If you're reading this in the UK, you might like to raise a glass this evening - either in celebration or commiseration, depending on your point of view.
From today, you are living under the only law in the world that obliges your government to reduce greenhouse gas emissions by the kind of figures that the Intergovernmental Panel on Climate Change (IPCC) implies are necessary if the world's inhabitants are to avoid "dangerous climate change".
The UK's Climate Change Bill, which has just received Royal Assent, sets "legally binding" targets of reducing emissions by 26% by 2020, and by 80% by 2050.
Phew. Take a breath: these are big-sounding numbers.
The government's primary aim is, obviously, to slash UK emissions. But there is a second - to present a template that other countries could follow as they too look for effective policies.
Other leaders have already endorsed the 80% by 2050 target, most notably the US leader-in-waiting Barack Obama.
(Turning that into US law, incidentally, is a much bigger ask. Everything is measured against 1990 levels and since then, UK emissions have fallen by about 16% while the US's have risen by a similar percentage.)
One of the big questions is whether the bill will work; another is whether it is the most sensible way to achieve the size of reductions that the government considers necessary.
Firstly, that phrase "legally binding": what does it mean, exactly?
Well, one thing is for certain: no minister is going to be carted off to jail, turned out of their home or sent to the stocks for failing to meet the target, either now or in 2050.
They will be called into the head teacher's study - sorry, the House of Commons - and given a painful public verbal flogging; but that will be about all.
Energy and Climate Secretary Ed Miliband acknowledges that the lack of sanctions is a problem with the bill; but it is hard to imagine that any group of politicians anywhere would have gone further.
Then there are the political realities.
On Monday, the Climate Change Committee, the government's new advisory body, will present suggestions for five-yearly "carbon budgets" that can take us to the 2020 target.
Mr Miliband is obliged to take its advice into account when deciding the budgets, but not to follow it. Specifically, he must also take such factors into account as "economic circumstances, and in particular the likely impact of the decision on the economy" as well as fiscal social circumstances, "in particular the likely impact of the decision on fuel poverty".
So there is wiggle room.
And Mr Miliband - a relatively junior cabinet minister - will have to persuade or charm or beat colleagues from other departments, including the Treasury, to implement policies designed to achieve the carbon budgets.
It is also legitimate to ask why the UK needs its own target-based bill given that the EU sets targets across the entire bloc - targets which do have teeth, given the power of the European Commission to fine states that do not comply.
And there is another question, one sometimes asked about the Kyoto Protocol: is setting targets the best way to cut emissions?
After all, countries such as Spain, Greece and New Zealand all have Kyoto targets and intend to abide by them, but they have a very long way to go very quickly if they are to do so.
There is an argument for saying that targets are less important than just getting on with measures such as increasing vehicle efficiency, investing in home insulation and building a big fleet of low-carbon power stations that will each bring real reductions.
The Climate Change Bill, by itself, is an architect's drawing. It shows how the government wants its edifice to look; but by itself, it is a paper house.
Building the real thing is going to be a much harder task. And it will be interesting to see, in the months and years to come, what the rest of the world's would-be carbon cutters make of the UK's approach.