Each of these reputable scientific institutions is staking its reputation on the claim that human carbon pollution is dangerously altering our climate. Credibility is their most valuable asset to risk.
The list of reputable scientific institutions staking their reputation on the denial of that claim can be counted on the fingers of one head.
Arguments from authority are a legitimate secondary form of argument, and are quite relevant for non-specialists such as myself (and, I assume, most of my readers).
This question is discussed in more detail by Kate at ClimateSight: "Is there a consensus in climate science?"