// climate change: 14th century -> temperatures fell. Part of general, long-term trend -> began in artic in 13th century. Weakening winds -> changes in rainfall patterns. Lack of moisture in deep interior regions with continental climates: three detectable levels of climate change. Long-term alternation between ice ages & global warming. Periods of a few hundred years of relative cold & warmth -> alternative within eras of global warming. Sudden interruptions of normal conditions -> occur irregularly & unpredictably. Normal distribution of atmospheric pressure disturbed for unknown reasons. El ni o effect: mongol world began to contract. // the coming of the age of plague: most attempts to write the history of disease -> foundered on a false assumption. We can recognize past visitations of identifiable diseases from symptoms historical sources describe: unrealistic for two reasons. People in the past looked at disease with perceptions different from ours. Diseases themselves change: pathology of black death.