Not many people can attract a decent sized crowd to give what is essentially a maths lesson.
Bill McKibben, a journalist turned global leader of climate change campaigns, is an exception.
His message is blunt: to avoid dangerous climate change, the Earth’s temperature must not rise by more than two degrees.
To have any chance of achieving this goal – the planet has already warmed 0.8 degrees since the industrial revolution, no more than about 560 gigatons of carbon dioxide can be pumped into the atmosphere before mid-century.
But buried in Australia’s soils are coal reserves that, if dug up, sold and burned as fossil fuel companies plan, represent more than 150 gigatons of C02.