Several different types of proxy data provide climatic information to us, particularly on a regional scale, such as pollen preserved in ice or sediment. However, biotic markers are not sufficient to estimate temperature, and are highly responsive to other factors like humidity and inter-species competition. The principal means of estimating temperature, in ice cores, sediment cores and sedimentary rock samples, is d18O (“delta-18-O”).
O stands for oxygen, and the superscript 18 refers to the isotope. An atom consists of a nucleus of both protons and neutrons, and a cloud of orbiting electrons. An element is defined by the number of protons in the nucleus: 1 for hydrogen (H), 6 for carbon (C), 8 for oxygen (O), and so on. Usually the count of neutrons is equal to the number of protons, but not always. “Isotope” is a term describing the number of neutrons in the nucleus. Different isotopes act the same way chemically—they form the same bonds with other atoms—but the different atomic masses lead to some differences in behavior.
We refer to different isotopes with a superscript number: 16O (8 protons, 8 neutrons, typical), 18O (8 P, 10 N, rare); 12C (6 P 6 N, typical), 13C (6 P, 7 N, rare), 14C (6 P, 8 N, also rare). 18O occurs in the present atmosphere at a known concentration (roughly 0.2%), but this concentration depends on global temperature. The concentration of 18O (also known as “heavy oxygen”) in the atmosphere increases as the climate warms, and decreases as the climate cools.
In science and math the term d, or delta refers to a numerical change. So d18O refers to the change in atmospheric 18O concentration over time. By correlating estimates using this method with other proxies, we have been able to construct a temperature record spanning the last half-billion years (out of 4.7). Change is the norm, and the stability our species has been accustomed to during the last 2500 years of recorded history is not.
This post was previously published on dailykos.com and is republished on Medium.
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