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How much climate variability have humans faced since we evolved and since we began to settle (Neolithic times)? How important was migration to human survival during these times?
The climate always fluctuates because the variation in heat from the Sun reaching the Earth causes glacial-interglacial cycles. Over the past 420,000 years, there have been at least four major transitions between ice ages and relatively warmer interglacials.
Modern humans emigrated from Africa to populate the rest of the globe between 120,000 and 80,000 years ago, which means our species has had to adapt to many massive climate transitions.
Heating and cooling
The Last interglacial 129,000 to 116,000 years ago, it was a period of intense global warming (from about 2 ℃ more than today to as much 11 ℃ higher in the Arctic), resulting in a sharp reduction in the ice caps of the Arctic, Greenland and Antarctica, and a sea level rise of 6 to 9 m.
The last ice maximum of 26,500 to 19,000 years ago coincided with a significant drop in atmospheric CO₂ and a global cooling of 4.3 ℃.
Read more: The climate explained: will the tropics eventually become uninhabitable?
Low temperatures turned much of the world’s water to ice and enlarged glaciers.
This drop in sea level up to 130m compared to today. These exposed continental shelves joined land masses and created vast coastal plains, such as Beringia which linked Russia to North America, and Sahul which linked Australia to New Guinea.
After a brief period of warming, the northern hemisphere abruptly returned to near-glacial conditions about 12,900 years ago which lasted for 1,300 years. Known as the Younger dryas, this period recorded a climatic cooling down to 15 ℃ and the giant ice caps have advanced again. The end of the Young Dryas was just as brutal, marked by a rapid warming up to 10 ℃ in a few decades.
The most recent period of climate instability was the transition from Medieval Hot Period to the Little Ice Age. Cold conditions between 1580 and 1880 were characterized by a 0.5-4 ℃ cooling and expanding mountain glaciers in the European Alps, New Zealand, Alaska and the Andes.
What climate change meant for humans
Despite our impressive ability to adapt to a wide range of environments, humans have a privileged environmental envelope in which we thrive. These conditions would have been characterized by a mixture of open savanna-type forests, wetlands and rocky habitats.
Dense, humid tropical forests made access to resources difficult, while deserts were often too dry to provide enough food and materials.
The climatic conditions of the last interglacial could have encouraged waves of human expansion outside Africa when a humid and hot climate favored vegetated corridors across Eurasia.
The ensuing cooling period connected land masses previously separated by oceans and allowed human travelers to access Sahul from the Indonesian archipelago.
Read more: An incredible journey: the first people to arrive in Australia came in large numbers, and on purpose
Entering America from Asia via Beringia was more difficult because humans only got there during the last ice maximum when a huge ice cap blocked the new land bridge.
This warming created new accessible routes along the Pacific Northwest, followed by another ice-free corridor that formed 3,000 years later when the ice sheet retreated.
The need for food
Due to the cold temperatures and the scarcity of food at this time, humans had to improve their hunting efficiency by targeting large animals to maximize the return of food.
In the southern hemisphere, modern humans had already lived in Australia for 30,000 to 40,000 years before the last glacial maximum, so such drastic cooling and drying likely pushed human populations decline and retreat to smaller refuges closer to reliable sources of fresh water where game also congregated.
Read more: Did man or climate kill megafauna? In fact, it was both
After the last ice maximum, modern humans continued to spread across North America. The warmer and wetter climate of the southern hemisphere has also contributed to human migration to South America.
At the same time, the young Dryas of the northern hemisphere forced the populations either to return to a nomadic way of life, or to take refuge in a few hospital areas. After the harsh conditions of the Younger Dryas, the first testimony of agriculture have emerged in various parts of the world.
The settlement of remote Oceania between 3,500 and 730 years ago required ocean voyages of thousands of kilometers across the Pacific to New Zealand temperate and subantarctic waters.
Although these migrations are not clearly linked to any of the previous climate change events, the wind regimes at the time were particularly sailing friendly.
Read more: Climate Explained: Sunspots Affect Our Weather A Little, But Not As Much As Other Things
But the Little Ice Age could have reduced the size of the population and pushed the first Maori settlements north.
The Little Ice Age probably hit people in the northern hemisphere much harder. The cold climate caused many poor harvests, famines and population declines.
In the past five years alone, Earth is already ~ 1.1 ℃ hotter than 150 years ago and temperatures should be + 4.5 ℃ more than today by 2100. Today, we live in the hottest climate since our species began to populate the globe.
Climate fluctuations that lasted for millennia now occur in less than 100 years, affecting freshwater availability, food supply, environmental health and integrity.
Past climate change has paved the way for people to demonstrate immense adaptability and resilience by developing new skills, agricultural techniques, business models and political structures, but above all by abandoning their old unsustainable lifestyles.