Nasa's temperature record is showing temperature increase. When I confronted a climate change skeptic with it, he replied that the data has been tempered with, and that there are different data sets which do not show temperature increase.
The exact reply was:
According to RSS, UAH and radiosonde global temp anomaly datasets, there hasn't been a global warming trend in 20 years;
GISS and HADCRUT4 datasets have added heat to raw temperature data to artificially keep the warming trend going as NOAA freely admits:
This sounds reasonable to me.
Can anyone familiar with climate science explain why there are several data-sets, and why they are saying different things, and why NASA's set is the correct one?