When it comes to better understanding the impact of emissions from offshore oil and gas storage and production, there are several factors that must be accounted for.

On a hot Louisiana morning in 2021, the Diamond Gas Rose glided down a narrow channel between marshes and swamps towards one of the state’s liquefied natural gas (LNG) terminals.

The Gulf Coast is no stranger to LNG tankers like this one. Since the first LNG export terminal opened in Sabine Pass, Louisiana in 2016, four new terminals have been constructed in the region, turning the United States into the world’s largest exporter of natural gas in just a matter of years. The physical footprint of this rapid ascent is hard to miss: ten-story storage tanks dominate the landscape and flares burn throughout the night. Experts have raised concerns about the environmental hazards posed by these LNG processing plants; however, the hundreds of LNG tankers that frequent these terminals have quietly brought their own problems.

On April 26, the Diamond Gas Rose docked at the Cameron LNG terminal, one of the newer terminals in the area. While loading LNG two days later, methane started leaking from a vent aboard the ship, eventually forming a mile-long plume over the ship, terminal, and the surrounding marsh, according to a National Response Center (NRC) report collected by SkyTruth Alerts. The Global Airborne Observatory (GAO), a monitoring plane studying methane in the Gulf of Mexico, detected the plume that afternoon and estimated that at least 2783 kilograms of methane were released every hour by this source, making it one of the larger emission events detected by GAO that year. In an email statement, Cameron LNG said it was aware of the  methane plume and notified the NRC, but was unwilling to comment on the cause or source.

Leaks like this from fossil fuel infrastructure are not uncommon. According to an analysis of Carbon Mapper methane data, in 2021, the GAO detected over 2000 other plumes releasing nearly 600 metric tonnes of methane from oil and gas infrastructure across the United States over the course of seven months. While significant progress has been made to monitor these point sources of onshore methane, the offshore oil and gas platforms that produce natural gas, and the fleets of vessels that transport LNG around the world, are difficult to track and even harder to monitor.

Recent advances in maritime methane monitoring suggest that the offshore production, liquefaction, and transportation of this ‘green’ fuel is far more polluting than previously imagined and that the problem may be growing, but more information is still needed about these murky methane sources.

The Danger of Methane

Methane is an odorless, flammable gas that is produced through the decomposition of organic matter. Because burning methane produces less carbon dioxide and fewer air pollutants than other fossil energy sources like coal, it has become a popular substitute for dirtier fuel sources. But, if released uncontrollably, methane can create a bigger problem than the fuels it replaced.

In the near term, methane has over 80 times more impact on our atmosphere than carbon dioxide, but it breaks down faster than carbon dioxide, meaning its impact is massive, but short lived. Methane already accounts for roughly 30% of rising global temperatures and emissions are only expected to increase, so while a decrease in carbon dioxide emissions remains an important long-term climate solution, methane’s significant warming potential and short lifespan means any reduction in atmospheric levels would have “a rapid and significant effect” making it especially important to reduce the most immediate effects of climate change, according to the Environmental Protection Agency (EPA).

In response to growing recognition of the importance of combating methane emissions, 150 countries have signed a multilateral pledge to reduce methane emissions by 30% by 2030. In pursuit of this goal, the United States declared a temporary pause on approving new LNG exports and the EPA has proposed to tax companies that produce excess methane. Yet methane emissions from the energy sector continue to reach record levels.

One of the biggest challenges to reducing methane emissions globally has been the availability of national emission data. Most countries have little or no methane measurement data so developing methane monitoring capacity “will be an important marker of early progress,” in reducing global methane emissions, says the International Energy Agency (IEA). Even among the countries with emission data, current estimates offer an incomplete picture of methane emissions.

An Incomplete Picture

National methane emission inventories often use standardized emission rates and the activity of infrastructure to estimate the scale of emissions. While these estimates are easier to calculate, generalizations about the infrastructure can lead to discrepancies between the estimated rate and the actual rate of emissions.

Leaks and other large emission events, for example, are unpredictable and may not be consistently incorporated in bottom-up inventories. As a result, the IEA estimates several countries may be underestimating their methane emissions by up to 50%. Industry estimates are similarly unreliable; methane emissions in oil and gas company reporting may be 95% lower than IEA estimates.

Remote monitoring by specially equipped airplanes and satellites can help bridge this gap by independently measuring methane emissions from above. An instrument aboard the International Space Station, for example, has been used to detect atmospheric methane. In 2022, the sensor detected dozens of methane plumes across the globe, including several plumes in Turkmenistan that were releasing over 50,000 kilograms per hour. Turkmenistan has not submitted greenhouse gas data to the United Nations since 2010, so measurements like these are the only accurate data available to track these emissions. Data from satellite imagery have either been limited in their resolution and scale or limited by commercial contracts but new satellites, like MethaneSAT, which launched earlier this year, will be providing the public with more frequent readings of higher resolution over larger areas.

While inaccuracies and deficiencies in onshore methane data are being overcome by new technologies and wider coverage, measuring offshore emissions presents a bigger challenge.

Challenges of Offshore Monitoring

As light passes through methane gas, certain wavelengths are absorbed by the molecules. These “spectral fingerprints” are how sensors aboard satellites and aircraft can distinguish methane from other gasses, but there must be enough light passing through the gas to be picked up by sensors.

Clouds, which are more abundant over the ocean than land, can block the light that has passed through the methane plume and obscure satellite readings. Water also absorbs more light than land, which can reduce the amount of light that passes through methane emissions and the detectable spectral fingerprint. As a result, current maritime methane detection methods require sensors to pass over potential plumes at a specific angle to capture sunlight that reflects off the ocean surface and through the gas. Additionally, range limitations for methane monitoring aircraft makes monitoring distant infrastructure and emissions from LNG shipping unfeasible.

Despite these challenges, existing measurements have shown that offshore methane leaks may be a bigger problem than their terrestrial counterparts and growing fast.

More Offshore Pollution

While leaks from onshore oil and gas infrastructure have captured the attention of activists and regulators, persistent leaks and super-emitting events could make offshore infrastructure more polluting than some onshore facilities. A Carbon Mapper study of methane plume data in the Gulf of Mexico found that offshore infrastructure produced higher and more persistent emissions than onshore facilities. Many of the targeted oil and gas platforms with observable emissions lost 8 to 22 times more methane than operations in the United States’ most productive oil fields. Many of these plumes were also detected repeatedly and just a handful were responsible for most of the methane released. For example, in April 2021, the GAO detected a methane plume from a Chevron oil and gas platform in the Gulf of Mexico. Eight days later another plume was detected from the same platform. In total, both plumes emitted at least 1600 kg of methane per hour. The incidence of maritime methane pollution like this is only likely to increase as the industry invests more into offshore oil and gas platforms.

Over the past two years, spending on new offshore fossil fuel projects has risen rapidly, driven primarily by state-owned oil companies in the Middle East. With 12,000 offshore oil and gas installations worldwide and hundreds more in development, methane emissions from offshore infrastructure may continue to grow.

Experts believe that methane may also be leaking from the global fleet of LNG tankers where measurements are much more elusive. Research on fugitive methane emissions from ships by the International Council on Clean Transportation found that the unloading processes of the largest LNG tankers can emit between 24 and 40 kilograms per hour. Although these individual emissions are more difficult to detect by satellites, in aggregate, small leaks from the thousands of LNG cargoes unloaded each year could create a large effect. But the movement of LNG tankers can make any emissions difficult to track by satellite and expensive to monitor from the ground. The leak from the Diamond Gas Rose is just one of a few ship-based leaks to be caught on camera.

The need for maritime methane monitoring

The IEA estimates that three-fourths of methane emissions from oil and gas infrastructure can be reduced by repairing leaks. The first step, however, is to identify where leaks are happening. While terrestrial methane monitoring continues to improve, our current understanding of when and where offshore leaks occur remains limited even as the use of tankers like the Diamond Gas Rose and construction of new offshore infrastructure to supply the world with cheap natural gas continue to increase.

Luckily, recent advancements in offshore monitoring have improved our understanding of marine methane emissions. Since 2022, researchers have demonstrated how sensors aboard existing Earth-observation satellites can use sunglint to detect large offshore methane emissions and, more recently, researchers successfully demonstrated how offshore emissions below 100 kilograms per hour can also be detected using the commercial GHGSat constellation. While existing missions either measure methane globally or assess specific point sources, the MethaneSAT program–launched in early 2024–promises to provide regional methane data. With plans to equip the satellite with sunglint measurement capabilities, the program may offer the first holistic picture of methane in large marine areas and, together with Carbon Mapper’s closeup satellite mission, will provide significantly more publicly-accessible offshore methane data.

“The ability to detect–and measure–methane leaks and emissions from space is a real game-changer,” says John Amos, SkyTruth’s founder. “But methane detection over the ocean is much more technically difficult than methane detection over land. That’s why we’re working with organizations like Carbon Mapper to identify high-priority targets in the ocean, helping them focus their data collection efforts where it will produce the biggest impact.”