Trapping Heat

The Earth would be a frozen block of ice if there were no heat-trapping gases. It’s not surprising that when industrial activity increases the concentration of heat-trapping gases like carbon dioxide, the planet gets hotter.

The atmosphere is mostly just nitrogen (N₂, 78% of dry air) and oxygen (O₂, 21%). Although they make up 99% of the atmosphere, neither is a heat-trapping gas. Some gases, though, are intensely excited by the low infrared frequencies. Usually made of three or more atoms, these molecules dance by vibrating or rotating when they absorb particular frequencies of light. Heat-trapping gases are relatively small in abundance but are huge in climatic importance. The main ones on Earth are

• Water vapor (H₂O), which is increasing rapidly as the world heats up. It manifests as the stifling humidity within heat waves, and creates torrential downpours when squeezed out within storms.
• Carbon dioxide (CO₂), which is soaring due to fossil fuel burning (90%) and deforestation (10%). A sizable fraction of carbon dioxide stays in the atmosphere for tens of thousands of years after it is emitted from industrial smokestacks.
• Methane (CH₄), which has a larger heat-trapping ability than CO₂ but a shorter lifetime of around a decade. Methane comes from leaking fossil fuel infrastructure like fossil gas wells, and microbial processes, including those coming from the bellies of cows and landfills.
• Nitrous oxide (N₂O), or laughing gas, emitted from industrial agriculture and the chemical industry. Nitrous has lifetime over a century.
• Forever chemicals, also known as fluorinated gases, synthetic molecules that did not exist in nature before their mass production in the chemical industry. Forever chemicals have lifetimes ranging from a few years to tens of thousands of years.
• Ozone (O₃), a two-faced molecule that is a toxic pollutant near the ground but crucial for life on Earth in the protective ozone layer far above.

Compare the concentration of five heat-trapping gases in the interactive plot above by clicking their names to hide and show their concentration. You’ll have to hide some of the higher concentration gases to see the other curves. 

Heat-trapping gases all are “trace gases,” meaning they have small concentrations in the air. Some are much smaller in concentration than others; forever chemicals, for instance, are just a trace of a trace. There are 4 million molecules of carbon dioxide in the air for every molecule of forever chemical HFC-134a.

We’ll need to know how much warming each gas can cause, which depends on three quantities:

1. Its effectiveness at trapping infrared radiation (indicated with chili peppers in the music video below),
2. Its lifetime, and
3. How much of it is emitted.

To assess the rate of heat-trapping from the increase of a given gas, we calculate a quantity called radiative forcing. Carbon dioxide levels are currently 420 parts per million (ppm), up from 280 ppm in preindustrial. That increase in carbon dioxide is causing 1170 TW of extra heating, meaning the radiative forcing from carbon dioxide is 1170 TW.

The current radiative forcing due to each heat-trapping gas is given below. In total, heat-trapping gases are locking in a radiative forcing of 2030 TW of power, equivalent to 4.0 W/m² over the Earth’s surface. This is over 500 times the world electricity usage, and is around 1.5% of the absorbed sunlight on Earth.

CO₂ is the biggest contributor right now, causing around 60% of the global heating. This percentage will likely rise substantially in future decades. N₂O is the smallest contributor in this list, at around 5% of the present-day warming.

Heat-trapping rate from carbon dioxide, methane, nitrous oxide, forever chemicals, and ozone, measured in terawatts. Data from NOAA and Inputs4MIPs.

Methane, forever chemicals, and ozone are more difficult to quantify because they are chemically active. Methane itself reacts away within about a decade, but leaves behind carbon dioxide and traces of water vapor within the normally dry stratosphere, in addition to a variety of other influences on the chemistry of the air. A certain class of forever chemicals called CFCs caused the ozone hole. And ozone itself results from a variety of precursor pollutants, like NO_x and volatile organic carbons (VOCs). The estimates above only include the heat trapped by each gas directly, not the chemical reactions they cause or the breakdown into the pollutants that cause ozone.