The Global Carbon Web
Countries use fossil fuels in many different ways. Most simply, they extract them domestically and use them domestically. Since coal fields and oil deposits are not equally distributed, there is also a major international trade in fossil fuels. Thirdly, some countries use fossil fuels (either their own or imported from elsewhere) to manufacture products that are then exported (think about the mountains of plastic items shipped to the United States from China).
In a heroic effort at number crunching in a recent Proceedings of the National Academy of Sciences, Steven J. Davis, Glen P. Peters, and Ken Caldeira have attempted to map these various streams, the result of which you see above. The map shows the 37% of world carbon emissions that come from fossil fuels that are traded internationally, and the further 23% that are "embedded" in manufactured products. It also makes very clear who's responsible: Most of those arrows are pointed at the United States and Europe. This suggests, the authors argue, that the easiest way to put a price on carbon is to do so at the point of extraction: the wellhead in Saudi Arabia, or coal mine in Wyoming.
The geographical concentration of carbon-based fuels and relatively small number of parties involved in extracting and refining those fuels suggest that regulation at the wellhead, mine mouth, or refinery might minimize transaction costs as well as opportunities for leakage.
Now we've just got to get it past the U.S. Senate. . .