The Earth was receiving the same energy from the Sun, and the orbital configuration was distributing it over the planet in the same way during the Last Glacial Maximum as today. Consider the period from the Last Glacial Maximum (20 kyr BP) to the HCO when atmospheric CO increase from the coldest point of the last glacial period to the warmest point of the present interglacial.Why is the climate so different for the same energy input? Almost a third of the glacial-interglacial span cannot be considered insignificant for the increase in CO concentrations (Kobashi et al., 2007; figure 38).The four millennia of warmer temperatures are called the Holocene Climatic Optimum which was 1-2°C warmer than the Little Ice Age. Another classification divides the Holocene climatically into two periods: the Holocene Climatic Optimum (HCO, also known as Hypsithermal or Holocene Thermal Maximum), between 9,000 and 5,500 yr BP (although some authors only consider it from 7,500 yr BP after the 8.2 kyr event), and the Neoglacial period, between 5,000 and 100 yr BP, separated by the Mid-Holocene Transition (MHT) that roughly coincides with the start of the Bronze Age.This climatic optimum was when global glaciers reached their minimum extent. Finally other authors divide the Holocene in three periods.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.
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.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.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.