Since 1979, NOAA satellites have been carrying instruments which measure the natural microwave thermal emissions from oxygen in the atmosphere. The signals that these microwave radiometers measure at different microwave frequencies are directly proportional to the temperature of different, deep layers of the atmosphere. Every month, John Christy and DR. Roy Spenser update the global temperature data sets that represent the piecing together of the temperature data from a total of eleven instruments flying on eleven different satellites over the years. As of 2008, the most stable instrument for this monitoring is the Advanced Microwave Sounding Unit (AMSU-A) flying on NASA’s Aqua satellite.
The graph above represents the latest update; updates are usually made within the first week of every month. The smooth curve in the graph is a fourth-order polynomial fit to the data, which smooths out the large amount of monthly variability in the data and helps reveal the underlying ‘trends’. (There is no claim that this curve has any predictive power for the coming months or years.)
(Directly taken from Dr. Roy Spenser's excellent blog)