Short answers, pulled from the story.
The World Meteorological Organization established a technical commission for climatology in 1929. This body later designated the thirty-year period from 1901 to 1930 as the reference time frame for climatological standard normals at its 1934 Wiesbaden meeting.
The next set of climate normals published by the WMO covers the years 1991 through 2020. The organization agreed to update these climate normals using data collected between the 1st of January 1961 and the 31st of December 1990 in 1982.
The Köppen climate classification is the scheme that was first developed in 1899 and remains one of the most used methods globally. Modern methods divide into genetic approaches that focus on causes and empiric methods that focus on effects.
Scientists infer past climates from proxy variables because very few direct observations existed before the 19th century. Non-biotic evidence includes sediments found in lake beds and ice cores drilled from glaciers while biotic evidence comes from tree rings and coral samples collected from ocean floors.
Suggested causes for ice age periods include continental positions variations in Earth orbit changes in solar output and volcanism. Climate oscillations correlate closely with astronomical factors such as barycenter changes solar variation cosmic ray flux and Milankovitch cycles.