I’m not a logician or philosopher of language, but on the off chance anyone reading can smell bullshit in this argument I’ve just written, let me know:
The argument that individual emissions make no difference faces a form of the Sorites paradox. If it is true that my individual emissions make no difference, it is also true of your individual emissions and anyone else’s that they also make no difference, since these are relevantly similar. However, if nobody’s emissions make any difference, then total emissions make no difference, because total emissions are just everyone’s individual emissions. But total emissions do make a difference, so an individual’s must too.
Update: This also feels possibly problematic, but I’ve been staring at this thing for too long to keep sight of its flaws by now:
If it is true that we have no duty to reduce or offset our individual emissions because they make no difference, it is hard to see how Sinnott-Armstrong and Sandberg can consistently claim that the state should nevertheless pass laws that force us to reduce our emissions, and that we should obey these laws. This would require the state to forcibly prevent me from doing something that is harmless and that I have no duty to stop doing. But on an ordinary understanding of state legitimacy, states should only use force to prevent harm, and to make us do what we ought to do. On Sinnott-Armstrong’s and Sandberg’s view, neither of these conditions hold, so state coercion would be illegitimate. But state coercion wouldn’t be illegitimate, so their view must be wrong. Our individual emissions do cause harm and we ought to prevent them from doing so.
Update 2: I feel much prouder of this paragraph:
If we campaigned for the state to make it illegal for us to emit greenhouse gases at the level we currently do, whilst continuing to eat meat, fly, take long showers and so on, then our campaign will never be persuasive. If Sinnott-Armstrong is correct, it may be philosophically consistent to act in this way. But since few people are perfectly rational, the appearance of hypocrisy will impede an effective campaign. If the British branch of Greenpeace flew to Australia for their conference and discussed their opposition to current levels of greenhouse gas emissions whilst eating steaks, then this would be laughable. The ordinary phrase claiming that we should ‘lead by example’ is true. Crucial to civic campaigning is effectively demonstrating individual commitments.