Michael Mann points us to a "great" piece by cartoonist Neil Wagner, who writes that he [Wagner] believes James Hansen partly because Mann believes James Hansen

Twitter / MichaelEMann: Great piece in @HuffingtonPost ...

Great piece in @HuffingtonPost by Neil Wagner "Global Warming Has Arrived: The Ultimate Unwanted Houseguest" http://fb.me/2ll32xHxw

Neil Wagner: Global Warming Has Arrived: The Ultimate Unwanted Houseguest

Third, Hansen has the support of world-class experts, such as fellow climate guru Michael E. Mann, who says of Hansen:
...
With that kind of gravitas, why would one doubt the man's assessment of our warming climate? Is he some kind of left-wing extremist with a "very partisan agenda?" Not entirely likely since Hansen is a registered Independent with a solid Republican background.

To wrap up, let's assess the credibility of Mr. Dr. James E. Hansen:

Time and data-proven accuracy of his work
Support of world-class experts
Grandfather (C'mon, what's more trustworthy than a grampa?)...