Little Ice Age
By Jon Hare
and global warming
layered in a lake
Varves are annually layered sediments. I first came across the word in a study of fish scales preserved in anoxic basins off Santa Barbara, California (Baumgartner et al. 1992). Sediment cores were taken and the number and species of fish scales in the varves (layers) were used to estimate the number of anchovies and sardines off California over the past 1,700 years. The conclusion was that populations of these fish varied greatly long before commercial fishing, indicating the importance of natural variability and commercial fishing in fish population abundance.
A recent use of varves to document the past comes from Lapointe et al. (2020). They examined layered sediments in South Sawtooth Lake, Nunavut, Canada. Previous analyses demonstrated that sediment grain size in each layer (aka year) was correlated with summer temperatures; finer grained sediments were associated with cooler summers. In addition, finer grained sediments have more titanium, so by measuring titanium through the varves of sediment cores, the authors were able to reconstruct a history of summer temperatures at the site. The concept is the same as that for scales and anchovies but in this case is titanium and temperature.
The authors verified their proxy through comparison of titanium from the varves in South Sawtooth Lake to measurements of summertime North Atlantic sea surface temperature. Temperature measurements are available annually from 1854 to the present and Lapointe and team show that temperature record is significantly correlated with their titanium measurements. They then use this verified relationship to develop a 2,900 year reconstruction – one of the longest reconstructions of North Atlantic sea surface temperatures to date.
The reconstructed temperature record shows multidecadal and multicentury variability and recent change. Multidecadal variability has been observed in measurements of North Atlantic sea surface temperatures from 1854 to the present. The study by Lapointe and co-authors provides evidence that this multidecadal variability has been occurring for almost 3,000 years. Multicentuary variability was also evident; the warm and cold periods noted in history are seen in the reconstructed temperatures – the Roman Warm Period (250 BCE – 400 CE) and the Dark Ages Cold Period (400-800 CE). The coldest temperatures in the reconstruction are from the middle of the Little Ice Age (1400-1600 CE). The warmest temperatures in the 2,900 year reconstruction are from the past decade – the authors state that “the rate and magnitude of warming over the last few centuries are unprecedented in the entire record, leading to the last decade which was the warmest of the past ∼2,900 y.” Thus the authors provide strong evidence for natural variability in the climate system and evidence for rapid – unprecedented – change over the last 50 to 100 years.
Original research:
Francois Lapointe, Raymond S. Bradley, Pierre Francus, Nicholas L. Balascio, Mark B. Abbott, Joseph S. Stoner, Guillaume St-Onge, Arnaud De Coninck, and Thibault Labarre. Annually resolved Atlantic sea surface temperature variability over the past 2,900 y. Proceedings of the National Academy of Sciences, 2020; 202014166 DOI: 10.1073/pnas.2014166117. https://www.pnas.org/content/pnas/early/2020/10/06/2014166117.full.pdf
Baumgartner, T. R. (1992). Reconstruction of the history of the Pacific sardine and northern anchovy populations over the past two millenia from sediments of the Santa Barbara basin, California. CalCOFI Rep, 33, 24-40. https://www.calcofi.org/publications/calcofireports/v33/Vol_33_Baumgartner_etal.pdf
Dr. Jon Hare is a scientist who works in Woods Hole, Massachusetts. His research background is fisheries oceanography and climate change impacts on marine fisheries. Check out Jon’s previous sciku ‘Owls of the Eastern Ice’, ‘Cobwebs to Foodwebs’, ‘Signs of Spring’ and ‘Glacier Mice‘.