Dating of speleothems
The answer is the huge thermal inertia of the planet due mainly to its water content. But instead of an increase in temperatures, what we find is a progressive decrease from the HCO to the LIA driven by changes in insolation. Temperature and greenhouse gases changes during the Holocene.
21,000 years ago the increasing obliquity had been adding energy to the poles for 10,000 years, reducing the insolation latitudinal gradient (Raymo & Nisancioglu, 2003), and adding energy to the summers (Huybers, 2006; Tzedakis et al., 2017), and was on its way to overcome the huge cold inertia with the help of precession changes that were about to take place. Black curve, global temperature reconstruction by Marcott et al., 2013, as in figure 37. Red curve, CO levels as measured in Epica Dome C (Antarctica) ice core, reported in Monnin et al., 2004.
The four millennia of warmer temperatures are called the Holocene Climatic Optimum which was 1-2°C warmer than the Little Ice Age. Another classification divides the Holocene climatically into two periods: the Holocene Climatic Optimum (HCO, also known as Hypsithermal or Holocene Thermal Maximum), between 9,000 and 5,500 yr BP (although some authors only consider it from 7,500 yr BP after the 8.2 kyr event), and the Neoglacial period, between 5,000 and 100 yr BP, separated by the Mid-Holocene Transition (MHT) that roughly coincides with the start of the Bronze Age.
They used the terms Boreal for drier, and Atlantic for wetter (figure 33). These changes increase or decrease seasonality or the difference between summer and winter.A comparison between temperatures and obliquity over the past 800,000 years shows that while variable, the thermal inertia of the planet delays the temperature response to obliquity changes by an average of 6,500 years (figure 35). Grey curve changes in obliquity of the planetary axis in degrees. This general pattern of Holocene temperatures was already known by the late 1950’s from a variety of proxy records from different disciplines (Lamb, 1977; figure 36 A). Green curve, simulated global temperatures from an ensemble of three models (CCSM3, FAMOUS, and LOVECLIM) from Liu et al., 2014, show the inability of general climate models to replicate the Holocene general temperature downward trend. The mean temperatures of an ensemble of three models (CCSM3, FAMOUS, and LOVECLIM; Liu et al., 2014; figure 38) show a constant increase in temperatures during the entire Holocene, driven by the increase in GHG.The drop of obliquity always terminates interglacials. Greenland ice cores confirmed this pattern, when corrected for uplift (Vinther et al., 2009), and greatly improved the dating of temperature changes (figure 36 B). This disagreement between models and data-derived reconstructions of Holocene climate has been termed by the authors the Holocene temperature conundrum (Liu et al., 2014).Precession changes do not alter the annual amount of insolation at any latitude, since whatever insolation they take from one month at a particular location, they give back in another month within the same year.Precession changes are also asymmetrical, as their effect is opposite in each hemisphere, so the Northern Hemisphere summer (June-August, N-JJA thick red line in figure 34) has become progressively cooler during most of the Holocene, while Southern Hemisphere summer (December-February, S-DJF thick blue line in figure 34) has become progressively warmer during most of the Holocene. Changes due to obliquity have the effect of redistributing insolation between different latitudes following an obliquity cycle of 41,000 years.