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.
liberal reflections 
So I’m not sure that the first one is a Sorites thing as such, and as a result I think it suffers from a fallacy of composition: just because one part makes no difference does not automatically mean that a bunch of parts put together make no difference.
Also, it’s a bit much to say that total emissions are just the sum of all individuals’ emissions. Unless you have a very clever way of accounting for, y’know, airplanes and cruise ships and factories and whatnot, there seem to be plenty of emissions that don’t sensibly attach to individuals. Moreover, these emissions are not relevantly similar to your or my emissions; often, from what I understand, they differ quite significantly, even to the point of being orders of magnitude more plentiful.
All those things exists and emit their emissions because 1) individuals are willingly paying them to do so, or 2) individuals have elected governments which are willingly paying them to do so, thus individuals bring about those emissions indirectly.
However, since no individual knows precisely what their own emissions are (world is too complicated for one person to understand) you could argue it does not make sense to attribute indirect emissions beyond one or two steps from the consumer to the individual.
Also it is unclear to me how to attribute emissions resulting from collective decisions (eg. emissions from gov’t subsidized tar-sand development in Canada). Should they be attributed to all voters (most did not vote for the government in power)? all voters for the party in power (most voted on other issues)? All elected leaders (opposition parties have limited power to stop it)? government politicians only (many backbenchers may personally disagree but are whipped into line)? etc…