P&O going green - What they aren't telling you.......

If you've booked a cruise with P&O lately and had a look at your cruise personaliser you may have noticed something new. There is now an option on there for you to go green.
Now we all know about global warming; we all now about deforestation; so the automatic reaction is to tick the yes box on something like this; yes please I'd like to go green!
But what aren't P&O telling you?