of the electromagnetic spectrum than the sun’s rays. This energy also goes out to space, but some of it is blocked by gases in the atmosphere. Carbon dioxide and a few other molecules block the escape of infrared energy and deflect it back to the Earth's surface where it causes even more heating.
Carbon dioxide has been part of Earth’s atmosphere for most of the time since the planet was created. It makes Earth hospitable to life. Earth’s average surface temperature would plummet to about -19°C if the atmosphere somehow lost all its carbon dioxide. But too much carbon dioxide would be just as undesirable as too little. The planet Venus roasts at a blistering average of 470°C, a temperature higher than that of molten lead, primarily because its atmosphere is thick with carbon dioxide.
Researchers have measured precisely the composition of Earth’s atmosphere hundreds of thousands of years into its past by examining bubbles of air trapped in layers of ice found in the polar ice sheets. They’ve also determined the planet’s temperature back then, by studying molecules of water in the same layers. They’ve shown that the amount of carbon dioxide in the air and global temperature rise and fall in near-perfect synchrony.
For these reasons, scientists have been warning for more than half a century that carbon dioxide (and other gases) produced by industrial activities could warm the planet artificially, with harmful consequences. These predictions have proven correct in recent years. Earth is now about 1°C hotter than it was before the Industrial Revolution. Some parts of the planet, such as polar regions, have warmed several times faster. The effect of this extra heat has been especially notable in the world’s frozen areas. The Arctic Ocean’s cap of sea ice has thinned. Today it covers only about half as much of this northern ocean each summer as it did in the late 20th century. The world’s mountain glaciers have retreated dramatically. The much larger ice sheets at the poles have also started to shrink.