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I think it's because some Nations with higher per-capita emmissions (like Qatar) are teensy and can't do much about their heavy outputs (desalination, aircon take a lot of energy). The US is huge, is visible on the top per-capita list and is also the 2nd largest emitter overall. Given that the US is a first world economy, is extremely consumerist and has a love affair with gas-guzzling cars, it's likely that by simply making better choices, the US could likely cut more % per-capita than Qatar. And given their size, that would have impact on a global scale.