&gt;We have an objective measure of climate change that virtually everyone agrees on (temperature rise).


Sorry but we don't. We have various opinions on what that rise may be, it's cause, etc.


The position you have taken is reminds me of the is ought fallacy - If an objective morality exists, it *ought* to be agreed upon by people (who are by definition subjective).


&gt;I’ll be specific: Is murder morally acceptable?

Its pretty much universally considered a crime and immoral, probably to a greater degree than the consensus on climate change. Yet there is an objective truth to climate change nonetheless:

"Murder is the unlawful killing of another human without justification or valid excuse... Wikipedia"


&gt;Now maybe it does exist, and large swathes of humanity devoutly believe in a religion that it turns out is objectively morally wrong. 

Exactly. You can't say it doesn't exist, simply because people disagree over aspects of it.