Yes, there is plenty of information available online about how the Earth's average yearly temperature is derived. The main source of this data is NASA's Goddard Institute for Space Studies (GISS), which compiles years of global air and sea surface temperature data to produce maps and estimates of global temperature changes. Additionally, scientists from the National Oceanic and Atmospheric Administration (NOAA) use a combination of data from satellites, weather stations, ships, buoys, and weather balloons to measure sea surface temperatures and other indicators of climate change. You can also find a variety of articles and research papers on the subject if you do some research.