The Science of Sustainability

The earth has sustained a wide and abundant variety of life for millions and millions of years, with the origin of life on Earth estimated to have begun 3.5 billion years ago. Humans only arrived in the last 500,000 years, or 0.01 percent of the total history of life on the planet. The forms of life and the climate have changed continuously and sometimes drastically over that time. The earth's history is divided into three geological eras, but the most recent and the one that we currently live in is the Cenozoic, which began 65.5 million years ago. The most significant features of the Cenozoic era are a long-term cooling of the earth, the rise and diversification of mammals, and the evolution of humans. Previous eras featured a much warmer planet, a different atmospheric system, and a wide variety of life.

A significant feature of the climate over the last 2.6 million years has been "ice ages," which are a continuous fluctuation between relatively cold (glacial) and warm (interglacial) periods. Glaciations are periods of extremely cold climate where a greater portion of the earth is covered with ice and snow. Interglacials are intermittent warming periods between glaciations, which feature a melting and retreat of ice from large portions of Earth. In the last 800,000 years there have seen eight glacial cycles - one occurring about every 100,000 years - with the last glaciation ending about 12,500 years ago. Currently we are in an interglacial time period. This last interglacial period is known as the Holocene and features the rise of modern human civilization and the current climate and earth system.

The glacial cycles are the result of a complex interaction of factors, including atmospheric composition (such as the concentration of carbon dioxide and methane), variations in the earth's orbit, movement of tectonic plates, and changes in solar output. Of the different factors, periodic changes in the earth's orbit, tilt, and precession (wobbling around its axis) are believed to be a major driver for ice ages when they occur in a specific combination that reduce the amount of sunlight reaching the earth. These changes are also known as Milankovitch cycles, named after the Serbian mathematician who theorized that these orbital changes were the cause of the ice ages. It is important to understand natural past climate change in the context of man's impact on global climate change, which will be discussed later on in this chapter.