to form two rings within one year, e.g. one in a warm early spring and then, after a cold phase, one more in a warm fall (H. Özbal and P. Kuniholm, priv. comm.); and (iii) there are age offsets between the tree ages of different laboratories, e.g. $\pm$ a few yr by comparing Irish oak trees with Seattle measurements (e.g. Reimer et al. 2004). Even though those effects can partly cancel each other, the exact year of the $^{14}$C increase and a possible event may be somewhat uncertain by up to a few years. And even though the $^{14}$C variation in AD 774/5 was detected in several different trees on different continents, always for the same year AD 774/5, that does not neccessarily mean that the year is exactly correct (nor that, e.g., no tree rings are missing), because it could be that in a cold year, no trees at all form a new ring, e.g. due to a large vocano eruption with global effects, see Mann et al. 2012.

Given this timing uncertainty, it is neccessary to search for unusual highly energetic events around the time of AD 774/5, and even if none were found, this would constrain the 