Policymakers might not be receiving accurate estimates for methane leakage from off-shore oil and gas platforms, leaving them unable to properly assess global emissions as they tackle climate change, according to research led by scientists from Princeton University.

Using a laser-based instrument mounted on small fishing boats, the researchers estimated methane emissions from eight North Sea production platforms off the coasts of England and Scotland. On average, methane leakage occurring during normal operations more than doubles each installations’ reported emissions to the U.K.’s National Atmospheric Emission Inventory.

The large number of offshore platforms around the globe make careful measurement and analysis of leakage critical for global emissions inventories and for mitigating climate change, the researchers said.

The difference between the estimates and findings in the North Sea study is the equivalent of having an additional 33,000 cars on the road. Previously, offshore rigs were viewed as nearly 100% efficient, with no leakage of methane except during certain actions including releasing gas to control pressure inside wells or moving oil from rigs to ships.