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Improved estimates of Nord Stream leaks

Ill.: NILU

12 October: NILU scientists provide new estimates of the methane emitted from the Nord Stream leaks. The new estimates suggest that in total, between 56 000-155 000 tonnes of methane were released into the atmosphere.

Estimates of the methane released into the atmosphere from the Nord Stream leaks were made using atmospheric observations at sites in the ICOS network and one additional site at Kjeller, Norway, operated by NILU.

“The method uses a model of atmospheric transport to relate the observations of atmospheric concentrations to emissions”, explains senior scientist Rona Thompson at NILU.

The four Nord Stream leaks occurred at approximately two locations on 26. September 2022. The first leak started at 02:03 local time (Nord Stream 2, Pipeline A). Later leaks started at 19:03 local time (Nord Stream 1 Pipeline A and B, Nord Stream 2 Pipeline A) and occurred over a small area, approximated by a single location in the NILU estimates.

Breaks in 3 the pipelines located close to Bornholm, Denmark, effectively resulted in 6 separate sections of broken pipeline: 3 shorter sections on the German side and 3 longer sections from Bornholm to Russia.

A first estimate of the gas leaking from the pipeline into the sea can be obtained based on the initial pressure of gas in the pipelines.

Figure 1 (left) shows the first estimate of the emission rate into the sea from a single pipeline. The emission rate is higher at the start because the pressure in the pipeline drops over time, and because the shorter sections of the broken pipeline empty more rapidly. Days are counted from the start of the leak. Figure 2 (right) shows the pressure in the two sections of broken pipeline over time. The pressure in the short length of pipe (German side) is very consistent with observations, namely, pressure dropped from 105 to 7 bars over several hours, while the pressure in the longer section decreased more gradually. Source: NILU

“At NILU, we have been studying natural and anthropogenic emissions of methane from the sea for many years; this amount is unprecedented”, adds senior scientist Ignacio Pisso, also NILU.

After getting new data from the Birkenes observatory and doing a rerun of the model based on the first 3 days of the leak, he saw that the amount of methane leaking out could reach 155 000 tonnes.

Challenging estimates

However, the amount emitted to the atmosphere is more difficult to quantify as not all methane released into the sea will reach the atmosphere.

According to senior researcher and team leader Bénédicte Ferré from CAGE/UiT – The Arctic University of Norway, naturally occurring microbes feed on methane coming out of the ocean floor.

“However, in this case, the amount of methane is orders of magnitude larger than that occurring naturally. It is also being released with such power that the microbes won’t have time to consume much of it”, she says.

To get an estimate of how much actually reached the atmosphere, scientists at NILU used atmospheric observations, but “these estimates are challenging to make, owing to uncertainties in the modelled atmospheric transport”, explains Thompson. “The diameter of the emission sources is only in the order of a hundred meters, but the atmospheric observations were recorded at sites up to hundreds of kilometers away.”

“Differences between the actual and modelled timing and location of the methane plume influence our emission estimates. Atmospheric flow is chaotic, and we are looking at a particular feature,” adds Ignacio Pisso.

Negligible climate impact

“If we compare the 155 000 tonnes of methane suspected to leak out of the Nord Stream pipes with the 22 000 tonnes of methane expected to be released from the Norwegian-western Svalbard margin per year, we see that the Nord Stream would leak between 2.5 and 7 times more than this natural phenomenon in just a few days,” continues Bénédicte Ferré. “The Nord Stream leak also occurred in shallower water.”

Although the Nord Stream leaks were large, the overall climate impact is still negligible. The approximately 56 000-155 000 tonnes released represents only about 0.01-0.03% of the methane emitted globally every year. The marine impact is also small since it lasted only for a short time period.

“What this event does highlight is the vulnerability of gas transport and storage infrastructure and its associated climate risk”, says senior scientist Stephen M. Platt from NILU. “Although natural gas as an energy source is considered to have a lower climate impact compared to coal, this advantage will be lost if a significant amount of methane leaks out.”

Methane is 30 times more powerful than CO2 as a greenhouse gas over a 100-year period and has a relatively short-lived lifetime in the atmosphere – about 10 years.

“Thus, reducing methane emissions needs to be a global priority this decade if we are to have any chance at keeping warming below 1.5°C by 2030”, concludes Rona Thompson.

Figure 3 shows the methane emissions from the inversion (average for 26-28 Sep) but interpolated to 0.5 x 0.5 degrees. The leak emissions are “smeared out” owing to the limited resolving power of the inversion due to the uncertainties in the transport but also to limited information provided by observation. Source: NILU