Here's some good news about climate change: emissions of greenhouse gases other than carbon dioxide have slowed and, in some cases, begun to decline. That means fewer molecules drifting in the atmosphere and blocking the escape of heat radiated by an Earth warmed by sunlight. The bad news is no one knows why.
Now a new study suggests that declines in ethane—a simple hydrocarbon molecule and component of the fossil fuel known as natural gas—can be attributed to companies stopping the practice of simply releasing the gas that comes up with every barrel of oil. Atmospheric measurements stretching from 1984 to 2010 suggest that ethane emission rates have fallen by a full 21 percent. The study is published in Nature on August 23. (Scientific American is part of Nature Publishing Group).
That's important in its own right but also because of what it suggests about the second most abundant greenhouse gas—methane, another simple hydrocarbon and the most abundant molecule in natural gas. Over the span of a century, a methane molecule traps 25 times the amount of heat compared to the most abundant greenhouse gas carbon dioxide.
So how much methane human activities put into the atmosphere has a big impact on how much global warming we end up creating this century (and beyond)—and represents a shortcut to restraining climate change, buying time to decrease CO2 emissions.
The 60 to 80 two-liter canisters bearing air samples collected from all around the Pacific Ocean since 1984 suggest that the ethane decline is a result of a decline in so-called fugitive emissions from fossil fuel production. As the name implies, fugitive emissions are molecules of natural gas that got away (usually on purpose). Given the value of natural gas as a fuel, however, the practice of releasing natural gas molecules has declined since the 1970s. As a result, oil companies—while funding efforts to question the reality of climate change—may have been inadvertently helping keep global warming from getting even worse.
There are other sources of ethane as well, such as wild (and human-set) fires or the burning of biofuels. But ethane emissions from those sources have actually increased, meaning the overall decline has to be attributed to some other source. The remaining possibility—ending the practice of simply releasing ethane when producing oil—is the most likely explanation, the new research argues.
Methane and ethane emissions have been strongly correlated over the period of the study, suggesting that what happens to ethane will happen to methane. In fact, the delay in a more significant drop in methane may be a result of that molecule's longer lifetime in the atmosphere—methane lasts roughly a decade whereas ethane breaks down within a few months.
As mentioned, that's good news. It may also give scientists a way to identify particularly catastrophic climate change scenarios before they set in, like a release of the trillions of molecules of methane trapped in Arctic permafrost or ice cages on the seafloor. If methane levels suddenly rose without a corresponding rise in ethane that might signal that such a geologic burp was happening.
Unfortunately, the rate of the ethane (and possibly methane) decline has slowed over the last decade, according to the record. And it may slow yet further if fugitive emissions from natural gas wells—like those currently being fracked in the U.S.—or pipelines grow. There's also an opportunity to stop the escape of methane from coal mines in China, oil wells in Africa and the Middle East, and pipelines in Russia if we want to slow global warming. To combat climate change, the decline in atmospheric levels of ethane cannot prove temporary.
Image: iStockphoto.com / HeliRy