BP's Deepwater Horizon catastrophe is commonly referred to as the Gulf oil spill, but liquid oil wasn't the only hydrocarbon that gushed out of the Macondo well for 84 days.
Up to 40 percent of the leak was gas, mostly methane invisible to the naked eye, reported scientists who last month in the research journal Nature Geoscience.
The study authors — Samantha Joye, Ian MacDonald, Ira Leifer and Vernon Asper — calculate the total volume of discharged gas as between 260,000 to 520,000 tons. That is enough, if burned, to supply the same amount of energy as 1.6 to 3.1 million barrels of crude oil.
Gas and oil occur together in deep ocean deposits, so it should come as no surprise that the Macondo well released large amounts of gas.
"Natural gas was a huge fraction of the hydrocarbon discharge ... and we don't know what happened to it," said lead author Joye, a professor of marine sciences at the University of Georgia.
The impact of the gas on marine ecosystems is largely unknown, and it has reignited a scientific debate about the behavior and whereabouts of hundreds of thousands of tons of natural gas. The discussions will have consequences for both the energy industry and climate change research and policy.
When Joye and her colleagues visited the spill site in May-June 2010, they measured concentrations of dissolved hydrocarbons at up to 75,000 times higher than background levels. This gas-rich layer was about 3,300 feet below the ocean surface.
Microbes in the ocean can break down those hydrocarbons, but the process uses up a lot of oxygen and could trigger depletion of the life-giving gas, though the extent and impacts remain unknown, said co-author Ian MacDonald, an oceanography professor at Florida State University.
"We're not saying this is a measure of doom to the Gulf ecosystem," MacDonald continued. "Rather, if we're to understand the environmental impacts of [the spill], we need to understand all the inputs — not just the oil but also the gas and dispersants."
Co-author Ira Leifer, a researcher at the University of California, Santa Barbara, said the gases could change the microbial balance of the sea. "It can be a cause for alarm [because] the bottom of the food chain is in the deep sea."
A sudden surge in methane-consuming bacteria might crowd out other microbes that feed organisms higher up on the food chain. "So in terms of [ecosystem impacts] it would've been better if the methane had mixed with the atmosphere."
Methane: 20 Times More Potent than CO2
But methane is a greenhouse gas twenty times more potent than CO2, and it enters the atmosphere from both natural and human sources. Wetlands, forests and oceans all emit gaseous methane, as do fossil fuel plants and livestock. According to a 2007 report, man-made emissions from sources such as landfills, agriculture and biomass combustion account for 60 percent of global methane output.
The high-end estimate of the gas discharge from the Macondo well — 520,000 tons — is equal to 2.6 percent of annual net global methane emissions.
In the atmosphere as a whole, which contains some 4,800 teragrams of methane, the leak would have increased overall methane concentrations by 0.01 percent, Joye explained in an email to SolveClimate News.
But in the case of the BP spill, most of the gas was trapped in the ocean, and very little made it to the atmosphere, said Joye. Some of the methane was observed as methane hydrates, cage-like structures where methane molecules are trapped in crystals of water ice molecules.
Gas hydrates form under high pressure and low temperatures, and they're commonly found in deep underwater sediments. According to from the (USGS), worldwide hydrate reserves could hold twice as much energy as all the fossil fuels found on Earth.