It's not about methane stored in gas tanks - the problem is methane trapped underground which we intentionally release in order to store it. The way we release it is far from a controlled process and it's impossible to capture all of the released methane. In any developed country they are required to monitor (and minimise) methane releases however there are widespread allegations of under-reporting and failures to capture as much as they could.

If you ask the gas mining companies, they claim it's a minor issue but if you actually measure methane in the atmosphere, which we are doing (it's easy to do), then it's pretty clear this is a massive problem. There's far too much methane in our atmosphere for all of it to be coming from other sources.

When you burn gas, it becomes CO2 and is released into the air. When methane is released without being burned, it's so much worse than CO2 that even with very low rates of methane release it still has a bigger impact on the climate than all of the burned gas in the world.

Exactly how much methane is released by mining is unclear but what we do know for sure is how much methane is in the atmosphere right now, and we know that it accounts for about a third of the climate change we are experiencing.

Having said that - gas is still better than coal. There are several reasons but one of them is coal mining also releases methane.