Made of melted snow, the Callery River tumbles cold and cobalt from the ramparts of South Westland’s Main Divide. There’s no such thing as easy country here: the river squirms between flanks so steep that the contour lines on a topomap form solid blocks of orange. You might call it ‘tiger country’. It used to be rat, stoat and possum country too. But no longer.

It’s the most abundant element in the universe, but just a tiny breath of hydrogen is a tailwind driving Predator Free 2050.

Suddenly, carbon’s a thing. Everyone wants to know how much we’re sitting on. But how do you measure something you can’t see?