Holocene Climate Optimum (HCO)

Understanding climatic fluctuations since the last ice age provides a context for evaluating the extent to which the present global warming trend is an anthropogenic phenomenon. Correlation of archaeological and historical records allows projections of the impact of global warming on human society.

Background

Climatic changes since the end of the last ice age form the backdrop for much of human prehistory and are viewed by some as a driving force in the rise and fall of civilizations. The retreat of continental glaciers began in earnest 13 million years ago, with a gradual warming trend that reached its peak around 6,000 years ago during a period known as the Hypsithermal or Holocene climatic optimum. Proxy records, supplemented by historical data in more recent times, suggest six periods of abrupt cooling in the Holocene, 9,000-8,000, 6,000-5,000, 4,200-3,800, 3,500-2,500, 1,200-1,000, and 650-150 years before the present. Within warm periods and cold periods, there is considerable fluctuation on scales ranging from decades to centuries. Temperature variations as measured by a variety of are more dramatic near the poles, while variations in rainfall associated with temperature-induced fluctuations in oceanic currents predominate in the tropics. Overall, climatic variability in the is considerably less than it was in the Pleistocene, and what fluctuations have occurred in the Holocene have decreased over the course of the period.

Climatologists are continually modifying the prevailing picture of climate change in the Holocene as more high-resolution studies become available from areas other than Europe and eastern North America. Climatic shifts typically appear earlier in the Southern Hemisphere than in the Northern Hemisphere. Signs of the 8.2ka event, a period of dramatic European cooling due to disruption of currents in the Atlantic Ocean, are much less evident in western North America and are absent in New Zealand.

Measuring Holocene Climate

Systematic instrumental records of weather in parts of Europe and North America exist for the last 150 years. Agricultural records, historic narratives, and even legendary sources chronicle catastrophic events throughout human history.

Until recently, much of the available information about climate in prehistoric times came from archaeological investigations. The study of artifacts and settlement patterns reveals a great deal about the climate in which ancient people lived. For example, prolonged drought in the American Southwest, corresponding to the culturally benign Medieval Warm Period in Europe, is evident in shifting patterns of cultivation, declining population, skeletal deformities due to malnutrition, compressed in construction timbers, and eventual abandonment of cliff dwellings.

Vegetation is a good climatic indicator. Leaves, woody material, and particularly pollen occur in abundance in bogs, lake sediments, and areas of human settlement. Pollen analysis is a powerful tool, because pollen is extraordinarily decay-resistant. Many pollen grains can be identified to genus, and relative abundance provides a fairly complete picture of a region’s flora. Wind-pollinated plants with narrow ecological niches are particularly useful. In Europe, the arctic-alpine herb Dryas octopetala indicates arctic-alpine conditions, spruce (Picea) indicates a cold, humid climate, and oak (Quercus) provides evidence of a drier, warm climate. Pollen of Plantago, a weed in grain fields, suggests cultivation. In marine sediments, relative abundance of planktonic types serves as a proxy for water temperature.

Various geological formations permit high-resolution analysis of local climate. Moraines and scouring document the advance and retreat of glaciers. Varves, which are layers of sediment in lakebeds, provide a record of stream flow into lakes. When precipitation is high, increased runoff and create thick varves and rapid deposition of alluvial fans at the mouths of rivers. Terraces along lake and ocean shores document rises and falls in water level. In some areas, the land may also be rising or subsiding relative to sea level.

Ice cores taken from glaciers in Greenland and Antarctica provide evidence of climate over the last 400,000 years, including rates of precipitation, amounts of atmospheric dust, and concentrations of in trapped air. Analysis of the ratios of carbon and oxygen isotopes in carbonates and of oxygen in ice also provides clues to climate, since both biotic and abiotic processes use isotopes selectively. Isotope ratios can also be used as proxies for sunspot activity, a suspected factor in warming and cooling trends.

Climatic Change in Human Prehistory

The tenure of modern humans on Earth encompasses the last Pleistocene glaciation and the ten thousand years of the Holocene, during which the Earth’s climate has fluctuated, with a temperature maximum roughly six thousand years ago. There are many studies correlating prehistoric cultural changes with climatic changes. For agricultural societies, the droughts associated with colder periods are more devastating than lower temperatures themselves. The Holocene historical and geologic records contain no compelling evidence of rapid rises in temperature such as the Earth is currently experiencing: Global warming in geologic time appears to be a gradual process to which life adapts. Cooling, on the other hand, can be extremely rapid and catastrophic.

Gradual warming can also produce catastrophic results when rising waters overwhelm a natural dam, unleashing a flood of biblical proportions. Indeed, a controversial theory postulates that the biblical flood was just such an event, and that the fertile agricultural land surrounding the Black Sea was suddenly inundated when rising waters in the Mediterranean breached the Bosporus roughly seven thousand years ago. Similar floods occurred in the lower reaches of the Tigris and Euphrates rivers and the northern Red Sea at about the same time.

In North America, rapid draining of Glacial Lake Agassiz through the St. Lawrence River caused local devastation, as well as disrupting oceanic currents. The Missoula floods in the Columbia River basin resulted from periodic breaching and reforming of an ice dam at the glacial margin. The widespread occurrence of devastating warming-induced floods in prehistoric times holds a lesson for the present. If there is a dam protecting a city from rising waters, whether it is natural or artificial, every incremental temperature rise increases the likelihood of disaster.

The causes of cooling episodes are various. They include changes in solar radiation levels, volcanic aerosol concentration, greenhouse gas (GHG) concentration, the hydrologic cycle, sea level, extent, and forest cover. The six defined periods of cooling in the Holocene most probably all derive from several such factors acting in concert.

The rapid cooling that occurred between nine thousand and eight thousand years ago took place during a decline in and a high level of volcanic SO₂ production. A massive infusion of glacial meltwater into the North Atlantic disrupted thermohaline circulation. A weakening Afro-Asian monsoon contributed to tropical aridity. Atmospheric methane concentrations declined because of the drought; this depletion of GHGs created a feedback loop prolonging cooling conditions.

Proxy records based on isotope ratios suggest a decline in solar radiation during the four subsequent cooling periods. The cooling corresponds to lows in solar radiation that correspond to variation in the Earth’s orbit. In the case of the 8.2ka event and the onset of the Little Ice Age, there is also evidence of increased volcanic activity. Massive volcanic eruptions cause by ejecting fine ash and sulfates into the atmosphere. However, in the absence of reinforcement from orbital forcing, this effect dissipates in about three years.

Cultural Effects of Holocene Climate Change

As several analysts have pointed out, climate has helped shape civilization, but not by being benign. Human technological and social advances appear in the archaeological and historical record as responses to climate change that rendered older ways of life maladaptive. The general pattern appears to involve a buildup of population and associated infrastructure during periods characterized by warm temperatures, adequate rainfall, and low variability, such as the Hypsithermal or the Medieval Warm Period. This is followed by a population crash due to famine, war, and pestilence when the climate changes, and a period of rapid technological and social innovation as the population adapts to the new conditions.

In the Middle East, the 8.2ka event, which occurred at a time when agriculture had not yet entirely supplanted the hunter-gatherer economy, spurred the transition to a permanently settled mode of life, more intensive methods of cultivation, and the rise of towns. The 6ka cooling period corresponds to the beginnings of civilization in the Middle East, China, and Southeast Asia. The need to coordinate a large population base over a wide area to manage irrigation during a prolonged drought has been postulated as a key factor in the rise of city-states in Mesopotamia.

During the Hypsithermal, large portions of the Sahara Desert were dotted with seasonal lakes, supporting abundant game and hunters who left rock art and artifacts in areas that are completely barren today. The present aridity of the Sahara and much of the Middle East was well established in classical times (twenty-five hundred to fifteen hundred years ago) and has persisted through warm periods of increased rainfall, in part as a result of human activities such as overgrazing. Global warming could increase rainfall in regions such as the Sahel. Although this would ultimately be beneficial, immediate effects would include flash flooding, erosion, and the proliferation of invasive species.

The cultural pattern is less clear-cut in the humid tropics, but studies of the rise and fall of lowland Mayan civilization in Central America suggest that, contrary to expectation, the peak of this civilization corresponded to the cooling period that occurred twelve hundred to one thousand years ago. Its subsequent decline came during the Medieval Warm Period. In the tropics, cold conditions at the poles cause a weakening of monsoons. Seasonally dry conditions advance toward the equator. Climates with strongly marked wet and dry seasons are more favorable to intensive, highly productive agriculture than is a permanently wet tropical rain forest.

Modern Western civilization is the product of the Little Ice Age. (The term “Little Ice Age” is used differently by different writers. Many use it to refer to the climate cooling from about 1300 to 1850, while others use it for the latter half of that interval, when cooling was greatest, beginning around 1550 or 1600.) The abrupt drop in temperatures that occurred in 1315 produced first a famine, then war, and finally a pestilence that wiped out one-third of Europe’s population. Without a famine-weakened population and the disruptions of war, the bubonic plague might well have remained localized, as it did in North Africa in the eighteenth century. Temperatures remained low during the recovery period. Thus, a return to the population levels and living standards of the thirteenth century required first technological innovation and later exploitation of warmer areas through colonialism. For reasons not clearly understood, the did not produce increased drought in the tropics.

In 2021, a study by scientists at Rutgers University showed that the first half of the Holocene was colder than scientists had originally predicted. Additionally, they showed that an increase in greenhouse gases in the atmosphere played a significant role in the warming of the Holocene. According to the scientists, this eliminates any doubt that greenhouse gases are responsible for modern global climate change.

Context

Probably the most important global warming lesson to be learned from the Holocene record is that of the disruption of North Atlantic currents and resulting deep freeze in Europe eighty-two hundred years ago. Very rapid melting of the Greenland and release of freshwater into the Atlantic could well produce a similar effect. Short-lived episodes of warming and cooling are documented in the climate proxy record. On a local level, such episodes undoubtedly produced dramatic effects, but they left no lasting impression on the world’s flora and fauna, nor on human culture as a whole. In today’s overpopulated and environmentally degraded world, the consequences of a temporary drop in global temperatures due to either massive volcanism or a disruption of in any of the Earth’s oceans present a much grimmer prospect than they did at any other time in history. Both scenarios have sufficiently high probability to attract the attention of Pentagon analysts.

Key Concepts

• positive feedback loops: self-accelerating processes, such as increased snow cover increasing planetary and promoting additional cooling, leading to more snow cover
• proxies: preserved, measurable parameters that correlate with climate and serve as evidence of past climatic conditions
• thermohaline circulation: the rising and sinking of water caused by differences in water density due to differences in temperature and salinity

Bibliography

Alverson, Keith D., Raymond S. Bradley, and Thomas Pedersen, eds. Paleoclimate, Global Change, and the Future. Berlin: Springer Verlag, 2003.

Fagan, Brian. The Long Summer: How Climate Changed Civilization. New York: Basic Books, 2004.

"Important Climate Change Mystery Solved by Scientists." Rutgers University, 16 Feb. 2021, www.rutgers.edu/news/important-climate-change-mystery-solved-scientists. Accessed 13 Dec. 2024.

Mayewsky, Paul, et al. “Holocene Climate Variability.” Quaternary Research 62 (2004): 243-255.

Saltzman, Barry. Dynamical Paleoclimatology: Generalized Theory of Global Climate Change. New York: Academic Press, 2002.

Stipp, David. “The Pentagon’s Weather Nightmare.” Fortune, February 9, 2004.