Last Interglacial

• 1. Before the Last Ice

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  The Last Interglacial began about 130,000 years ago, at the close of the Penultimate Glacial Period. It gave way to a new glaciation around 115,000 years ago. In those roughly 15,000 years, the planet moved through one of the warmest spells of the past 800,000 years.

  Carbon dioxide in the atmosphere held at about 280 parts per million. Temperatures at the peak ran up to 2 degrees Celsius warmer than the Holocene we inhabit today. Hippopotamuses had pushed as far north as Stockton-on-Tees, in northern England. The world was recognizable as ours, yet built on different terms.

  Neanderthals occupied Europe. Modern humans were present in southern Africa and West Asia. The sea stood several meters higher than today. A 2-degree difference, measured as a global average, remade coastlines and rerouted the migrations of animals. It left its fingerprints in sediment, ice, and coral on every side of the planet. The record it left was first recognized from a handful of shells pulled from an inland borehole. To one Dutch geologist, those shells raised a question that would take more than a century to answer fully.

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• 2. Named for a River

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  In 1875, boreholes drilled near the Dutch city of Amersfoort yielded a sediment deposit that caught Pieter Harting's eye. He named it the Système Eémien, after the river Eem on whose banks the city stands. His attention had been drawn by the marine shells embedded in those layers. They bore no resemblance to the fauna then living in the North Sea.

  South of the Strait of Dover, many of those ancient mollusk species still survive today, but only in warmer waters. Their modern range runs from Portugal into the Mediterranean. Harting had uncovered evidence of seas once warm enough to host a distinctly southern fauna this far north.

  Lorié extended the molluscan record in 1887, and Spaink analyzed it further in 1958. Van Voorthuysen described the foraminifera from the type site in 1958; Zagwijn followed with a pollen analysis in 1961. By 2000, a team that included Cleveringa had revisited the original site using new interdisciplinary methods. That same year, Van Leeuwen and colleagues examined the Amsterdam-Terminal borehole in the Amsterdam glacial basin. Their uranium-thorium dating placed late Last Interglacial deposits at 118,200 years ago, with an uncertainty of 6,300 years.

  In Britain the period is called the Ipswichian. In Russia it is the Mikulino interglacial, sometimes spelled Milukin; in Ukraine, the Kaydaky; in the Alps, the Riss-Würm; in Chile, the Valdivia interglacial. Geologists in North America use the term Sangamonian, though different publications define its scope differently. In the global ocean record, the interval corresponds to Marine Isotope Stage 5e, identified from oxygen isotope ratios in seafloor microfossils.

  What Harting had fixed in geological time was the edge of an interval whose warmest point lay thousands of years before what his boreholes had captured.

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• 3. Peak Warmth

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  Around 128,000 to 123,000 years before the present, the Last Interglacial climbed to its hottest. During northern summers, Arctic temperatures ran about 2 to 4 degrees Celsius above their 2011 values. At approximately 125,000 years ago, forests reached North Cape, Norway. That latitude sits above the Arctic Circle and today is open tundra.

  Hazel and oak grew as far north as Oulu, Finland. Trees extended across southern Baffin Island in the Canadian Arctic Archipelago, further north than today's treeline at Kuujjuaq in northern Quebec. Saint Lawrence Island, now tundra, carried boreal forest during the interglacial, sustained by reduced sea ice in the Arctic Ocean. In the interior of Alaska and the Yukon Territory, however, inadequate precipitation limited forest cover despite the warmer air.

  The prairie-forest boundary in the Great Plains of the United States lay further west than today, near Lubbock, Texas. The modern boundary runs closer to Dallas. In Northern Europe, winter temperatures rose over the course of the interglacial while summer temperatures fell, reflecting how orbital changes distribute heat unevenly across seasons.

  Kaspar and colleagues, writing in 2005, modeled the European climate of the Last Interglacial using observed greenhouse gas concentrations and orbital parameters. North of the Alps, their model found temperatures about 1 to 2 degrees Celsius above modern values. South of the Alps, it found conditions 1 to 2 degrees Celsius cooler. Three overlapping shifts in Earth's orbit drove the warming: a more elongated orbit, a greater axial tilt, and perihelion at a different point in the year. Together these factors, known as Milankovitch cycles, amplified seasonal contrasts across the Northern Hemisphere.

  As the interglacial cooled during its latter half, atmospheric carbon dioxide held steady. What did not hold steady was the internal circulation of the climate system.

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• 4. Cold Within the Warm

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  The geological record at Beckentin preserves a cooling event during the E5 phase of the Eemian. It struck precisely 6,290 years after the start of interglacial afforestation. Pronounced temperature swings recorded in Greenlandic ice cores confirm that the Arctic climate of the Last Interglacial was highly unstable throughout.

  Meltwater pulse 2B, approximately 133,000 years before the present, substantially weakened the Indian Summer Monsoon. Between 133,000 and 130,000 years before the present, during an insolation maximum, meltwater from the Dnieper and Volga rivers connected the Black Sea to the Caspian. During the middle of the interglacial, the Atlantic Meridional Overturning Circulation weakened. That weakening gradually cooled the eastern Mediterranean even as northern latitudes remained warm.

  A 2018 study based on soil samples from Sokli, in northern Finland, identified abrupt cold spells around 120,000 years ago. These spells were triggered by disruptions to the North Atlantic Current and lasted hundreds of years. They caused temperature drops of a few degrees and produced visible changes in vegetation across those regions.

  A 468-year-long aridity pulse swept central Europe around 116,000 BC, one of several markers of the interglacial's slow close. By 112,000 BC, ice caps had begun forming in southern Norway. The Eemian lasted roughly 1,500 to 3,000 years longer in Southern Europe than in the north. On Antarctica, interglacial conditions ended before the Northern Hemisphere had fully cooled, a reminder that different parts of the planet do not always move in step.

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• 5. The Higher Sea

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  Sea level at the peak of the Last Interglacial stood probably 6 to 9 meters higher than today. Greenland contributed somewhere between 0.6 and 3.5 meters of that rise. Thermal expansion and the melting of mountain glaciers added up to 1 meter more. Antarctica's contribution remains uncertain.

  A 2007 study of the Greenland ice core site Dye 3 found evidence that the site was glaciated during the Last Interglacial. If Greenland held substantial ice there, it could have contributed at most 2 meters to sea level. Research on marine sediment cores from offshore of the West Antarctic Ice Sheet suggests the sheet melted during the Last Interglacial. That melt may have driven ocean waters to rise as fast as 2.5 meters per century at times.

  Fennoscandia became an island during the Last Interglacial. A seaway connecting the Gulf of Finland of the Baltic Sea to the White Sea of Russia had opened between it and the continent. The Baltic, sometimes called the Eemian Sea during this period, held a much higher salt content than today. Its coastlines lay further inland; Jutland, too, was surrounded by water. Vast areas of northwestern Europe and the West Siberian Plain were inundated.

  Fossil coral reefs exposed today in the tropics mark the former waterline. The Caribbean and the Red Sea coastlines are particularly rich in them. Many contain internal erosion surfaces, evidence that sea level fluctuated within its elevated range rather than holding steady. Along the Central Mediterranean coast of Spain, ancient sea levels were comparable to those of the present, a local exception shaped by the geometry of that basin.

  Jutland, now a peninsula, was an island during this period. The animals that crossed those altered geographies filled an interglacial world that reached into every corner of the continents.

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• 6. An Altered Bestiary

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  Straight-tusked elephants (Palaeoloxodon antiquus) roamed the temperate landscapes of Europe during the Last Interglacial. Two species of rhinoceros shared the continent: the narrow-nosed rhinoceros (Stephanorhinus hemitoechus) and Merck's rhinoceros (Stephanorhinus kirchbergensis). Irish elk (Megaloceros giganteus) and aurochs (Bos primigenius) moved through forests alongside red deer, fallow deer, roe deer, and wild boar. Predators included cave lions (Panthera spelaea), cave hyenas (Crocuta spelaea), brown bears, and wolves.

  The Mediterranean, warmer than today, was colonized during this period by tropical marine invertebrates from the Atlantic. These species, collectively called the Senegalese Fauna, included the conch Thetystrombus latus, the cone snail Conus ermineus, and the sea snail Linatella caudata. Today they live only off the coast of West Africa and in the tropical Atlantic.

  The melting of the Laurentide Ice Sheet opened northern North America to a wave of migrations. The American camel Camelops hesternus moved into northern Canada and Alaska. Alongside it spread mastodons of the genus Mammut, the giant ground sloth Megalonyx jeffersonii, and the bear-sized giant beaver Castoroides. At lower Canadian latitudes, Columbian mammoths (Mammuthus columbi), stag-moose (Cervalces), and the llama Hemiauchenia also pressed northward.

  The steppe bison (Bison priscus) migrated from Alaska into the heartlands of North America at the start of the Last Interglacial. From that lineage came the giant long-horned bison Bison latifrons. The oldest known Bison latifrons specimen comes from the Snowmass site in Colorado, dated to around 120,000 years ago. All modern North American bison species descend from that lineage, and its appearance marks the beginning of the Rancholabrean faunal age.

  The American lion (Panthera atrox) appeared widely across North America during this period. It descended from populations of the Eurasian cave lion (Panthera spelaea) that had crossed into Alaska during the Penultimate Glacial Period. The woolly mammoth (Mammuthus primigenius), adapted to cold, contracted toward refugia as the warmth held sway.

  Two kinds of hominin moved through those landscapes. Their separate stories lead in different directions, but both begin in the warmth of the Last Interglacial.

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• 7. Two Kinds of Human

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  Skhul and Qafzeh, sites in what is now Israel, contain fossils of anatomically modern humans living in West Asia during the Last Interglacial. These people ranged as far east as the Persian Gulf in Arabia. Their presence, combined with populations in Southern Africa, marks the earliest split of living modern human lineages, preserved in mitochondrial haplogroup L0.

  Neanderthals re-entered the higher latitudes of Europe as temperatures rose, having retreated from those regions during the Penultimate Glacial Period. Britain was absent from their range. High sea levels had made Britain an island, severing it from the continent. Within Europe, Neanderthal groups fished and hunted big game, including straight-tusked elephants, the largest animals then living on the continent.

  Both populations occupied what archaeologists designate the Middle Paleolithic. As the most recent period with a climate comparable to the Holocene, the Last Interglacial serves researchers as a concrete reference point. For ecologists, those interglacial European ecosystems mark the last time the continent carried its full complement of large animals. They have been formally proposed as a baseline for studying how a fully stocked warm-world landscape might be restored.

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