Global warming - (n.)
The phrase "global warming" is used in a number of different ways. Observations show that the Earth has warmed overall in the last 150 years by almost 2°F (nearly 1°C), especially in the northern hemisphere (Observations in the southern hemisphere are not good enough to determine this.). The reasons for this warming are not clear, however, such warming is within the range expected for an increased greenhouse effect from human activities, including the emission of carbon dioxide from burning wood, coal and petroleum, and the emission of methane from the stomachs of cattle and from the soils of rice fields. "Global warming" is also used to describe the unusual warming seen for the last two decades, when maximum temperatures and also average temperatures were commonly outside the range experienced for the last 1000 years. Much or most of this warming seems to be a result of human activities resulting in an increase in greenhouse gases in the atmosphere. A third use of the phrase is in the framework of expectations for the future. Continued release of greenhouse gases is expected to lead to "global warming" according to the best available computer models of climate dynamics. Many climate scientists eschew the phrase "global warming" because the warming is not necessarily general. In some areas there may be no change, and in others there may be cooling associated with an average warming. Also, warming by itself may not be the most important effect of the change seen. When taking account of these caveats, the phrase "global change" is used in preference to "global warming."