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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).

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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.

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.

This transition displaced the climatic equator, ended the African Humid Period and increased El Niño activity. However, it captures the essence of Holocene climate as four periods of roughly 2500 years each. Although this is currently the most popular subdivision, in my opinion, it fails to properly capture the climatic trends of the Holocene.

Background color represents changes in annual insolation by latitude and time due to changes in the Earth’s axial tilt (obliquity), shown in a colored scale. Obliquity changes contribute to the lack of warming of Antarctica during the Holocene, despite increasing Southern Hemisphere summer insolation. Greenland temperature reconstruction based on an average of uplift corrected δO of seawater and calibrated to borehole temperature records. I have also rescaled the temperature changes to make them congruent with the vast literature and consilience of evidence from different fields that indicates that the Holocene Climatic Optimum was on average between 1 and 2 °C warmer than the Little Ice Age (figure 37 b). The resulting temperature curve is extraordinarily similar to H. The averaging method does not correct for proxy drop out which produces an artificially enhanced terminal spike, while the Monte Carlo smoothing eliminates most variability information. Black curve, global average temperature reconstruction from Marcott et al., 2013, using proxy published dates, and differencing average.

This figure essentially shows how global temperature changes respond mainly to persistent changes in insolation caused by changes in obliquity that are symmetrical for both poles. Ultimately obliquity changes will be responsible for the glacial inception that will put an end to the Holocene interglacial in the distant future. Lamb regional reconstruction from the 1970s (figure 36 A), with significant temperature drops at 5.5, 3, and 0.5 kyr BP. Temperature anomaly was rescaled to match biological, glaciological, and marine sedimentary evidence, indicating the Holocene Climate Optimum was about 1.2°C warmer than LIA. Purple curve, Earth’s axis obliquity is shown to display a similar trend to Holocene temperatures. The controversial role of greenhouse gases during the Holocene What role, if any, have greenhouse gases (GHG) played in Holocene climate change?

As we saw in the previous article, botanists studying peat stratigraphy were among the first to notice, in the late 19th and early 20th century, abrupt climate changes reflected in peat layers. Changes due to precession (modulated by eccentricity) have the effect of redistributing insolation between the different seasons of the year by latitude.

These sudden transitions were later confirmed by changes in sediment pollen composition. The 23,000-year precession cycle determines the direction each hemisphere is pointing towards at perihelion and aphelion, and thus the amount of insolation received by each hemisphere at any point of the orbit.

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.

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