Per-capita is absolutely a useful metric when you're asking other countries to take on a burden. When we ask developing nations to reduce their emissions, they

Per-capita is absolutely a useful metric when you're asking other countries to take on a burden. When we ask developing nations to reduce their emissions, they rightly point out that this will harm their citizens' living standards. A very good response to this objection would be to prove that we can maintain US-quality living standards while emitting much less per-capita CO2 than China or India do today. Unfortunately (and despite some modest reductions) we're giving them exactly the opposite example, and they're making decisions accordingly.