Not much. Unlike oil, the gas either dissolves in water or rises into the atmosphere - assuming that 100% of the gas rise up, it's the CO2 equivalent of two weeks of Germany's emissions [1].

Yes, it's a lot, but unlike an oil spill there won't be much other impact.

[1] https://www.zeit.de/wissen/umwelt/2022-09/nord-stream-pipeli...

Minimal. Its only the pipes leftover gas that is escaping, compared to a oil field. Together with that gas rises, contamination of water is minimal, again compared to what an oil leak would do. Adding up to that experts say that long-term effects of these leaks are essentially not going to be noticeable.[1]

https://news.err.ee/1608730996/scientist-minimal-lasting-env...

1.3 million cars driving for a year [0]

[0]https://www.theguardian.com/environment/2022/sep/28/nord-str...

According to this tweet [1] by someone at a German climate NGO it is about 15 Mt CO2e, looking at the greenhouse effect of the Methane in the next 20 years.

[1] https://twitter.com/SaschaBay/status/1574664872293715968