A “greenhouse gas” in general is any gas that traps infrared rays and thus promotes atmospheric warming. Water vapor is, in fact, the most abundant greenhouse gas in the earth’s atmosphere, but human activities do not substantially affect its abundance, and it is in equilibrium with surface water and oceans. Excess water in the atmosphere readily falls out as rain or snow. Some of the excess carbon dioxide is removed by geologic processes (see chapter 18), but since the start of the socalled Industrial Age in the mid-nineteenth century, the amount of carbon dioxide in the air has increased by an estimated 40%— and its concentration continues to climb (figure 10.2). If the heat trapped by carbon dioxide were proportional to the concentra- tion of carbon dioxide in the air, the increased carbon dioxide would by now have caused sharply increased greenhouse-effect heating of the earth’s atmosphere.
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