What that article tells us is that there was a 1.2% spike in Carbon-14 in 774 in Japan.
What it doesn't tell us is if that spike was local to Japan, to the Northern Hemisphere, or was a global spike.
There are three possible reasons for an increase in Carbon-14 (and combinations thereof); more Carbon in the upper atmosphere (either local to the cell above Japan or more extensive), a loop in the Earth's geomagnetic field increasing the amount of cosmic rays interacting with Carbon (with no change in external cosmic ray levels), or an increase in the external cosmic rays striking the Earth (with various possible extra terrestrial causes).
More data and a better understanding of the Carbon cycle are needed to move beyond speculation.
There are several articles in play here, the OP BBC News article, the wikipedia article (linked by the person I replied to and the one to which I referred) and the primary source papers about Carbon in Japan and Beryllium in Antartica.
My points stand, increased external cosmic rays (for whatever reason, flares / nova / etc.) are not the only explanation for relatively low spikes in isotopes.
When the scientists checked the carbon-14 levels in the cedar tree rings, though, they noticed something strange: a spike in carbon-14 corresponding to the years 774 and 775 AD. It's normal for carbon-14 concentrations to vary a little bit from year to year, but the normal variation is just five-hundredths of a percent. During 774 and 775, the levels jumped by more than 1 percent -- a dramatic leap.
What kind of event would have showered Earth in the kind of radiation necessary to make so much carbon-14? Could a supernova do it? Supernovas do sizzle with gamma rays, which can produce carbon-14 in the atmosphere. Yet even the supernovas that happened in 1006 and 1054 AD -- which, unlike the 774 event, were dramatic enough to catch the eyes of ancient astronomers -- weren't big enough to leave a carbon-14 fingerprint in tree rings. If it was a supernova, it would have to be a strange one: the Invisible Man of supernovas, incredibly powerful yet unseen.
Indeed, astronomers can still see remnants of the 1006 and 1054 supernovas today. If a bright supernova occurred in 774, we should see a correspondingly glaring source of X-rays and radio waves somewhere nearby -- but we don't.
Could our own sun be the culprit? Solar flares douse the atmosphere in high-energy protons, which can also produce carbon-14. But a solar flare capable of making enough carbon-14 to explain the tree rings would be bigger than any solar flare ever recorded, say the scientists.
Though it is not beyond the imagination that our sun could have belched out such a "super flare," someone on Earth would surely have noticed and made a record of the resulting auroras, which would be unlike anything ever seen before. And if the auroras weren't worth writing home about, surely the mass extinction would be, as it's likely that the flare would destroy the Earth's protective ozone layer.
Not everyone is convinced that we should rule out the solar flare option, though. Maybe conditions were just right for a ho-hum flare to create a surge of extremely high energy protons. Or maybe, instead of just one flare, the sun let loose a series of smaller ones over a period of years.
Whatever happened, there are hints that it was a global phenomenon, not a local one. Carbon-14 levels in trees from North America and Europe were elevated around that time, too. All the way in Antarctica, layers of snow and ice were laced with traces of another isotope made by cosmic rays, beryllium-10. Though it's difficult to put a precise date on the sudden peak in beryllium-10, it seems to have occurred around the same time as the carbon-14 spike.
Again, an increase in cosmic ray levels striking the Earth is only one possible cause for the observations.
Oddly enough, while no expert in the field, I did spend a decade working with high resolution radiometric and magnetic survey equipment measuring the global magnetic field, local and diurnal variations, ground radiation and performing numerous high altitude stacks to plot the falloff of cosmic ray intensities through the atmosphere. This was performed in both hemispheres and near both poles and totalled several million line kilometres. Writing the software to process all that data gave me a little insight into the issues and the opportunity to work with people that had spent their careers in the field.
What the spikes tell us is that there was minor increase in isotope production, most probably in the upper atmosphere.
Clearly this could be driven by either an increase in raw material in the upper atmosphere (equatorial volcanic explosion is one possibility), or an increase in cosmic ray levels within the atmosphere (variations in the global magnetic field lines could account for that) OR an increase in the amount of external cosmic rays (which brings us to the guesswork being expounded in the article you so kindly quoted).
I'm no expert either, but I was under the impression that the paleomagnetic record is pretty good for at least the last thousand years. So a magnetic variation which could cause a 20-times-normal variation in the production of carbon-14 (from 5/100 of a percent to 1 percent) would have left some evidence?
Likewise for volcanic explosions, especially considering the narrow time scale; enough material was pumped into the atmosphere to cause that 20-fold increase in variation, but its effects were gone in a year or less?
Anyway, I know that mainstream science reporting is not the best, but just because scientists don't explicitly say "we've considered alternate explanations and found them less likely" doesn't mean they haven't done that.
From the wikipedia account, assuming it's accurate;
> The 774–775 carbon-14 spike was an increase of 1.2% in the carbon-14 content of tree rings during the years AD 774 or 775, which was about 20 times higher than the normal rate of variation.
A somewhat convoluted way of saying that the increase was only 1.2%, but this was an uncommon variation (a spike) as most of the time (year to year)(?) variations where 20 times smaller.
Without delving into the mechanics of possible transfer functions and localised concentration for other reasons and just going for a simple linear coupling ... this could simply mean that 1.2% more material was raised up or that 1.2% more cosmic rays were being let through ... and that this unusual (like an eruption, like field lines hiccuping, like a flare or a nova) as "normally" you see a much smaller variation in isotope levels.
Now the paleomagnetic record isn't continuous and year by year globally complete, it's formed from spot observations of the magnetic orientation at ground level in rock flows as they cooled. Inference of field strength at ground level from this is difficult, and inference from that to field orientation a few kilometers overhead is, umm, akin to reading tea leaves.
The IGRF (International Geomagnetic Field) is a richly structured and constantly fluctuating 3D beast that pulses on a daily basis and can only be coarsely approximated with confidence over five year intervals (the spherical harmonics used for this are fun).
> but just because scientists don't explicitly say "we've considered alternate explanations and found them less likely" doesn't mean they haven't done that.
A moot point as nor does it mean they have; what we have is mainstream reporting on the speculations of those scientists that are hypothesizing on the assumption that increased isotope production was caused by an increase in external cosmic ray levels.
There are possibilities that arise from the assumption of no such increase that are equally interesting and have similar probabilities (ie equally difficult to quantify)