Ancient sea level as key to the future

Studies of ancient sea levels provide insights into the mechanisms and rates of sea level changes due to tectonic processes (e.g., ocean crust production) and climatic variations (e.g., insolation due to Earth’s orbital changes and atmospheric CO₂). Global mean sea level (GMSL) changes since the Middle Eocene (ca. 48 million years ago [Ma]) have been primarily driven by ice volume changes paced on astro-nomical timescales (2400, 1200, 95/125, 41, and 19/23 thousand years [kyr]), modu-lated by changes in atmospheric CO₂. During peak warm intervals (e.g., Early Eocene Climatic Optimum 56–48 Ma and the early Late Cretaceous ca. 100–80 Ma), atmospheric CO₂ was high and Earth was more than 5°C warmer and mostly ice-free, con-tributing ~66 m of GMSL rise from ice alone. However, even in the warmest times (e.g., Early Eocene, ca 50 Ma), growth and decay of small ice sheets (<25 m sea level equivalent) likely drove sea level changes that inundated continents and controlled the record of shallow-water deposits. Ice sheets were confined to the interior of Antarctica prior to the Oligocene and first reached the Antarctic coast at 34 Ma, with the low-est sea levels –20±10 m relative to modern GMSL. Following a near ice-free Miocene Climatic Optimum (17–13.8 Ma), a permanent East Antarctic Ice Sheet (EAIS) devel-oped in the Middle Miocene (ca. 13.8 Ma). During the Pliocene (4–3 Ma), CO₂ was similar to 2020 CE (Common Era) and sea levels stood ~22±10 m above present, requiring significant loss of the Greenland Ice Sheet (~7 m of sea level), West Antarctic Ice Sheet (~5 m after isostatic compensation), and vulnerable portions of the EAIS. The small Northern Hemisphere ice sheets of the Eocene to Pliocene expanded into continental scale in the Quaternary (past 2.55 million years). Sea level reached its low-est point (~130 m below present) during the Last Glacial Maximum (ca. 27–20 thousand years before 1950 [ka]), episodically rose during the deglaciation (ca. 20–11 ka) at rates that at times were in excess of 47 mm yr^–1 (vs. modern rates of 3.2 mm yr^–1), and progressively slowed during the Early to Middle Holocene from ca. 11 ka until ~4 ka. During the Late Holocene (last 4.2 kyr, including the CE), GMSL only exhib-ited multi-centennial variability of ±0.1 m. The modern episode of GMSL rise began in the late nineteenth century, with most of the twentieth century rise attributable to global warming and ice melt. Under moderate emissions scenarios, GMSL is likely to rise 0.4–1.0 m in this century, with ancient analogs suggesting a longer term (centen-nial to millennial scale) equilibrium rise of ~10 m. Under higher emissions scenar-ios, twenty-first century GMSL will rise greater than 2 m, and in the long term, tens of meters cannot be excluded.