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.