Climatic Research Unit : Information sheets

6: The Holocene

Jean Palutikof

The Holocene is the most recent period in the geological record. It began at the time of the retreat of the ice sheets at the end of the last glaciation. Various dates are given for this retreat, but many sources place it at around 11,500 calendar years before present (BP), as shown in Table 1.

Table 1: The late-glacial stratigraphy of northwest Europe (after Roberts, 1998)

Age (Cal. years BP)	Britain	Northern Europe	Climate
After 11 500	Holocene	Holocene	warm
11 500 - 13 000	Loch Lomond stadial	Younger Dryas stadial	cold, glacial re-advance
13 000 - 15 000	Windermere interstadial	Allerød interstadial Older Dryas Bølling interstadial	moderately warm a brief cool interval warm
15 000 - 18 000	Devensian glaciation	Oldest Dryas	cold
Before 18 000	Devensian glaciation	Weichselian glaciation	glacial

The potential causes of the succession of glacial-interglacial periods throughout the Quaternary period (Milankovitch cycles) are discussed in Information Sheet 2. The Holocene is a relatively warm period, probably an interglacial between ice ages. The large-scale fluctuations in climate during the Holocene are most likely related to Milankovitch cycles, and allow us to divide the Holocene into Early and Later stages:

The Early Holocene

The early Holocene extends from around 11 500 BP up to 5 000 BP, and was a period in which the climate became warmer and wetter. Major features are:

Sea level started to rise at around 20 000 BP as glaciers began to melt. The rise was rapid, >30 mm/yr at some sites. Present sea level was typically reached at around 6 000 BP, although wide regional variations in this date exist.
The increase in rainfall meant that desert and tundra regions of the Earth became much less extensive. The rainfall increases in sub-tropical regions were related to a strengthening of monsoon systems, and it is estimated that the rainfall in sub-tropical deserts such as the Sahara increased by around 20%.
Forest ecosystems spread to cover a much larger area in the warmer, moister conditions.
Human beings moved from a primarily meat-based diet to an increased reliance on grain, as shown in the figure below. This was related to the reduction in open habitats such as tundra, which were most suited to the early hunter-gatherer economies. A warmer wetter climate encouraged the development of sedentary farming.

Graph: The changing contribution of domestic plants to diet, Techuacan, Mexico

The Later Holocene

Following the thermal optimum at around 9 000 - 5 500 BP and up to around 500 BP, there has been progressive deterioration in the climate. This has been marked by:

Cooling in temperate latitudes.
Drying in the sub-tropics between 6 500 BP and 4 500 BP, leading to the development and expansion of the Saharan, Arabian and Thar deserts.
As a result of these changes, human societies have seen the rise and fall of the great irrigation civilizations such as in Mesopotamia, and also the development of nomadic pastoralism.

Short-term events

Superimposed on these slowly evolving trends, there were shorter perturbations in climate during the Holocene. Some of these were caused by volcanic explosions (see Information Sheet 13). For example, the explosion of Santorini (Thera) in about 1628 BC (3628 BP) is thought from tree-ring evidence to have led to a period of cooler, wetter climate. Information Sheet 5 looks at short-term fluctuations in climate during the last millennium.

In the early Holocene, there were at least three occasions when the climate deteriorated abruptly, as shown by rapid falls in many African lake levels, in 12 000, 8 200 and 5 200 BP. The cause of these perturbations is unknown, although they may have been linked to the switching off of the Atlantic Ocean conveyor (Information Sheet 7).

The most recent past