Author, educator and environmentalist Bill McKibben has just wrapped up "Do the Math": a campaign aimed at spreading ‘the most important numbers you’ll ever hear’ across the US.

The first of those numbers is 2.

2 degrees Celsius is the global average temperature rise that the Earth can handle before destructive weather events, changing weather patterns and sea level rise make our current way of living challenging at best and impossible at worst - for some plants and animals, even a 2 degrees increase will sadly be too much.