Environmental Issues: Global Warming

The phrase global warming refers to a phenomenon in which the Earth's surface temperature increases from its long-term averages generally because of an atmospheric blanket of greenhouse gases (GHGs; primarily carbon dioxide, methane, and chlorofluorocarbons) that serve to trap reradiated solar energy from escaping into space. This blanket of greenhouse gases is responsible for providing Earth a generally temperate, stable, and life-sustaining climate. In common parlance, global warming is often used interchangeably with climate change. In the present context, though, it is used in a more limited sense as a driver of global climate change.

The Science

In all of our solar system, Earth is the only planet known to support life. This uniqueness derives in great part from an atmosphere that regulates the Earth's surface temperature within a range conducive to the development of living organisms, including humankind. The explanation for this phenomenon was suspected as early as 1824, when French mathematician and physicist Jean Baptiste Fourier postulated that gases in Earth's atmosphere might influence its surface temperature. In 1859, physicist John Tyndall suggested that changes in the concentrations of some atmospheric gases could result in changes to Earth's climate. The Swedish chemist Svante Arrhenius published an article in 1896 demonstrating that the amount of carbon dioxide in Earth's atmosphere would significantly affect its surface temperature. Arrhenius coined the phrase greenhouse effect and predicted that a geometric (nonlinear) increase in atmospheric carbon dioxide would result in an arithmetic (linear) increase in the Earth's surface temperature.

Based on this thermal blanket of greenhouse gases, scientists have long understood that the Earth has undergone a series of long-term cyclic cooling and warming phases. The former account, in part, for periods in which glaciers have covered vast areas of the planet; the latter for long periods of regional desertification during which man and beast have populated a greater portion of the Earth and during which their numbers have greatly multiplied. The most recent glacial period ended approximately 10,000 years ago; Greenland and Antarctica are vestiges of that period. The duration of our current interglacial period will be determined in no small part by the extent of warming caused by the greenhouse effect.

By testing polar and glacial ice cores at continuously increasing depths, scientists can determine the composition of Earth's atmosphere as a function of time. For example, an Arctic ice sample will contain minute pockets containing a small amount of air—and its constituent gases—that were trapped at the time the ice froze. Ice samples taken from deeper cores were formed earlier in time. Using these historic data, modern computer models of Earth's climate systems are able to calculate Earth's surface temperature over time. Using this and other proxy data, we have a longitudinal history of Earth's surface temperature stretching back hundreds of thousands of years.

An example of the type of data scientists are able to coax from ice core samples is shown in Figure 1. The top three curves show that present-day carbon dioxide and methane concentrations are comparable to, if not ...