5 Heat Traps: Earth’s Increasing Greenhouse Effect

The heat-trapping effect, or greenhouse effect, on Earth is very strong, as a couple of key observations can demonstrate. First, consider the fraction of longwave radiation from the Earth’s surface that makes it all the way through the atmosphere and out to space. Of the 398 W/m² of longwave emitted from the Earth’s surface, only 40 W/m² makes it out to space. The 10% that escapes is through something called the atmospheric window, while the other 90% is trapped by gases or clouds.

Another measure of the greenhouse effect is the amount of downward longwave radiation absorbed at the Earth’s surface. This quantity, sometimes referred to as the back radiation, is 340 W/m² (173 PW). When compared to the shortwave radiation absorbed at the surface (163 W/m²), one can see that the Earth’s surface is heated twice as much by longwave radiation than shortwave radiation. This fact is often surprising to people, given our experience with being heated up on a sunny day. But the back radiation from the greenhouse effect is always present, day and night, summer and winter.

The greenhouse effect is intuitive if you pay attention. There’s an old mariner saying, “Stars bright, cold night.” Clouds have a heat-trapping effect, so on a cloudless night, temperatures plunge much more quickly, leading to a colder overnight low. Cloudy nights, on the other hand, stay warm for longer. Watch the video below by the UW Atmos Outreach group that illustrates this principle with time-lapse images in infrared and visible.

Other heat-trapping gases like water vapor have an effect as well. Dry regions like deserts tend to have large diurnal temperature swings, while humid regions stay warm during the night. We’ll next examine the heat-trapping gases on Earth and their strengths, as well as how quickly they’re increasing.

What traps?

The atmosphere is mostly just nitrogen (N₂, 78% of dry air), oxygen (O₂, 21%), and argon (Ar, 1%). Although they make up pretty close to 100% of the atmosphere, none of the three are gases that interacts with longwave radiation. 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, or greenhouse 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 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 the effective radiative forcing, here calculated using models as the extra heating that occurs due to the increase in concentration of a given gas. Carbon dioxide levels in 2019 (when radiative forcing was last assessed by the IPCC) were 410 parts per million (ppm), up from 280 ppm in preindustrial. That increase in carbon dioxide caused 2.16 W/m² of extra heating, meaning the radiative forcing from carbon dioxide is 2.16 W/m². This heating is mostly due to the prevention of longwave radiation from making it to space. Stratospheric cooling, which occurs as a direct radiative response, independent of the surface temperature change, helps to amplify the forcing. Changes in clouds also amplify the heat trapping effect slightly.

The current effective radiative forcing due to each heat-trapping gas is given below. In total, heat-trapping gases are locking in a radiative forcing of 3.84 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.

Notes: All numbers from IPCC AR6 WGI Chapter 7.  Total includes 0.05 W/m² from stratospheric water vapor. The “well-mixed greenhouse gas” total, which excludes ozone and stratospheric water vapor, is 3.32 W/m². 

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 heating.

Heat-trapping rate from carbon dioxide, methane, nitrous oxide, forever chemicals, and ozone. 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. We’ll study refinements to the radiative forcing calculation that include this effect later in the book.