The earth has always had a shifting climate with some periods having a colder climate then what is considered normal. This essay aims to investigate how the research of cold periods have changed. The purpose with this study is to investigate the most common beliefs researches has for the events of the climate catastrophe 536 and what factors lead to a possible population reduction. I will investigate three different reasons for this: Volcanic eruptions, plague, and climate. Another cold period the “little ice age”, 1300-1850, is used as an example and a theoretic comparison to the catastrophe of year 536 because the “little ice age” is a well-documented cold period of the later historical era. These two periods are interesting because they are booth cold periods. The first cold period lasted for a couple decades while the second cold period lasted for five hundred years. The bad climate situation depends of several different factors especially the fall in summer temperatures because the sun was hidden in a dust veil. The dust veil had been created after the big eruptions. The result of this study is that in general, the research has gone from more simple research to a more complex when knowledge of the complexity of the situation has evolved. The newer research has better access to natural science sources. The most common believes is that the volcanic eruptions or the Justinian plague were the biggest reason for a possible population reduction.