It's not just because there is no warming. Here's a bunch of emails along with a summary, including quotes like this classic:
“I’ve just completed Mike’s [Mann] Nature trick of adding in the real temps to each series for the last 20 years (i.e. from 1981 onwards) and from 1961 for Keith’s [Briffa] to hide the decline.” Dr. Phil Jones, Director of the Climatic Research Unit, disclosed Climategate e-mail, Nov. 16, 1999