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All 4 of the measurements listed in the discussion of "What is global warming?"--direct surface measurments, indirect surface measurements, balloon (radiosonde) measurements, and satellite (MSU) measurements--have shown increases over the course of the 20th century. However, the balloon and satellite data show far, far less warming than the direct surface measurements, over the time period from 1979 to the present. This fact has important implications when we investigate whether humans have caused the observed warming.
The magnitude of the surface temperature increase over the 20th century has been calculated to be approximately 0.6 degrees Celsius (1.1 degree Fahrenheit).
It is also important to note that global temperatures are never constant. For several million years, the earth has undergone ice ages (of approximately 80,000-100,000 years duration) and interglacial (warm) periods of approximately 10,000 years. Even within these warm periods, the temperature rises and falls. None of these past glacial or interglacial periods were caused by changes in CO2 concentrations in the atmosphere (although global temperature changes have appeared to cause changes in atmospheric CO2 concentrations, with CO2 concentrations falling during ice ages, and rising during warm periods).
To summarize: Yes, "global warming" (a global rise in temperature, at least at the surface) seems to have occurred in the 20th century. The next key question is: "Are human greenhouse gas emissions causing global warming?"
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