Without the greenhouse effect, Earth would be a frozen ball. The average surface temperature would be around minus 18 degrees Celsius instead of the current 15. The effect works because certain gases in the atmosphere absorb infrared radiation and re-emit it in all directions, including back toward the surface. This traps heat and keeps the planet warm enough for life. The process is natural and has been operating for billions of years.

The problem is the enhancement of the effect. Human activity has added greenhouse gases to the atmosphere, mainly carbon dioxide from burning fossil fuels. More gases mean more heat trapped. The result is global warming. Carbon dioxide levels are now higher than at any point in at least 800,000 years, and probably several million. The increase is fast. Natural changes in carbon dioxide usually unfold over thousands of years. Human emissions have raised levels by about 50 percent since the industrial revolution, in just two centuries.

Main greenhouse gases

• Carbon dioxide: from fossil fuels and deforestation.
• Methane: from livestock, landfills, and gas leaks.
• Nitrous oxide: from fertilizers and industry.
• Water vapor: the most abundant, amplifying other effects.

The greenhouse effect is not a problem in itself. The problem is that we have turned up the dial too far and too fast.