During the Mid-Holocene, when CO2 concentrations were stable and low (270 ppm), Antarctica’s massive Ross Ice Shelf naturally collapsed, adding the meltwater equivalent of 3-4 meters to sea levels.
Because CO2 concentrations changed very modestly during the pre-industrial Holocene (approximately ~25 ppm in 10,000 years), climate models that are predicated on the assumption that CO2 concentration changes drive ocean temperatures, ice sheet melt, and sea level rise necessarily simulate a very stable Holocene climate.
In contrast, changes in ocean temperatures, ice sheet melt, and sea level rise rates were far more abrupt and variable during the Holocene than during the last 100 years.
Modern ocean changes are barely detectable in the context of natural variability
Image Source(s): Rosenthal et al., 2013; Climate Audit
According to Levitus et al. (2012), the global ocean’s 0-2000 m layer warmed by 0.09°C during 1955-2010, while the 0-700 m layer warmed by 0.18°C during that span.
In the context of the Pacific Ocean’s 0-700 m temperature changes during the last two millennia (Rosenthal et al. 2013), that 0.18°C change in 55 years is barely detectable.
Mid-Holocene centennial-scale sea level fluctuations were much higher than today’s
During the Early Holocene, when continental ice sheets were still in the process of melting, sea levels rose at rates that ranged between 10 to 60 mm/yr, or 1 to 6 meters per century (Ivanovic et al., 2017; Zecchin et al., 2015; Hodgson et al., 2016).
During the Mid- to Late-Holocene, when relative sea level was about 2 meters higher than today’s levels, sea levels rose and fell at rates of a half-meter to a meter per century, with 13 mm/yr reached on decadal timescales.
Image Source: Meltzner et al., 2017
Image Source: Mörner et al., 2011
In contrast, the modern record indicates that sea levels only rose at a rate of 1.5 mm/yr during 1958-2014, or about 0.15 of a meter (6 inches) per century.
Image Source: Frederikse et al., 2018
Widespread collapse of ice sheets from 5000-1500 years ago
A new paper (Yokoyama et al. ) suggests that the Antarctic (and/or Greenland) ice sheets melted to such an extent around 5000 years ago that they added between 3 and 4 meters to sea levels.
The Ross Ice Shelf (Antarctica) underwent “widespread collapse” during this period (Yokoyama et al., 2016), subjected to rates of retreat and sub-ice shelf water temperatures much higher than present.
These melting events occurred while CO2 concentrations were a low and quiescent 270 ppm.
Image Source: Yokoyama et al., 2019
Image Source: Yokoyama et al., 2016