1.1: Entering the Age of Humans

Before diving into the problem of anthropogenic climate change, it’s helpful to understand what the planet’s climate was like before modern humans (Homo sapiens) became a major force.

The history of Homo sapiens extends as far back as 160,000 years ago. However, it was only in the last 10,000 years, with the advent of the Agricultural Revolution, that Homo sapiens began modifying the land surface for growing food. The Agricultural Revolution roughly coincides with a warm interglacial that began about 11,700 years ago and continues to the present. Scientists call this period of warm and relatively stable climate the Holocene. Although there were significant regional climate fluctuations, the stable climate of the Holocene enabled the Agricultural Revolution. During this period humans transitioned from hunting and gathering to agriculture. Cities, writing, and major human civilizations all developed during this time. The last 270 years of the Holocene ushered in the Industrial Revolution, and with that, climate pollution began to increase dramatically.

Figure 1.1.1 Homo sapiens history. Image from V. Ramanathan.

Figure 1.1.2 Effects of the Great Acceleration on six different measures. As human population and gross domestic product (GDP) have risen sharply, so have water use, carbon dioxide emissions, deforestation, and species extinctions. Adapted from Steffen et al. 2015.

The “Great Acceleration”

The Industrial Revolution, which started in Britain and evolved during AD 1750–1850, was powered by fossil fuels. Now nearly all nations in the world consume them. Beginning around 1950, after World War II, the production and consumption of goods increased dramatically, leading to accelerated burning of fossil fuels. This acceleration is illustrated in every measure we can think of: population, gross domestic product (GDP), water use, fertilizer consumption, the number of motor vehicles in circulation, and many more. Because of this, scientists call the period from 1950 to the present the Great Acceleration.

Not only did the Great Acceleration increase production, consumption, and population, it also left a huge imprint on the ecosystem. For example, the rate of change in carbon dioxide concentration, which was already steadily increasing, accelerated in 1950. By the 1980s, this increase in carbon dioxide had already led to a significant rise in global temperatures, which in turn brought huge ecological impacts. Depletion of the ozone layer again became a major factor in the 1980s. After 1950, the biodiversity of species started decreasing and the extinction of species accelerated.

Figure 1.1.2 shows a few examples of the rapid increase in human activity and its ecological impacts during the Great Acceleration. Notice that nearly every measure—population, total real GDP, water use, biosphere degradation (species extinctions), and the loss of tropical rain forests—takes off dramatically beginning around 1950.

No part of the planet remains unaffected by the Great Acceleration. The planet we see today is vastly different from the planet our ancestors inherited a century ago.

The Anthropocene

We are still in the Holocene climatologically, but the impacts of the Industrial Revolution and Great Acceleration, combined with a massive population increase, have caused many scientists and other thinkers to argue that we have entered a new period called the Anthropocene, or age of humans. Humans have emerged as a major force modifying the environment. The activities of all humans combined are comparable to geological forces, such as earthquakes and volcanoes, that modify the Earth’s surface and atmosphere on large scales and in visible ways.

Scientists have not yet agreed on the time period that marks the beginning of the Anthropocene. Many propose the period started sometime during the Industrial Revolution. Others argue that it began with the advent of the Agricultural Revolution about 10,000 years ago. Still others suggest that its start date should be set to 1945, the date of the first atomic explosions. Irrespective of how scientists settle the starting date of the Anthropocene, it’s the Great Acceleration that began in the 1950s that marks the beginning of Homo sapiens’ truly massive global imprint on the planet. The climate has already warmed by 1°C since 1900. If we continue with business-as-usual consumption of fossil fuels, the warming between 1900 and 2100 could exceed 4°C.

Why worry about a warming of 4°C?

We know that the planet has been both significantly warmer and significantly cooler in the past. Why bother about a human-induced warming of 4°C? How do we judge whether this amount of warming is large or small? One way is by looking at the glacial to interglacial cycles the planet has undergone many times over the last 2.5 million years. The difference in global average temperature between a glacial and an interglacial period is only about 4°C to 5°C, but this difference results in a dramatically different climate. At the coldest point of the last glacial period (the Ice Age, about 20,000 years ago), an ice sheet covered most of what is now Canada and the northern United States. This ice sheet was about 1 kilometer thick over the current location of New York City. In contrast, during the previous warm interglacial period about 120,000 years ago, called the Eemian interglacial, there was substantially less ice in Greenland and Antarctica than at present, and global sea levels were at least 6 meters higher. The temperature at that time was only about 1°C warmer than the preindustrial average of the early 1800s. It’s clear that a 4°C increase in the global temperature is, in fact, a big deal.

It’s also important to understand that the planet is already in its warm state (interglacial period), having warmed by about 4°C from the glacial temperatures of about 20,000 years ago. Warming it by another 4°C would push the planet, along with all of its ecosystems and glaciers, beyond any temperature experienced in the last 25 million years. In short, more than just the 4°C warming, the fact that this warming is happening on top of the current warm interglacial period is the bigger concern.

Finally, past changes in global temperature by as much as 4°C occurred over periods of thousands of years or more. Compare that to the projected Anthropocene warming, which will happen within a single century. This rate of warming is at least 100 times faster than naturally occurring changes—and far too rapid for social systems, natural species, and ecosystems to adapt.