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Birkenesobservatoriet. Photo: Chris Lunder, NILU

New estimates of Nord Stream methane emissions: An event of historical significance

Since the Nord Stream leaks in September 2022, scientists have tried to determine the exact amount of methane released into the atmosphere.

“The Nord Stream methane leak is the largest recorded amount of methane released from a single event. Still, with the amount we have now calculated it is only equivalent of 0,1 % of the total anthropogenic methane emissions for 2022,” says senior scientist Ignacio Pisso from the climate and environmental research institute NILU.

A dramatic event

If it is such a small part of the total, why are scientists still trying to pinpoint the exact amount of methane released?

The rupture resulted in gas bubbling up through the ocean, creating week-long mounds of foamy seawater up to several hundred meters in diameter.

“It was a very dramatic and visible event of historical significance. With such unprecedented amounts of methane released we did of course want to understand the potential climatic impact,” explains Pisso.

From a scientific viewpoint, it is worth quantifying the leak not only to understand whether there was a climate impact. It is also important to understand the scale of what leaks are possible from such events, for future reference.

New and higher estimate

In a new study published in Nature 15 January 2025, an international team of scientists have collaborated to determine exactly how much of the pressurized methane gas released from the pipelines ended up in the atmosphere.

According to their estimates, around 500 kilotons (kt) of methane were contained in the Nord Stream pipelines before rupturing. The amount is derived from volumetric calculations and simulations based on the pipelines’ physical parameters and information from pipeline operators.

The new estimate is that between 445 and 485 kt methane reached the atmosphere, which is significantly higher than the 75-230 kt estimated in an initial report from February 2023.

Wide approach

The scientists reached this conclusion based on initial modelling of the atmospheric emissions from the leak throughout its duration and a synthesis of several different approaches: Measurements from planes, point-source imaging satellites, and cumulative emission estimates based on satellite and tall-tower observations.

NILU contributed with an estimation of the leak based on inversion modeling.  In inversion modeling, they already know the concentration of the substance in the atmosphere. Then, they calculate back to get an estimate of what the original emission was.

After doing so, their initial estimate of the leak was refined to better match the tall tower observations.

“Using ICOS data and NILU’s own instrumentation, we provided an estimate that turned out to be very close to the initial modeled leak. We were even able to accurately predict the arrival of the methane gas several days later to as far away as the Zeppelin Observatory in Svalbard,” says senior scientist Stephen M. Platt.

Will the exact numbers be revealed one day?

Based on these numbers, between 55 and 15 kt of methane was never emitted to the atmosphere.

“Any methane that did not reach the surface will have dissolved into the ocean. From there, it would be consumed by bacteria and converted to carbon dioxide. It might also have been carried by ocean currents and escaped from the ocean some distance from the leak site,” explains Platt.

As of now, this is probably the best methane emission appraisal of the Nord Stream leaks. Platt finds it intriguing to think that the true value might be known by involved parties, and that one day it might be made public. Then, it will be interesting to see exactly how accurate their estimates were.

Simulation showing how the methane gas cloud is released from the source and transported through the atmosphere. The simulation shows the location of relevant ICOS monitoring stations and additional NILU data. It does not show the total gas cloud, only the part of it that is close to the surface. Each grid cell used in the simulation is about 20 km x 10 km. ECMWF is acknowledged for providing the meteorological data from the IFS model. The simulation is not intended as and should not be used as a basis for public health advice. Source: NILU