Mandatory Goals and Binding Emissions Commitments

There's lots of things that I do understand about the climate change debate and, of course, a number (quite possibly a larger number) that I don't. One that really doesn't make sense to me at all is this idea of "mandatory" and "binding" emissions commitments:

The environment secretary, Hilary Benn, yesterday called on the US to agree to mandatory goals for reducing greenhouse gas emissions, warning that the alternative was dangerous climate change.
...
"The only way forward must involve developed countries taking on binding emissions commitments because a voluntary approach ... isn't going to do the job," he said. "And that means all of us, including the largest economy in the world, the United States - taking on binding reduction targets. It is inconceivable that dangerous climate change can be avoided without this happening."
Let us, for the moment, accept the entirety of the debate: even then what the fourth generation MP (and third generation Cabinet Minister...there's social mobility for you!) says is nonsense. Targets, whether binding, voluntary or mandatory are irrelevant. Within the terms of the debate what will avoid climate change is a reduction in said emissions. Not the posturing of politicians about them.

Even if we leave that aside there's still no sense in this "mandatory" idea. For who is the enforcing power? If a sovereign government some years in the future has allowed emissions to be above said target, what is anyone going to do? Invade? Impose trade sanctions? Wibble at an international meeting?

The plain fact is that whatever the words used, such targets are always going to be voluntary. So why quibble over those words?