What do you imagine when you hear the phrase “greenhouse gases?” If you think of a factory belching out coal or a packed freeway filled with idling cars comes to mind, you’re on the right track: Emissions from these and other human-driven processes vomit tens of billions of tons of carbon dioxide into the air each year. But it turns out that CO2 isn’t the only game in town. It’s one of several greenhouse gases that trap heat in the atmosphere, driving global warming and climate change. Here’s what you need to know about CO2’s cousins—greenhouse gases that get less air time, but are no less important to Earth’s atmosphere.
Scientists have known about greenhouse gases since Joseph Fourier, a French physicist and mathematician, theorized that the planet’s temperature must be regulated by something that both absorbs the sun’s rays and emits some of the resulting heat back to Earth. Fourier theorized that gases must be that something, and his work in the 1820s was soon continued by other scientists determined to find out which gases trap heat from the sun on Earth. Eventually, people started comparing the work of those gases to that of glass that covers a greenhouse, bouncing its internal heat back toward the building that emits it and warming itself even when it’s cold outside.
Over time, scientists began to develop a more nuanced view of how gases form and act. Not all gases on Earth are greenhouse gases. The amount of greenhouse gases in the atmosphere depends on sources (natural and man-made processes that produce them) and sinks (reactions that remove the gases from the atmosphere). Carbon dioxide is only part of that equation, and only the second most abundant greenhouse gas on Earth.
At the top of the list is water vapor, the granddaddy of all greenhouse gases. Water vapor is present wherever there’s measurable humidity. Clouds aren’t water vapor—water vapor is invisible. But that doesn’t mean it’s not abundant: About 80 percent of the atmosphere’s total mass of greenhouse gas is water vapor.
Water vapor sounds pretty nonthreatening, but it’s part of a cycle that is warming Earth. Here’s where it gets confusing: Water vapor doesn’t cause global warming, but it worsens it. As carbon dioxide and other emissions grow, water vapor increases, too. More concentrated water vapor and higher evaporation rates means more global warming.
The phenomenon is called stratospheric water vapor feedback, and its concerning to Sean Davis, a CIRES research scientist working at the National Oceanic and Atmospheric Administration whose research focuses on the gas. “It’s really a complicated problem,” he tells Smithsonian.com. In 2013, Davis and colleagues showed evidence of that vicious cycle—and suggested that it contributes significantly to the sensitivity of Earth’s climate. Though satellites and space-based radar that monitors precipitation are now available to researchers, he says, they still need more data about how water vapor and carbon dioxide interact in Earth’s atmosphere.
Methane, the third-most abundant greenhouse gas, presents a similar quandary for researchers. In recent years, they’ve learned much more about how the gas, which is the second most emitted in the United States, contributes to global warming. Methane is emitted by everything from farting cows to wetlands and natural gas systems, and industry, agriculture and rotting trash make sure plenty is spewed into the atmosphere. But even though the gas warms Earth by an order of magnitude more than CO2 (up to 86 times as much), both sensors and environmental watchdogs often underestimate .
Other gases contribute to climate change and global warming—there’s nitrous oxide, which is emitted by fertilizer and has become one of the biggest ozone depleters in the atmosphere. You may know the gas better in its incarnation in dentists’ offices and whipped cream dispensers, but there’s plenty of nitrous in the atmosphere, too. Since the beginning of the industrial era in the 1700s, nitrous oxide levels have grown, and atmospheric levels of the gas could nearly double by 2050.
Nitrous oxide isn’t alarming just because of its warming power (one molecule traps as much heat as 300 CO2 molecules). It can take over a century for a molecule of N2O to degrade. In the meantime, it contributes to ozone loss in the atmosphere, which in turn spurs warming on Earth. There’s still plenty scientists don’t know about N2O: For example, its ozone-depleting potential seems sensitive to different environmental conditions. It may take decades before it’s clear just how the gas reacts with other GHGs and the changing climate.
Though chlorofluorocarbons, or CFCs, are non-toxic to humans and are inert in the lower atmosphere, things are different once they reach the stratosphere. There, the man-made chemicals eat up ozone, and they are still present in today's atmosphere despite sweeping regulation aimed at closing the ozone hole.
Like N2O, CFCs last long periods of time in the upper atmosphere. They’re being phased out with good reason: On a molecule-by-molecule basis, CFCs have a much higher global warming potential than carbon dioxide. For example, CFC-13 (also known as Freon 13), which cools some industrial freezers, is 16,400 times as warming as carbon dioxide over a 500-year period. CFCs are banned in the United States, but plenty made their way into the atmosphere before the Montreal Protocol, which was agreed to in 1987. Though they are no longer present in deodorant cans and spray bottles, they’re still up above, breaking down ozone. (It would hypothetically be beneficial for N2O and CFCs to "eat" ozone when it's in the troposphere, where it's technically considered a "bad" greenhouse gas. But once ozone makes it up to the stratosphere, it actually protects Earth from the sun's brutal rays.)
It’s tempting to think that because CO2 has so many counterparts, it’s not worth worrying about. But just because CO2 isn’t the only greenhouse gas doesn't mean it’s not cause for concern. “A lot of people use [greenhouse gases] to downplay the importance of carbon dioxide,” says Davis. “That’s the biggest issue we face.” Some gases may be more abundant, but none stand alone—and with CO2 rates rising at unprecedented levels, it’s difficult to estimate just how dire the consequences of unchecked emissions of any kind might be.