Over millions of years the face of the Earth has changed dramatically. Continents have formed and drifted apart; sea levels have risen and fallen again, atmospheric oxygen has increased and decreased, fossil deposits left behind by the oceans of prehistoric times have been folded into high mountains through the movements of plate tectonics—the same mountains that were later eroded, cut through by rivers, or shaped by glaciers. The history of our Earth’s movement and development is still visible today in many landscapes. As early as the 17th century, English scientist Robert Hooke recognized this geological evidence provides a clue to understanding a changing climate, attributing the existence of fossil turtles in Dorset to formerly warmer conditions. Today, paleoclimatologists and paleoceanographers are studying the climatic changes of past times, and the effects such changes bore on life. As climate records based on instrumental measurements trace back no further than 150 years, studying paleoclimate archives gives us better understanding of the Earth’s climate system and the scope of the climate change we are experiencing today. Many different things can serve as so-called climate proxies—the measurable indicators of past environmental conditions. Tree rings and ice cores from Greenland are two well-known examples. But the oceans also have a lot of information to offer; ocean sediments and the organisms preserved within them can be read as climate archives, too. Corals and many bivalves develop skeletal rings or bands very similar to tree rings. A paleoclimatic study about the warming bias of the last 66 million years, published in Science Advances journal in 2021, warns that even a modest initial warming can promote biological and chemical processes that ultimately lead to more warming.
Sources: Wanamaker, Alan D.; Steffen Hetzinger and Jochen Halfar, “Reconstructing mid- to high-latitude marine climate and ocean variability using bivalves, coralline algae, and marine sediment cores from the Northern Hemisphere”, Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 302, issue. 1–2, (2011): 1-9.
Constantin W. Arnscheidt and Daniel H. Rothman, “Asymmetry of extreme Cenozoic climate–carbon cycle events”, Science Advances, vol. 7, issue. 33, (2021).
Fairbridge, Rhodes, “History of Paleoclimatology”, Encyclopedia of Paleoclimatology and Ancient Environments, (2008): 414–426.