The opinion by Senior Judge Vanessa Ruiz states:

To the extent statements in appellants’ articles take
issue with the soundness of Dr. Mann’s methodology and conclusions —
i.e., with ideas in a scientific or political debate — they are
protected by the First Amendment. But defamatory statements that are
personal attacks on an individual’s honesty and integrity and assert or
imply as fact that Dr. Mann engaged in professional misconduct and
deceit to manufacture the results he desired, if false, do not enjoy
constitutional protection and may be actionable.

The opinion also found that making statements to gain advantage in a "no-holds-barred debate over global warming" are typically protected under the First Amendment. However:

... If the statements assert or imply false facts that
defame the individual, they do not find shelter under the First
Amendment simply because they are embedded in a larger policy debate.

http://www.dccourts.gov/internet/doc..._14-CV-126.pdf