Introduction

Modern climate change is consistently in the news and on the minds of scientists, policy-makers, and everyday people because of its threatening global impacts. But this is not the first time the earth has experienced abrupt climatic change. The Younger Dryas (YD) period, occurring around 12,800 years ago, is a recent (geologically speaking) example of a time of extreme cooling in the northern hemisphere. By studying this abnormal climatic period, we can learn more about current climate change.

The YD marks the transition from the Pleistocene epoch – the time from 2.6 million years ago to 11.7 thousand years ago – to our current climatically stable period, the Holocene epoch. What was the earth like in the centuries leading up to the YD? The late Pleistocene was punctuated by many climatic oscillations including glaciations – colloquially known as ice ages. Many species went extinct, notably megafauna like wooly mammoths, many Eurasian and Australian elephants, and some species of hippopotamuses, bears, and lions. The last archaic human species, like Neanderthals, also died out during this time.

Unlike most glaciations of the Pleistocene, only the northern hemisphere was affected during the YD. The climate conditions of the southern hemisphere were what we would expect for that time in the natural climatic cycles, but there were unexpected temperature drops over most of North America and some parts of Europe. At the end of the YD, temperatures rose extremely quickly, in a matter of decades. Since the speed of cooling and warming at the start and end of the YD is similar to the rapid anthropogenic climate change of today, studying this period may provide insight into the potential consequences of modern climate change.

What Caused the Younger Dryas?

The causes of the YD are not yet completely understood and further research may shed light on this important period that could help predict future (and current) abrupt climate changes and their effects on the earth and its ecosystems.

The most widely accepted explanation of the YD is that a flood of freshwater into the Arctic Ocean disrupted thermohaline circulation (THC) in the Arctic and North Atlantic. THC is a natural process by which the ocean circulates water, driven by salinity and temperature. THC is very sensitive to external conditions like meltwater, wind patterns, land runoff, and precipitation.

Just before the YD, global temperatures started to increase after the Last Glacial Maximum in the natural cycle of cooling and warming. At that time, North America was covered by a sheet of ice. The warming began melting this ice sheet and the meltwater created a huge freshwater lake just north-west of the Great Lakes. Flow from the lake to the ocean was blocked by an ice dam, but as the temperature increased further and more meltwater flowed into the lake, the ice dam suddenly broke and released approximately 9,500 km3 of freshwater into the Atlantic ocean.

The huge flow of freshwater into the Atlantic diluted the shallow saltwater which shut down THC. Without THC, warm water stopped moving north and cold water at the surface near the north pole got colder and colder.

The instability of THC probably made it easier for the YD to occur in the first place. This is something to consider when studying modern climate change. Ocean composition is changing because of rising temperatures, ice melt, and pollution. If these changes are sudden or drastic enough we may also risk disrupting ocean circulation and creating an unpredictable climate event. Other earth systems, like the carbon and nitrogen cycles and atmospheric composition, are also sensitive to small changes. One of the key characteristics of modern climate change is extreme weather events, which may be a result of alterations to these systems.

How did it affect the Earth’s ecosystems?

Natural climatic cycles operate on time scales of one hundred thousand to one million years. But, as in the YD, abrupt climate changes are possible and can have devastating effects on the earth’s ecosystems and human populations.

Ecology: biodiversity, vegetation, and megafauna

Climatic change during the Younger Dryas decreased deep-ocean biodiversity in the North Atlantic. This is likely related to the decreased ocean circulation minimizing flow of  available nutrients, the colder water temperature, and lower species migration. Similar drops in biodiversity have been observed at other times of reduced THC.

Climate-driven ecological change is not unique to the YD. Throughout the Pleistocene era (2.5 million years ago to the start of the YD), the variable climate strongly affected plant range and abundance which had secondary effects on herbivores. The extinctions of this period, as well as the megafaunal extinctions at the Younger Dryas-Holocene transition, were most likely the result of the combination of climate change and human hunting.

Climate-driven extinction is a primary concern of modern climate change as well. Habitats are being destroyed by human intervention directly, for example, from deforestation, and indirectly because of changes in temperature and weather resulting from greenhouse gas emissions. This has sometimes been called the “sixth mass extinction” and a recent study estimated that up to a third of all plant and animal species could go extinct by 2070 because of anthropogenic climate change.

Humans

Late Pleistocene humans used tools and lived in either nomadic tribes or small hunter-gatherer societies. The human inhabitants of North America during the YD were called the Clovis people and were potentially the first widespread human population in the American continent. The first human settlers of North America probably arrived on the continent around 14,000 years ago. From Alaska, where they arrived from the Bering land bridge, they spread throughout North and South America and are thought to be the ancestors of most native peoples of the American continent. Clovis people were highly mobile and hunted mammoths and other large animals like bison and mastodons. With the colder temperatures of the YD in North America, they settled down into villages and distinct cultures began to emerge.

This example illustrates that humans are also affected by climate events. We have seen that modern climate change has led to more extreme weather events including hurricanes, heat waves, droughts, and floods. These events pose a serious threat to coastal and island communities.  We will have to migrate and change our ways of life to adapt in the decades to come, just as the Younger Dryas forced the humans of 12,000 years ago to change.

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