Scientists have calculated the total greenhouse gases we can safely emit. Divide that by the number of people on the planet (currently 6.5 billion, heading towards 9) to get each person's fair share: about 1 tonne per year. Given the UK average is 10 and one return flight to New York is 3.5, it's very difficult. But the other option - letting millions die - is worse.
Cutting your personal emissions by a factor of ten can be a daunting and unappetising prospect - but sooner or later, it is something we are all going to have to do, and many of the changes bring all kinds of other benefits.
For example you can get fit cycling to work, get a healthier diet with lots of local fresh fruit and veg, treat a train journey as part of your holiday adventure and make your home cosier and cheaper to run.
But it's hard to know where your biggest emissions come from (and we are very good at kidding ourselves when it suit us!). So the very best thing you can do before you do anything else is work out what is going to work best for you and your life – and that means a carbon audit.