Greenhouse gas

Greenhouse gases are specific gases in a planet's atmosphere that trap heat. This process is called the greenhouse effect.

By absorbing radiation emitted by a planet, these gases raise the surface temperature. On Earth, this effect is essential for life. Without these gases, the average surface temperature would be about -18°C. Instead, the present average is much warmer. However, human-induced warming is now increasing at an unprecedented rate. Between 2015 and 2024, warming reached 0.27±0.1°C per decade.

To understand the mechanism, we must look at how molecules interact with energy. Most of Earth's dry atmosphere is nitrogen (78%) and oxygen (21%). These molecules consist of two atoms of the same element. This means they have no asymmetry in their electrical charge distribution. Consequently, they are almost totally unaffected by infrared thermal radiation.

In contrast, greenhouse gases contain atoms of different elements. This creates an asymmetry in their electrical charge. This allows their molecular vibrations to interact with electromagnetic radiation. Scientists call these molecules "infrared active."

When sunlight heats the Earth's surface, the surface radiates heat back toward space. Greenhouse gases absorb this infrared radiation.

Instead of letting all the heat escape, these gases exchange thermal radiation with the surface. This limits the rate at which heat flows away from the planet. This process creates a radiative forcing. A positive forcing means more energy arrives at the top of the atmosphere than leaves it. This imbalance causes the planet to warm up.

There are several distinct greenhouse gases with different roles. Water vapor is the most abundant and causes about 41% to 67% of the greenhouse effect. It acts as a climate change feedback. This means as temperatures rise, water vapor concentrations also increase.

Carbon dioxide is another major gas. It is responsible for about three-quarters of global warming. Methane is also significant, though it lasts in the atmosphere for an average of only 12 years. Methane is much more potent; its direct effect is about 84 times stronger than carbon dioxide over 20 years. Other gases include nitrous oxide, ozone, and chlorofluorocarbons (CFCs).

Human activities have significantly altered these natural levels. Since the Industrial Revolution began around 1750, carbon dioxide has increased by over 50%. Methane levels have increased by 150%.

Most carbon dioxide comes from burning fossil fuels, agriculture, and industry. Methane emissions originate from agriculture, fossil fuel production, and waste. These emissions have pushed greenhouse gas levels to an all-time high. In the last decade (2014–2023), emissions reached 53.6±5.2 Gt CO2e per year.

Historical data shows how much the atmosphere has changed. For one million years, natural carbon flows between the ocean, ecosystems, and the atmosphere were fairly balanced. However, carbon dioxide levels are now higher than they have been for three million years.

The 2023 annual update showed that human-induced temperature rise and energy imbalance reached new records. If current emission rates continue, the IPCC says global warming will surpass dangerous levels between 2040 and 2070.

These changes connect to many different global systems. The carbon cycle takes thousands of years to fully absorb carbon dioxide from the atmosphere. This long duration means the effects of current emissions will last for a very long time. The warming also affects the structure of our atmosphere. While the lower atmosphere warms, the upper atmosphere is actually cooling and shrinking. This complex interaction shows how a change in one gas can shift the entire balance of our planet's climate system.