Are we really in an unprecedented time of warming?

Comparing modern climate change to Dansgaard-Oeschger (D-O) events.

Dansgaard-Oeschger (D-O) events are one of the most striking discoveries in earth science related to past climate. They are named after the two climatologists, Willi Dansgaard and Hans Oeschger, who were instrumental in their identification. These events are rapid climate fluctuations that occurred frequently during the last glacial period, illustrating a planet capable of swift and dramatic temperature shifts.

The discovery of D-O events can be traced back to the ice core drilling projects in Greenland in the late 20th century. Analysis of the isotopic composition of ice cores, particularly the ratio of oxygen isotopes 18O and 16O, revealed evidence of abrupt climatic changes. These isotopes served as proxies for past temperatures, with higher ratios indicating warmer periods. The meticulous work of Dansgaard and Oeschger, along with their colleagues, in the 1980s, led to the recognition that the Earth's climate has not always changed at a steady pace but has seen dramatic swings, especially during the last glacial period around 115,000 to 11,700 years ago.

Comparison of temperature proxies for ice cores from Antarctica and Greenland for 140,000 years. Greenland ice cores use delta 18O, while Antarctic ice cores use delta 2H. Note the Dansgaard-Oeschger events in the Greenland ice core between 20,000 and 110,000 years ago, which barely register (if at all) in the corresponding Antarctic record. GRIP and NGRIP data is on ss09sea timescale, Vostok uses GT4, and EPICA uses EDC2. Source: https://commons.wikimedia.org/wiki/File:Ice-core-isotope.png

One of the most well-known D-O events is the Bølling-Allerød interstadial, a warm period that occurred around 14,700 years ago, punctuating the last glacial period with a rapid shift to warmer conditions. It was followed by the Younger Dryas, a sudden return to glacial conditions before the onset of the current Holocene epoch. These episodes are clearly recorded in Greenland ice cores, with corresponding evidence found in ocean sediment cores, showing worldwide effects.

Another notable example is the Older Dryas, which marked a brief return to colder conditions within the general warming trend of the last deglaciation. Each D-O event is represented in the ice core data by a rapid warming transition, followed by a more gradual cooling period.

The evidence for D-O events primarily comes from the analysis of ice cores, where they are marked by layers containing different isotopic compositions, suggesting rapid temperature swings over decades or centuries. For instance, the Greenland Ice Sheet Project 2 (GISP2) and the Greenland Icecore Project (GRIP) have provided detailed and high-resolution data that chart these fluctuations (refresher on oxygen isotopes).

Sediment cores from the North Atlantic also offer support, as they display changes in the deposition patterns, which coincide with the warming and cooling phases of D-O events.

D-O events are critical to our understanding of the Earth's climate system. They serve as a reminder that climate can change abruptly and with significant global impacts. While these events occurred during a glacial period, and their direct applicability to our current interglacial climate is debatable, they highlight the roles that oceanic and atmospheric circulations play in global climate and how these systems can undergo rapid transitions.

Modern climate change, of approximately 1.1°C since 1880, is rather mild when compared to past natural fluctuations like D-O events.