So, yet another thing to worry about. It might not sound like news, but something big hit earth back in the 770s. Researchers looking at the radioactive carbon traces in tree rings from that period have discovered evidence of a burst of intense radiation between 774 and 775AD. It might seem churlish to worry about something that happened 1,200 years ago, given the problems that the Rio+20 summit this month clearly was not even going to begin to address. Yet warning us of potential danger is part of the scientist’s job description. What we do with that information is up to us.

The source of that 1,200-year-old radiation burst – potentially the harbinger of a much bigger catastrophe than climate change – is a mystery to scientists. A burst of radiation of this sort would normally come from a spectacular solar flare or a supernova. Taking the second option first, the exploding star in question would have been bright enough to be visible in daylight – a second sun that would have been recorded by contemporary historians. It should also have been spotted by today’s astronomers: the explosion would have created what looked like a new star. Stellar explosions recorded in 1006 and 1054 weren’t big enough to cause a spike in radiation but we have spotted the remnants in the sky.

The solar flare explanation has been ruled out, too. A flare occurs when the sun spits out a gob of plasma, a roiling gas of charged subatomic particles. If that is composed largely of protons and fired towards earth, its interaction with particles in Planet Earth’s atmosphere creates a burst of intense radiation. But it also creates the Northern and Southern Lights and, again, a radiation spike of this intensity would have produced a show spectacular enough to be recorded by historians.

On the downside, it would also have wiped out much of the ozone layer, causing biological chaos. The extra radiation and intense ultraviolet light usually absorbed by ozone would have mutated genomes and led to significant extinctions. There is no evidence that this occurred.

Why does it matter? Until we understand the source, we face a significant unknown. If it is somehow a threat from the sun, the threat is far bigger than we have known. Such a spike in radiation could be associated with the kind of solar activity that could melt many of the world’s power grids, sending civilisations howling back to the Stone Age.

We already know that things from space could hurt us badly. There’s about a one-in-five chance that, in the next 100,000 years, an asteroid strike will do as much damage as the one that wiped out the dinosaurs. There is no reason for complacency: it’s highly likely that, in the same time frame, a chunk of space rock roughly 400 metres in diameter will hit earth. The impact will be significant enough to devastate an area the size of France.

Too much in the sun

The tree-ring work, published in the journal Nature this month, suggests that scientists don’t yet have a handle on all the threats to humanity. But it will almost certainly be dismissed as another curiosity, not worth following up.

Maybe that is the right response. The conclusions were a result of interpreting the amounts of radioactive carbon – created by collisions between particles in the upper atmosphere – trapped in the tree rings.

Anyone with responsibility to act over threats to his or her citizens can choose to find weaknesses and uncertainties in the data, the extrapolations and the conclusions and thus justify a non-response. Just as our political leaders (or, rather, their delegations) will do in Rio. l

Michael Brooks’s “The Secret Anarchy of Science” is out now in paperback (Profile Books, £8.99)