Holocene Period

The Holocene, derived from the Greek words holos and kainos meaning “entirely recent,” covers the most recent unit of geologic time. Marked by a major shift in climatic conditions, it spans the last 11,550 calendar years before the present time. The prior epoch, the Pleistocene, was characterized by large swings in climate between cold (glacial) conditions and warm (interglacial) conditions. In contrast to the instability of the Pleistocene climate, the Holocene climate is far more stable and warm. This mild climate coincides with the Mesolithic period and the development of human technology, and is thought to play a key, supporting role in the advancement of human civilization.

Despite the generally mild climate, paleoclimate proxies (indicators of past climate) show that climate fluctuations lasting for hundreds to thousands of years occured during the Holocene, albeit smaller amplitude changes than during the Pleistocene. Scientists use paleoclimate proxies such as ice cores, marine ocean sediment cores, ancient shorelines, fossil pollen, and tree rings, to understand and estimate these changes in climate.

These proxies have provided evidence for some of the significant Holocene climate fluctuations, including the Younger Dryas, the Neoglacial, the Medieval Warm period, and the Little Ice Age. Only a general timeline for these events is provided because the exact dates vary from one geographic region to the next.

The first significant climate change of the Holocene, the Younger Dryas, began during the transition from the Pleistocene to the Holocene when a final pulse of cold, glacial conditions interrupted the post-glacial warming. Major cooling in the North Atlantic region is evidenced by pollen records, which show that cold-tolerant plants reinvaded the landscape. The episode is named after Dryas octopetala, an arctic plant that spread to lower altitudes during this cold period. Greenland ice cores have also provided evidence of these colder temperatures.

By 6,000 to 7,000 years ago, climate had warmed to what is often referred to as the mid-Holocene thermal maximum. Vegetation zones moved to higher latitudes, and glaciers retreated in many regions of the world. This was followed by a second cooling episode, the Neoglacial, which lasted from approximately 2,500 to 4,500 years ago. The generally cooler and drier Neoglacial climate led to widespread birth and re-growth of mountain glaciers. These glacial advances are physically evidenced by the remnants of debris originally transported down-valley by advancing glaciers and then deposited as the glaciers melted away.

The Medieval Warm period (also known as the Medieval Climatic Optimum) was a time of unusual warmth around 700 to 1,200 years ago. This warmer climate resulted in more harvesting and prosperity for human civilizations. In Greenland, the unusually warm climate attracted Nordic tribes in the high altitudes of the North Atlantic to migrate and settle the area, growing wheat in areas previously covered in deep snow. Pollen records show that warm-tolerant plants flourished in these areas.

The Little Ice Age followed the prosperity of the Medieval Warm period. True to its title, the Little Ice Age was a cooling episode that lasted from approximately 200 to 600 years ago and was marked by moderate glaciation, crop failures, and famines. Substantial historical evidence, such as literature, maps, and art, is available to scientists as a record of this advance in alpine glaciers in many mountainous regions of the world. 

Climate and Human Civilization

The Holocene record shows a climate history with which scientists can compare recent changes in global climate. Evidence throughout the Holocene shows that changes in climate have a direct influence on human civilization, and will continue their influence into the future. There is also substantial paleoclimatic evidence that the present greenhouse gas concentrations and rates of global warming are greater than those seen any time previously during the Holocene. This evidence suggests that these changes in climate are unique in the Earth’s recent history

Scientists disagree about the exact nature of the Holocene climate changes and the mechanisms giving rise to them are not yet well understood. Researchers have proposed several mechanisms, of which three have been studied extensively. Solar variability (or changes in solar output) was theorized to be a mechanism; however, it is no longer widely accepted because of the large uncertainties in the solar-climate link. Another mechanism, changes in volcanism, has been shown to have a clear link to short-term climate changes, typically less than a few years. Less certain is if long periods of increased volcanism have a clear link with longer periods of climate change, or if short periods of climate change associated with volcanic events then give rise to longer periods.

Another mechanism, ocean circulation, has been shown to give rise to lower frequency climate variability. Research in this area focuses on finding evidence concerning changes in ocean circulation that could have been the primary driver of climate change during the Holocene, or if it only served as an amplifier of the smaller changes in climate. An additional area of active debate is whether these climate changes of the Holocene were global, hemispheric, or regional in nature. It is clear, however, that there were measurable fluctuations in Holocene climate over large regions, and that these changes in climate likely had a significant impact on the human civilizations in those regions.