Theories will sometimes reflect the particular concerns that historians might have on cultural, political, or economic trends in their own times. Gibbon's criticism of Christianity reflects the values of the Enlightenment; his ideas on the decline in martial vigor could have been interpreted by some as a warning to the growing British Empire. In the 19th century socialist and anti-socialist theorists tended to blame decadence and other political problems. More recently, environmental concerns have become popular, with deforestation and soil erosion proposed as major factors, and destabilizing population decreases due to epidemics such as early cases of bubonic plague and malaria also cited. Global climate changes of 535-536, perhaps caused by the possible eruption of Krakatoa in 535, as mentioned by David Keys and others, is another example. Ideas about transformation with no distinct fall mirror the rise of the postmodern tradition, which rejects periodization concepts (see metanarrative). What is not new are attempts to diagnose Rome's particular problems, with Satire X, written by Juvenal in the early 2nd century at the height of Roman power, criticizing the peoples' obsession with bread and circuses and rulers seeking only to gratify these obsessions.