Since then, he has devoted much of his attention to uncovering the causes of these global environmental downturns. He believes that impacts from cometary debris may account for most of the downturns, especially the AD 540 event. This hypothesis is supported in work by British cometary astrophysicists, who find that earth was at increased risk of bombardment by cometary debris in the AD 400-600 timeframe, based on the frequency of fireball activity in the Taurid meteor streams recorded in Chinese archives.