Ari didn't get any reply from most of the 30 societies (he said nine of the 30 replied to him confirming their support for the letter), which prompted him to falsely allege in his headline that "Ooops! Not all 31 scientific societies actually signed the AAAS ‘consensus’ letter". In other words, his evidence is absent. Non-existent. He just made that up.
Why wouldn't they support the letter? It was clear and simple and non-controversial. Here's an excerpt:
Observations throughout the world make it clear that climate change is occurring, and rigorous scientific research concludes that the greenhouse gases emitted by human activities are the primary driver. This conclusion is based on multiple independent lines of evidence and the vast body of peer-reviewed science.