by Tucker Lieberman

Created on: February 10, 2013

The metaphor of the Earth’s atmosphere as a greenhouse was born in the nineteenth century when it was discovered that atmospheric gases trap heat.  So-called greenhouse gases — including carbon dioxide, water vapor, and methane — permit short-wave radiation from the sun to pass through to the Earth but prevent long-wave radiation from the Earth from departing into space. (The metaphor is a bit of a misnomer, as part of what makes a greenhouse hot is the heating of the glass itself, whereas clearly Earth is not surrounded by glass.) 

All other things being equal, the temperature will rise as more greenhouse gases are added to the atmosphere. This is linked to a complex set of natural processes. For example, carbon dioxide, a byproduct of the combustion of carbon-based fossil fuels like coal and oil, is naturally removed from the atmosphere when it dissolves into the ocean or is absorbed into plants which return the carbon to solid form.

The first scientist to speculate that the carbon dioxide emissions from the burning of fossil fuels could heat the planet was Svante Arrhenius of Sweden in 1896. This is discussed in Spencer Weart's book The Discovery of Global Warming (2008).

In the 1930s, scientists knew that the planet had warmed over recent decades, but they didn’t know why. The English scientist Guy Stewart Callendar (1898-1964) said it was caused by the greenhouse effect, but his theory, proposed over three decades after Arrhenius's, was not taken seriously. Callendar’s archived correspondence is kept today at the library of the University of East Anglia.

In the 1950s, scientists began to investigate the possibility that carbon dioxide was indeed raising the average global temperature. Finally, in 1960, their results showed that the level of atmospheric carbon dioxide was rising each year and that this could indeed explain the rising global temperatures.

Atmospheric carbon dioxide is measured in parts per million (ppm). Scientists can get air samples from millions of years ago by drilling into glaciers that formed in the eras since the dinosaurs roamed the earth (when there was no ice on the planet). Samples obtained in this way indicate that carbon dioxide levels naturally fluctuated between 200 and 280 ppm for millions of years. However, this level has risen steadily since the beginning of the industrial revolution. Today, in 2013, the levels of atmospheric carbon dioxide are at 394 ppm, according to the advocacy

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