Cosmogenic dating definition
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.
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.
Summary: Holocene climate is characterized by two initial millennia of fast warming followed by four millennia of higher temperatures and humidity, and a progressively accelerating cooling and drying for the past six millennia. Rutger Sernander proposed that this climatic change was abrupt, even a catastrophe that he identified with the Fimbulwinter of the Sagas.
These changes are driven by variations in the obliquity of the Earth’s axis. At the time other scientists believed in a more gradual climatic change, but recent studies on the 2.8 kyr abrupt cooling event (Kobashi et al., 2013) agree with Sernander.
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).
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.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.Introduction A review of abrupt climate changes of the recent past provides a frame of reference for current global warming. Every period shows a characteristic vegetation pattern indicative of stable climatic conditions, separated from other periods by rapid vegetation changes suggestive of abrupt climate changes. Holocene general climate trend Broadly speaking the Holocene had an abrupt start at 11,700 yr BP, after the Younger Dryas cold relapse, and reached maximal temperatures in about 2,000 years.The glacial cycle was reviewed in the first article in the series. The dates and conditions generally accepted (Encyclopedia of Environmental Change) are: – Pre-Boreal, 11,500 – 10,500 yr BP. Since about 9,500 yr BP, a time that coincides with maximal obliquity of the Earth axis, the climate of the Holocene stopped warming and a few thousand years later started a progressive cooling.