4. What Is the Greenhouse Effect, and Why Does It Matter?

The greenhouse effect and greenhouse gases are important to climate change, but details about their workings are often not understood and are sometimes misunderstood. Helping students learn about them is a way to better grasp the conditions that make life on Earth possible and the conditions that currently pose threats related to climate change.

The Greenhouse Effect

The greenhouse effect is somewhat like what occurs in a greenhouse (both phenomena result in higher temperatures), and also significantly different. Light energy from the sun reaches the Earth in the form of visible light and shortwave ultraviolet radiation. About a half of the solar energy that reaches the top of the atmosphere is reflected back into space or absorbed by the atmosphere. The rest is absorbed by the Earth’s surface, which is warmed and radiates the energy, in much longer wavelengths, primarily in the infrared part of the spectrum.1

In an actual greenhouse, the glass walls and roof let the shortwave ultraviolet radiation in, but reduce airflow and trap the longwave infrared radiation, keeping the greenhouse warm. In the Earth’s greenhouse effect, some of the infrared radiation passes through the atmosphere and back into space, but some of the heat energy is absorbed by certain “greenhouse gases” (GHGs), and re-emitted in all directions. Some of that goes into space, some is absorbed by other GHG molecules, and some spreads downward and ultimately comes back into contact with the Earth’s surface, making it warmer than it would be if it were heated only by direct solar energy.2 Virtually all of the heat energy eventually goes into space, but the greenhouse effect causes enough of it to remain around the Earth to raise average surface temperatures substantially from what they would be otherwise.

The Earth’s greenhouse effect is crucial to life as it has evolved on our planet. Without it, temperatures on Earth might be much more like those on Mars. Despite having a day about as long as the Earth’s, and summer daytime temperatures up to 20ºC (70ºF), Mars’s summer nighttime temperatures plunge to -73ºC (-100ºF). Earth, on the other hand, with an atmosphere 140 times as dense as that of Mars, has an average surface temperature of 14ºC (57ºF), and day-to-night swings of only tens of degrees. Venus, meanwhile, with an atmosphere 93 times as dense as Earth’s, made up mostly of carbon dioxide, has a surface temperature of 462ºC (864ºF).3

The problem today, in the conclusion of the vast majority of climate scientists, is not the greenhouse effect per se. It’s that human activities have raised (and continue to raise) the level of greenhouse gases, warming the Earth so much—and so rapidly—as to seriously disrupt the planet’s climate, with impacts around the world.

Greenhouse Gases

The atmosphere is about 78 percent nitrogen and 21 percent oxygen. Because of their structures, nitrogen and oxygen molecules do not absorb infrared energy, but the molecules of several other gases—the greenhouse gases—which make up less than half a percent of the atmosphere, do.4 Climate Change, an online publication for high school students produced by the Lawrence Hall of Science at the University of California, Berkeley, explains:

Photons of visible light vibrate too fast to affect any of the molecules in the atmosphere. That is the reason visible light goes through the air. However, photons of infrared energy vibrate at just the right frequency to transfer their energy to molecules of [greenhouse gases], which causes those molecules to vibrate. We experience this vibration as heat.5

Climate Change includes an exercise (Investigation 2-3 in Chapter 2) in which students experiment with models of molecules to see what happens when they are energized with different frequencies of vibration.

The principal greenhouse gases are carbon dioxide (CO2), methane (CH4), ozone (O3), nitrous oxide (N2O), tropospheric ozone (O3), several fluorinated gases, and water vapor (H2O).

With respect to their effects on global warming, these gases differ from one another in two important ways. One is their ability to absorb energy (their “radiative efficiency”). The other is how long they remain in the air (their “atmospheric lifetime”). Global Warming Potential (GWP) is a metric developed to provide a common unit—the energy that one ton of the gas will absorb over a given period of time, relative to one ton of CO2. The time period typically used to calculate GWPs is 100 years, though researchers and activists sometimes argue for different periods for some gases (see below). By definition, the GWP of CO2 is 1. When someone says that a gas is 25 (or 36 or 300, or whatever) times more powerful a greenhouse gas than carbon dioxide, the number given is the gas’s GWP.

The figures below for atmospheric lifetime and GWP are taken from the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) of 2014, unless otherwise noted.6 Ranges reflect different methods of calculation. GWPs are for 100 years.

Note: Much of the data and other information in this essay is taken from the website of the US Environmental Protection Agency (EPA). At the time of the essay’s writing, that information is still accessible. Given the attitudes of the Trump administration, the future of material on websites such as those of the EPA and the National Oceanic and Atmospheric Administration (NOAA) is uncertain. For instance, a page to which readers are directed for the EPA’s most recent inventory of greenhouse gas emissions reads, “Thank you for your interest in this topic. We are currently updating our website to reflect EPA ’s priorities under the leadership of President Trump and Administrator Pruitt.”7

Carbon Dioxide (CO2)
GWP: 1
Atmospheric Lifetime: variable (sources and sinks for CO2 involve complex interplay among the hydrosphere, biosphere, and lithosphere). “About half a CO2 sample emitted today would be gone in a century, but a portion of the rest will persist for thousands of years,” says the American Chemical Society (ACS).8

CO2 accounts for about 81 percent of US greenhouse gas emissions from human activities. While CO2 emissions come from a variety of natural sources, human-related emissions are responsible for the increase that has occurred in the atmosphere since the industrial revolution.9 The main human activities leading to emissions are combustion of fossil fuels for energy and transportation and some industrial processes such as manufacture of cement. The primary food system contribution besides energy use and transportation is practices such as plowing and destruction of rainforests that expose soil, allowing carbon in the soil to combine with oxygen in the air to form CO2.

Methane (CH4)
GWP: 28–3410
Atmospheric Lifetime: 12.4 years

Methane accounts for about 11 percent of US greenhouse gas emissions from human activities. Methane is the primary component of natural gas, and some CH4 is emitted to the atmosphere during the extraction (including fracking), processing, storage, transmission, and distribution of natural gas. The food system’s primary contributions are enteric fermentation during digestion by livestock, anaerobic decomposition of manure, and rice cultivation. Methane is also released during the decomposition of food waste in landfills.

The conventional practice of calculating Global Warming Potential for a 100-year period is especially controversial in the case of methane, whose atmospheric lifetime is much shorter than CO2’s. As a result, some have argued that methane’s GWP should be calculated over 20 years. This can matter, because GWP is used in part to help decision makers set priorities for attention and resources in their efforts to respond to climate change.

Instead of a GWP of 28 to 34, methane’s GWP when calculated over 20 years is 84 to 86. Some activists argue that this figure should be used, because the next 20 years is an especially critical time for action, while others believe that 100 years is the better period, because mitigation should be understood as a long-term project.11 The choice of which figure to use also affects discussions of the impact of animal agriculture, which is a primary source of methane emissions.

Nitrous Oxide (N2O)
GWP: 298
Atmospheric Lifetime: 121 years

Globally, about 40 percent of N2O emissions come from human activities. N2O is emitted when transportation fuels are burned and in the production of adipic acid, which is used to make fibers and other synthetic products. With respect to the food system, almost 79 percent of N2O emissions in the US result from the application (especially the overapplication) of synthetic fertilizers. It is also a byproduct during the production of nitric acid, which is used to manufacture fertilizer. Another 4 percent of N2O emissions are from the breakdown of nitrogen in livestock manure and urine, particularly liquid slurry in manure lagoons or holding tanks.

Tropospheric Ozone (O3)
GWP: n.a.
Atmospheric Lifetime: hours–days12

Ozone is formed when an energy source such as ultraviolet radiation from the sun or lightning breaks the bonds in a normal oxygen molecule (O2), freeing an individual oxygen atom which can combine with an O2 molecule to create ozone. Ozone in the upper atmosphere (the stratosphere) plays a critical role by shielding living things from ultraviolet rays. In the 1980s, it was discovered that chlorofluorocarbon (CFC) gases produced by the chemical industry had drifted into the stratosphere and begun to destroy the ozone layer. This is the famous “hole in the ozone layer” that students may have heard of. Note that the ozone hole is not related to global warming. In the lower atmosphere (the troposphere), ozone acts as a greenhouse gas, as well as contributing poisonous chemical properties to smog. Automobile exhaust accounts for about 75 percent of the ozone in the troposphere.13

Fluorinated Gases14
Hydrochlorofluorocarbons (HFCs): GWP: 12–14,800; Lifetime: 1–270 years
Perfluorocarbons (PFCs): GWP: 9,390–12,200; Lifetime: 2,600–50,000 years
Nitrogen triflouride (NF3): GP: 17,200; Lifetime: 740 years
Sulfur hexafluoride (SF6): GWP: 22,800; Lifetime: 3,200 years

Fluorinated gases have no natural sources. They are emitted through a variety of industrial processes such as aluminum and semiconductor manufacturing. After CFCs were found to be destroying the stratospheric ozone layer, HFCs began to replace them as refrigerants, aerosol propellants, and in other uses. These are now being phased out, but are still a problem because they are being released through leaks, servicing, and disposal of equipment and have a long lifetime and high GWP. Sulfur hexafluoride, used in electrical transmission equipment, has the highest GWP evaluated to date by the IPCC.

Water Vapor (H2O)

Water vapor (an invisible gas, not steam, which consists of large water droplets suspended in the air) is the dominant absorber in the Earth’s greenhouse effect, but some climate scientists do not treat it as a “driver” of climate change. According to the ACS, “It’s true that water vapor is the largest contributor to the Earth’s greenhouse effect. On average, it probably accounts for about 60 percent of the warming effect. However, water vapor does not control the Earth’s temperature, but is instead controlled by the temperature. This is because the temperature of the surrounding atmosphere limits the maximum amount of water vapor the atmosphere can contain.”15 

Systems Perspective: The relationship between water vapor and global temperature is a good example of a positive (reinforcing) feedback loop. As temperature increases, the atmosphere can hold more water vapor; since the water vapor is a greenhouse gas, this leads to further temperature increase, which permits the atmosphere to hold more water vapor, and so on around the loop.

There is also a possibility, notes the ACS, that more water vapor in the atmosphere as the result of higher temperatures could produce a negative (balancing) feedback loop through cloud formation. “This could happen if more water vapor leads to more cloud formation. Clouds reflect sunlight and reduce the amount of energy that reaches the Earth’s surface to warm it…. But cloud cover does mean more condensed water in the atmosphere, making for a stronger greenhouse effect than noncondensed water vapor alone—it is warmer on a cloudy winter day than on a clear one.”16 It is not known whether these feedbacks balance each other or if one of them has a greater impact than the other. The subject is an area of active research.