Alternative methane model identifies shortfalls in current modelling

The amount of methane entering the atmosphere is a point of contention and there is no universal, standardised way of measuring it. Tracking the emission of this greenhouse gas in the US is the responsibility of the EPA. The EPA uses models which take a “bottom-up” approach; their inventory is found by counting the well heads, storage tanks, pipeline and other sources of methane, calculating an average annual release for each source and summing them up.

Other organisations taking a top-down approach, using satellite imaging or atmospheric measurement to calculate the total methane emissions, have results which contradict the EPA models and say that these are underestimating the amount of methane by as much as half.

“Top-down approaches were finding total emissions double the EPA’s emissions, but the reason why was not clear,” said Jeff Rutherford, who has been working on the problem since beginning his PhD in 2018. With his advisor, Professor Adam Brandt, he has been developing an alternative bottom-up model for methane emissions which they hope may bridge the gap between top-up and bottom-down approaches.

Brandt commented: “If our emissions-based models that we use to make important climate-related decisions are not correct, it is a big problem.”

The researchers used the latest component-level data to tabulate a more accurate total amount of methane. The data were gathered by directly sampling at various components throughout the oil and gas industry where methane is most likely escaping, such as storage tanks, connectors, valves, and hatches on well heads.

“We use a very similar approach as the EPA, but with different underlying data,” explained Rutherford. “The EPA’s inventory and their modelling are actually very good – the best there is. It took me two years of digging through it to understand and try to build on it.”

Results generated by the new model closely match those from the top-down models – coming within the margin of error of the top-down inventories – finding that current EPA estimates are low.

A major source of the missing emissions, the researchers explained, could be liquid storage tanks. Some emissions are intentional, such as flashing, during which dissolved methane escapes when pressure is reduced. “It’s like opening a beer,” Rutherford said. “It’s liquid as long as there is high enough pressure, but if you release the pressure, the gas quickly escapes.”

However, many emissions are accidental, such as when technicians leave open a hatch or separation equipment fails. The combination of deliberate and accidental emissions leads to very high emissions from liquid storage tanks: one of the sources of error in the EPA’s inventory.

The researchers identify two benefits of their new methane inventory. The first is awareness: it highlights a shortfall in the current modelling and points the finger at certain activities that should be targeted by remediation or regulation. The second is providing a useful baseline tool on which to base modifications to current models, to improve their accuracy. Rutherford has been discussing with US regulators at the state and federal level, as well as petroleum industry figures, about what lessons can be learned from the new model.

“It is helpful simply to identify that there is a problem,” he said. “Beyond that, our model offers up some clear actionable steps to improve our inventories and ways operators can adjust their practices that could really make a difference in reducing the amount of methane entering the skies.”

The landmark UN climate report published earlier this week recommended that climate efforts pay closer attention to cutting methane emissions, which causes air pollution as well as global warming.