evidence: Earth’s climate has always warmed and cooled, and the 20th century rise in global temperature is within the bounds of natural temperature fluctuations over the past 3,000 years. [...] A study published in Nature found that "high temperatures – similar to those observed in the twentieth century before 1990 – occurred around AD 1000 to 1100" in the Northern Hemisphere. [116] A study published in Boreas found that summer temperatures during the Roman Empire and Medieval periods were "consistently higher" than temperatures during the 20th century. [59] [...] Over the 20th century there have been two Arctic warming periods with a cooling period (1940-1970) in between. According to a study in Geophysical Research Letters, natural shifts in the ocean currents are the major cause of these climate changes, not human-generated greenhouse gases. [124] William Gray, PhD, Emeritus Professor of Atmospheric Science at Colorado State University, said most of the climate changes over the last century are natural and "due to multi-decadal and multi-century changes in deep global ocean currents." [122]