Beyond Carbon Dioxide: Study Points Out Other Greenhouse Culprits

The big Blue Marble, Earth, as seen from space. CREDIT: NASA View full size image

Global warming is about more than just carbon dioxide, a fact that scientists have recognized for some time. Although this infamous gas tends to dominate discussions of climate change, other gases make significant contributions, a new study points out. It suggests that cutting emissions of these other gases could kick-start efforts to reduce global warming. 

Some of these other gases, particularly methane, don't linger in the atmosphere for as long as carbon dioxide, meaning cutting their emissions would help put the brakes on the growing imbalance in the Earth's energy budget (the balance between the heat coming in and the heat going back out into space), according to the researchers.

"In my opinion, it would be nice to see some near-term results, and that might be something that society can grab onto," said study researcher James Butler, director of the National Oceanic and Atmospheric Administration's Global Monitoring Division. "But to ignore carbon dioxide is to fail." [Earth in the Balance: 7 Crucial Tipping Points]

Humans' emission of that behemoth greenhouse gas, carbon dioxide, have been increasing exponentially since the Industrial Revolution, when we began burning massive amounts of fossil fuels.

They looked at greenhouse gases that remained in the atmosphere for several years or more, long enough for them to become well mixed in the atmosphere. Of these long-lived greenhouse gases, carbon dioxide is responsible for roughly two-thirds of the excess energy that is trapped by long lived greenhouse gases. (The technical name for the change in the amount of energy that reflects back out into space is radiative forcing.)

About 20 percent carbon dioxide emissions remain in the atmosphere for more than 1,000 years. [How Two Degrees Will Change Earth]

The study considered four other categories of greenhouse gas:

• Methane is comparatively short-lived, hanging around in the air for about nine years, but molecule for molecule, it is 25 times more potent at trapping energy than carbon dioxide.
• Nitrous oxide sticks around for about 120 years, is 300 times more potent than carbon dioxide, but is present in the atmosphere at much lower levels.
• Ozone-depleting substances are the chlorofluorocarbons banned by the Montreal Protocol, as well as their replacements.
• Extremely long-lived gases, including nitrogen trifluoride, make up the final group. Although present at low levels, some can stay in the atmosphere for as long as tens of thousands of years. 

Their analysis did not include other greenhouse gases, such as ozone, that don't stick around in the atmosphere for long.

Butler and the other researchers looked at the impact of reducing carbon dioxide emissions versus the other gases. Not surprisingly, the best scenarios involved cutting both types of gases, but cuts in the shorter-lived non-carbon dioxide gases showed potential for more rapid effects curbing radiative forcing, the authors write. "Such a quick response is not possible from cuts in emissions of carbon dioxide alone."

Immediately cutting methane emissions could, for example, show results as soon as a decade now. The more likely, more gradual approach to reducing emissions would take a little longer to be detected, according to Butler.

But even completely cutting out all of the non-carbon dioxide gases would not offset carbon dioxide's hand in the continuing increases in the gap in the Earth's energy balance, the researchers write in the Aug. 4 issue of the journal Nature.

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