Why a trace of carbon dioxide can warm a whole planet
Nitrogen and oxygen make up 99% of Earth's dry air, yet they barely interact with heat radiation. Carbon dioxide, at just 0.04%, does. Without such trace greenhouse gases, the planet's surface would average about minus 18 °C instead of a comfortable 15 °C. The secret lies in lopsided molecules.
Sunlight warms the ground, which radiates heat back upward as infrared. Greenhouse gases absorb and re-emit radiation in that same long-wavelength band, so they slow the escape of heat. Molecules made of two identical atoms, like nitrogen and oxygen, have a symmetrical electric charge and are almost transparent to infrared; argon, a lone atom, is completely so. Molecules built from different elements, such as carbon dioxide, methane and nitrous oxide, have uneven charges that let their vibrations interact with radiation.
Water vapour is the biggest contributor, responsible for 41–67% of the greenhouse effect, but humans barely change its global amount directly because it cycles out of the air in about nine days. It acts instead as a feedback: warmer air holds more moisture, amplifying warming caused by other gases. Methane is potent: kilogram for kilogram, it traps about 84 times as much heat as carbon dioxide across a 20-year window, yet lasts only around 12 years, whereas a pulse of carbon dioxide lingers until natural cycles mop it up over millennia.
Since around 1750, human activity has raised carbon dioxide by over 50% and methane by 150%. Carbon dioxide causes about three-quarters of warming and methane most of the rest. The pace keeps quickening: the first 30 parts per million rise in carbon dioxide took roughly 200 years, the next 90 only 56. Levels are now the highest in three million years. For six decades, about 45% of each year's emissions has stayed in the air, with the rest soaked up by land and ocean, though those sinks may weaken if emissions stay high.
An odd side effect is that extra greenhouse gases cool the thin upper atmosphere even as they warm the surface. Human-caused warming reached about 0.27 °C per decade over 2015–2024, and at current rates it will pass 2.0 °C sometime between 2040 and 2070, a level the IPCC calls dangerous.
Source: Greenhouse gas
