One day, if we get it right, and we're lucky, humans will be able to look back and talk about 'the carbon age'. Starting with the industrial revolution, and ending at the end of this century, well be able to count the total amount of carbon we released during the carbon age. And if we've successfully managed the tricky situation we're in, that amount will be less than 1 trillion tonnes.

According to new research, that's the limit on what we can burn, ever, as a species, without severely messing up the planet. If we burn more than a trillion tonnes, well probably be too busy dealing with the effects of climate change to spend much time looking back into history. Normally, we talk about targets for reducing emissions - for NZ to do its bit globally we need to reduce our emissions here by 40% by 2020. But in a paper in the science journal Nature, a group of researchers decided to describe how emissions have to change in another way - in terms of a 'carbon budget'.

So what is a carbon budget, exactly?

Broadly, once carbon is in the atmosphere it stays there for centuries. How hot the planet gets is controlled by the total amount of carbon dioxide in the atmosphere - not how much were emitting at the moment. So, for a temperature rise of 2 degrees, there's a total amount of carbon we can put into the atmosphere. If you like, it's a carbon budget to buy us 2 degrees of warming.

The researchers reckon that putting 1 trillion tonnes of carbon into the atmosphere will warm the world by 2 degrees. (There are lots of caveats around that - we're in the world of scientific probabilities, remember but its a dependable result.) To keep climate change to levels that aren't dangerous (on the generally accepted definition of what dangerous means), then in total, for ever, we can burn 1 trillion tonnes of carbon, and no more.

A trillion tonnes sounds like a lot, even for the complete carbon budget of a species. But the reality is that we're halfway there already - we've emitted half that since the industrial revolution, and were burning the stuff faster and faster. Say we burned the remaining half of our budget - 500 gigatonnes of carbon - this century. That works out at 5 a year. At the moment we emit about 9 gigatonnes of carbon a year - nearly twice that. We're overspending badly.

The thing about a budget is that it's finite. The longer we continue to blow our yearly allowance, the less we're going to have to spend in the future. So here's the deal - we've got 500 gigatonnes of carbon left to spend, as a human society. We've got to spend that budget intelligently. We already know most of the things we need to do to cut emissions - and they're fairly straightforward, and only require technologies that we've already pretty well developed.

Global warming is telling us that we've got to break the link between carbon and how we live - cutting out the things we can no longer afford on our budget - coal, oil, jet fuel, deforestation, intensive industrial farming. Clean renewable energy doesn't spend from our carbon budget - neither does cutting the amount of energy we waste. New coal power plants, replacing forests with intensive dairy farming, or flattening the Amazon rainforest are sure-fire ways to blow the budget. And, by 2100 we've got to have cut out carbon emissions altogether. There are good reasons why we should do it faster than that, but that's the bottom line thats going to let us move out of the age of carbon, without causing catastrophic climate change.

This post was originally published by Greenpeace UK