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Black temperature proxy curve represents δO isotope changes from NGRIP Greenland ice core (without scale). Since then, and for the next 11,500 years, the poles will be receiving decreasing insolation.

The insolation curves are presented as the insolation anomaly for summer, winter, spring, and fall. Unlike precessional insolation changes, obliquity changes are symmetrical.

Scandinavian palynologists established the Blytt-Sernander sequence which divided the Holocene into five periods. An example of the Blytt-Sernander climatic zones established with the traditional pollen indicators, with the distinct elm-fall at the Atlantic/Sub-Boreal transition, and the rise of beech at the Sub-Boreal/Sub-Atlantic transition. Insolation changes due to precession are represented in figure 34 with three month insolation curves for a North and South latitude, relative to present values.

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.

Precession changes are responsible for sea surface temperature (SST) patterns, and thus oceanic currents. When obliquity was maximal 9,500 years ago, both poles received more insolation due to obliquity, while the tropics received less.

North-South differences set the position of the ITCZ (Intertropical Convergence Zone or the climatic equator). Obliquity also affects seasonality, at maximal axial tilt, there is an increased difference between summer and winter at high latitudes.

N (red) or S (blue) are the Northern or Southern Hemisphere and the three letters are the month initials. Although the annual insolation change is not too large, it accumulates over tens of thousands of years and the total change is staggering, creating a huge insolation deficit or surplus. There is only one Holocene global average temperature reconstruction available (Marcott et al., 2013; figure 37 a).

Northern and southern summer insolation represented with thick curves. This changes the equator-to-pole temperature gradient, and is largely responsible for entering and exiting glacial periods (Tzedakis et al., 2017) and for the general evolution of global temperatures and climate during the Holocene. To correct some of the problems it presents, I use this reconstruction averaged by differencing (explained here), without any smoothing, and with the original published dates for the proxies. Red curve, global average temperature reconstruction from Marcott et al., 2013, figure 1.

The Holocene Climatic Optimum corresponds to high insolation surplus in polar latitudes (red area), while Neoglacial conditions represent the first 5,000 years of a 10,000 year drop into a high glacial insolation deficit in polar latitudes (blue area). See in figure 34 how the thick red curve representing northern summer insolation reaches maximal values 10 kyr BP, almost coinciding with the center of the background polar red color, representing highest warming from maximal obliquity about 9.5 kyr BP. 19,000 years ago obliquity was the same as it is now (only increasing), and the precession cycle was at the same position as it is now (same 65 °N summer insolation; figure 34). This increase of 25 ppm represents about 10% of a doubling.

The Mid-Holocene Transition, caused by orbital variations, brought a change in climatic mode, from solar to oceanic dominated forcing. The Blytt-Sernander sequence fell out of fashion in the 1970s when new techniques allowed a more quantitative reconstruction of past climates. The Early Holocene, up to the 8.2 Kyr event, the Middle Holocene, between the 8.2 and the 4.2 Kyr events, and the Late Holocene since the 4.2 Kyr event.


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