The climate stability underpinning our modern society is exceptional, standing out from a climate history on Earth dominated by what we would consider extremes. Humans have only lived in a narrow range of temperatures, but in the deep past the earth was home only to organisms, plants, and animals that could thrive in climates that we might recognize from terrariums, saunas, steam rooms, and deep freezers.
Consider a period that fascinates children around the world, one of the warmest times in Earth’s recent history, 90 million years ago: Dinosaur Earth.
Picture the earth that dinosaurs inhabited. With carbon dioxide at almost three times the atmospheric concentration of today, heat couldn’t escape and temperatures never dropped low enough for the ice sheets to form, even in the Arctic or Antarctic.
In fact, the poles had a temperate, swampy climate while most of Earth experienced such hot and humid conditions that only reptiles and insects could thrive above water. The global temperature spiked to staggering highs and volcanic eruptions scarred landscapes around the globe.
Following the asteroid impact that drove dinosaurs to extinction, Earth’s climate fluctuated dramatically. Debris and dust from the impact lingered in the atmosphere preventing sunlight from warming the earth and making the climate more arid.
Far more recently in Earth’s history, about 25,000 years ago, variations in Earth’s orbit led to our planet’s most recent glacial maximum. Ice sheets stretched all the way down to the northern United States and across Europe and Asia. They reflected much of the incoming warmth from sunlight back out of Earth’s atmosphere, leading to drought and frigid desertification.
During this period, the earth was not only significantly colder than today, but also dusty, stormy and devoid of much of the life that would have filled its forests and oceans in hotter conditions. The climate was violent and unstable, with harsh temperatures and jarring climatic shifts. To survive, early homo sapiens inhabiting the planet lived in small, nomadic societies following their food sources while subsisting on hunting and gathering.
About 12 thousand years ago, changes in the earth’s orbit and the tilt of its axis led the planet out of a frigid glacial maximum into a milder period known as the Holocene epoch that proved to be unusually stable. The dominance of homo sapiens and the flourishing of civilization is owed to the particular climate of the Holocene. For the first time, our species could settle, plant crops, and enjoy comfortable temperatures. We flourished and came to dominate the planet.
The Holocene climate was the product of a delicate, intricate balance. A mirror of ice and snow in the Arctic, Antarctic, and in many mountain and tundra regions reflected sunlight, moderating the warming effects of solar radiation. In the Tropics, trees and plants cycled carbon, ensuring more carbon remained stored on land in forest and vegetation than escaped into the atmosphere. The result was an atmosphere that kept Earth at a stable temperature ideal for our species as well as for those glaciers and forests.
Climate change threatens to push our planet out of the Holocene. The driving force behind climate change is heat. Heat is familiar, it is intuitive, and understanding its attributes will help you understand our changing climate more clearly.