According to the ABC’s Vote Compass, a majority of voters has put the environment ahead of the economy as the top election issue. Yet the budget we haven’t heard about in this so-called "climate election" is the carbon budget. That’s the budget set by the laws of physics and chemistry to hold global warming to the safer side of 2 degrees.

A polar bear climbs out of the water in the Franklin Strait in the Canadian Arctic Archipelago. The Arctic is suffering dramatic loss of sea ice. Credit:AP

Barring some speculative technology deployed in the next decade on massive, unprecedented scales that pulls down more carbon from the atmosphere than we are putting up, the emissions budget that humans must not exceed is 1000 billion tonnes of carbon – give or take. That’s the total carbon budget – from the beginning of the industrial revolution – to keep global warming strictly below 2 degrees with at least a 2/3 chance.

But the amount we have left to “spend” is much less, for three reasons. First, humans have already emitted 585 billion tonnes of carbon over the course of history until the end of last year. That must be subtracted to see what’s left.

Second, other greenhouse gases, such as methane and nitrous oxide, also cause warming, so we need to account for their effects. That’s another 210 billion tonnes of carbon we can’t spend.