From a Danish perspective I don't really see any positive long, or short, term gains from the Fehmarn tunnel, but I also live in the wrong part of the country.

This is a tunnel for Sweden, Norway and Copenhagen, it's moving the center of everything in Denmark closer and closer to the center of Copenhagen, completely disconnecting the rest of the country. A few days ago a new train start running Copenhagen to Oslo, a seven hour trip. That's the same time it takes me to get to Copenhagen by train within Denmark. Everyone is happy that you can "Get on the train and just pop to Hamburg, Berlin or Prag", but you can't, only if you happen to live in a few select spots does that work. It's a multi-day journey with a layover within the country if I want to leave by rail.

Internationally this is a great project, internally in Denmark, it's going to make international train travel worse for the majority of the country.

I really hate how German environmental activists resort to hyperbole and alarmist language by default when voicing concerns. This only makes it harder for me to take them seriously.

And then there is this tried and true tradition of commissioning studies with the sole intent to support a predefined viewpoint rather than taking an unbiased approach. This makes it so hard to trust any information when political arguments become heated.

To make the connection back to the tunnel: it consumes a huge amount of concrete and that releases the associated amount of CO2. Thisnpart is fairly easy to estimate. But estimating the impact on traffic emissions is fraught with issues. There are so many assumptions about lifetime, amount of traffic, types of vehicles that I can easily imagine the error bars to stack up to the point where a little tuning of model parameters gives just about any desired result.