Temperature can be inferred from the isotope ratios for carbon (carbon-12 and carbon-13C) and oxygen (oxygen-16 and oxygen-18) in the skeletons of sea foraminifers, in the bottom deposits in Sargasso Sea (Northern Atlantic). These indicate that in the last 3,000 years, the climate on Earth has been constantly changing, and the scope of changes in modern times does not differ from those of the past.
Shown are the Medieval Optimum (1,000 years ago) the beginnings of the Holocenic Optimum (2,500 years ago), and also the Little Ice Age (ca. 500 years ago) from which we are still emerging. The Early Middle Ages also witnessed a strong climate cooling, which had an impact on Europe's economic and cultural decline in this period.
Source: Adapted from L.D. Keigwin, et al., 1994. "The Role of the Deep Ocean in North Atlantic Climate Change between 70 and 130 Years Ago" Nature, Vol. 371, pp. 323-326