Complex sentence: The claim that atmospheric data showed no warming trend was incorrect, as the published satellite data at that time showed a warming trend, though weaker than at the surface (earlier estimates published in the 1990s showed no warming); balloon data also showed warming.
Split into simpler sentences: The claim that atmospheric data showed no warming trend was incorrect. The published satellite data at that time showed a warming trend, though weaker than at the surface (earlier estimates published in the 1990s showed no warming); balloon data also showed warming.