I just don't understand why anyone can suggest that, even if there there did turn out to be natural causes for some degree of global warming, that would in any way reduce the need to do whatever we can to avoid making things worse by adding our own contribution.

Just because there are forest fires caused by lightning strike is not a valid reason to be relaxed about stuff like dropping lighted cigarette ends in forest areas, or pouring petrol on the flames.
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So far as those hacked emails go - don't all musicians use the way "trick" in just the same way, not to refer to a way of deceiving people, but rather as a term for a smart or elegant technique we have picked up or invented? That's the same way scientists are liable to use the word.