Cosmogenic dating definition
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?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.Changes in seasonality insolation caused by the precession cycle (modified by eccentricity) are asymmetric and less important for the global response, although they cause profound changes in regional climatic differences. In the Holocene, the precession cycle and the obliquity cycle are almost aligned so that maximal obliquity and maximal northern summer insolation are almost coincident at the beginning of the interglacial about 10,000 years ago. Available data indicates that despite significant changes in GHG concentration in the atmosphere during the period of 10,000 to 600 yr BP, their contribution to temperature changes cannot have been important. (2004), CO concentrations measured in Antarctic ice cores decreased from 267 to 258 ppm between 10,000 and 6,800 yr BP, and afterwards increased more or less linearly to 283 ppm by 600 yr BP, just prior to the LIA (figure 38).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.
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.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.The second article focused on the Dansgaard-Oeschger cycle that characterizes glacial periods. By far the main factor driving Holocene climatic change are the insolation changes due to the orbital variations of the Earth (figure 34).In this article I review the general features of Holocene climate, limiting it to the period ending in the 0 yr BP (year zero before present), that is established by geologists and paleoclimatologists as 1950. These changes are of two types that produce two different effects not always properly differentiated by Holocene climate researchers.