Whether it's man-made or not, I'm not sure there's much we can do at this point beyond adapting. Even if a country or continent went to zero emissions it wouldn't matter because of the rest of the world.
Your assumption is that if the US reduced their emissions as quickly as possible, it would likely not affect the emissions policies of other countries, but a lot of countries are refusing to cut emissions faster _because the US isn't doing so yet_.
The process of a unilateral, bilateral, or trilateral dramatic reduction would likely work as such:
1. The United States, China, or Europe, or two or all three would internally implement programs to drastically reduce emissions.
2. In order to maintain economic competitiveness, border adjustments (tariffs and subsidies) would be applied to imports and exports to and from countries without similar carbon emission reduction programs. If these tariffs and subsidies are sufficiently carefully applied, they would be legal under international trade agreements.[1]
3. Countries without similar carbon emission reduction programs would suffer economically by losing access to large export markets due to the border adjustments.
4. The countries would have their economic incentives shifted so that it becomes much more economically viable for them to apply similarly dramatic reductions themselves and regain access to the large markets.
5. As more countries apply similarly dramatic reductions, the total volume of border adjustments would decrease, reducing market distortions and increasing international trade efficiency, while maintaining each country's programs for dramatically decreasing carbon emissions.
Not just any country could do this, but the US could certainly start this process by itself. If the US started unilaterally, Europe would likely adopt similar measures and regain access to unadjusted imports and exports, which would create a very large bloc of high-consumption countries into which all countries (such as China and Australia) would want to regain cheap access.
[1] http://www.carbontax.org/nuts-and-bolts/going-global/ (Would the WTO approve of carbon tax-related border adjustments? If the primary consequence was to discriminate because of carbon emissions rather than because of being a foreign country, then yes. Possibly...)
Logically, there is a great deal that could still be done to mitigate global warming. It is more a question of if enough people are knowledgeable of the issues and are actually willing to change their behaviour and apply pressure on the rest of society to force change.
I think there is a huge difference between a +3.0C world in 2100, with stable cumulative emissions, and a +4.5C world in 2100, where the cumulative emissions are still rising. The former is probably very unpleasant (i.e. many people dead, particularly those unfortunate enough to be born poor or in the wrong country), the latter denotes some kind of worsening failure state for human civilisation.
It is unfortunate that climate change is a long-term, uncertain consequence, a collective side-effect of economic activity, overpopulation, etc. If enough of the painful consequences of our actions could be experienced in the short term, across society, we'd probably have a very different attitude and a sharp motivation to change.
> One blueprint for trouble, making collapse likely, is where there is a conflict of interest between the short-term interest of the decision-making elites and the long-term interest of the society as a whole, especially if the elites are able to insulate themselves from the consequences of their actions. Where what's good in the short run for the elite is bad for the society as a whole, there's a real risk of the elite doing things that would bring the society down in the long run. -- Jared Diamond
