How much coal, oil and gas can the world burn before climate change spirals out of control?

The latest science says that we can burn AT MOST one quarter of known reserves of fossil fuels.

That means we have a "safe" budget of just under 700 billion tonnes of carbon dioxide left.

Any more and we are likely to push mean global temperatures above 2°C.

Total known reserves are about 2,800 billion tonnes and the bulk of that would come from coal.

If you remember, 2°C is the figure most scientists and governments agree is the magic number. Above this and the climate will change so drastically that famines, droughts, destructive weather and pest outbreaks will become commonplace across the globe.

Who are the scientists?

This answer comes from a top team of researchers led by Myles Allen of the University of Oxford and Malte Meinshausen of the Potsdam Institute for Climate Impact Research in Germany.

Allen and Meinshausen are not environmental activists: they are top tier climate scientists. Their paper is in the latest edition of Nature magazine.

Even if we limit ourselves to that much we still only have a 50 percent chance of staying below 2 degrees.

What the research means?

The implications of this are enormous.

Firstly their research puts the science of stopping climate change that much simpler.

You can burn this much and no more. No parts per million, no emissions cuts below 1990 levels. Just this amount and no more.

And even more revolutionary is that their predictions clearly mean it’s no point us thinking we can burn all the fossil fuels just more slowly to avoid climate change. We simply can’t burn them so we have to find alternatives.

Here's a comprehensive list of 26 questions with answers on the research and what it means here.

So how long have we got?

The answer is scary.

At best 40 years, at worst 20 years.

Since coal is the biggest carbon dioxide producer we should stop using coal as soon as possible.

In the long term the two teams agree that we simply have to stop burning all fossil fuels.

What does this mean?

While the scientists did not say whether they thought it was likely that we could stay within our carbon budget, they did say it was possible.

That means it is imperative now that we put all systems go into developing renewable energy.

It also means that governments meeting in Copenhagen in December at the UN climate summit have to make a strong deal that pushes us towards a low-carbon future.