Plenty of people who would have reason to know think the IPCC's reports are overly conservative because they are trying to spur action / avoid defeatism, and because they have to reflect a baseline consensus of many researchers.

https://skepticalscience.com/ipcc-scientific-consensus.htm

https://www.washingtonpost.com/news/wonk/wp/2014/10/30/clima...

https://www.vice.com/en_us/article/43e8yp/the-uns-devastatin...