Asklemmy

If your house is too warm and you're running 20 electric heaters but for some reason think it's something else causing the heat, it'd still pay to turn off some of those heaters to compensate for the external cause since you actually have control over the heaters.

If you really think that anything you say will have any impact on them then maybe it doesn't matter what the cause is as long as you can get them onboard with the solution. The two are inextricably linked so if they're really adamant about this you'll probably not be successful in this endeavour but it's worth a try if you have some faith in them. If they think it's all long term cyclical solar patterns or some other non anthropogenic cause it might be worth pointing out that it's still happening and causing consequences. The trouble will be not implying too heavily as I have in my heaters example, that the solution is obviously linked to the cause, even if it is. Otherwise they will not believe you have common ground.