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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.
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.
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).
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.
The insolation changes for the last 40,000 years are represented. This is represented by the background color of figure 34, that shows how the polar regions received increasing insolation from 30,000 yr BP to 9,500 yr BP.
Climate modelers should take the opportunity to adjust their models to Holocene conditions.
It is clear that the main driver of Holocene climate has been changes in insolation due to orbital variation.
In the present, decreasing obliquity has been taking energy from the poles for 10,000 years, increasing the insolation latitudinal gradient that favors energy loss and increased polar precipitation, and reducing energy during summers. On a multi-millennial scale, global average temperatures follow mainly the 41,000 year obliquity cycle with a lag of several thousand years. Crosses represent dating and temperature uncertainty. Blue curve, methane levels as measured in GISP2 (Greenland) ice core from Kobashi et al., 2007.
These changes will also overcome the huge warm inertia even against precession changes, but will do so progressively for many thousands of years. Black curve, temperature anomaly in degrees centigrade at EPICA Dome C ice core for the past 800,000 years, lagged 6,500 years. Holocene temperatures are no exception, and a few thousand years after the peak in obliquity (9,500 years ago), temperatures started to decline. Summer (July-August) Central England temperature reconstruction from multiple proxies and sources by H. Notice the great effect of the 8.2 kyr event on methane concentrations. Climate models adjusted to explain present global warming do not reproduce the Holocene climate.