Scientists always work with ranges of data (due to measuring errors, inaccuracies and imperfect models).
To avoid being the boy who cried wolf, scientific advisors to the government have consistently used the most optimistic end of the range when calculating necessary measures to combat climate change.
The politicians then tried to find a “compromise” between the cost of those required minimum measures and the cost of the other things they want to spend money on.
Finally, they further compromised in meetings with other countries who flat out refused to do anything unless they could carve out exceptions for themselves and their industries.
In the end, most countries agreed on a set of promises for what they’ll do, which is a lot less than even the most optimistic evaluation demanded.
Then they actually did less than half of that.
And then the biggest contributors to climate change left the meetings entirely.