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Time series of seasonally averaged global surface temperature (December 1879–August 1999)
based on the Quayle et al. (1999) data set, computed as differences from the 1880–1998 mean.
The time series uses-an area-weighted average of the surface air temperature over land and
the temperature of water at the ocean's surface.
Temperature changes at and just above the earth's surface are of
singular importance from the standpoint of societal and human
impacts, and they are also widely regarded as an important
indicator of human-induced climate change. However, if global
warming is caused by the build-up of greenhouse gases in the
atmosphere, it should be evident not only at the earth's surface,
but also in the lower to mid-troposphere. Temperatures aloft can be
measured in a number of ways, two of which are useful for climate
monitoring: by radiosondes (balloon-borne instrument packages,
including thermometers, released daily or twice daily at a network
of observing stations throughout the world), and by satellite
measurements of microwave radiation emitted by oxygen gas in the
lower to mid-troposphere, taken with an instrument known as the
Microwave Sounding Unit (MSU).5
The balloon measurements are taken at the same Greenwich mean times
each day, whereas the times of day of the satellite measurements
for a given location drift slowly with changes in the satellite
orbits. The radiosonde network has been operative since the late
1940s and substantialcontinue