Fossil Fuels: Oil
Oil is currently the source of almost all fuel for transport, producing 20% of world carbon emissions. While alternative non-polluting fuels have been found that can fuel some forms of transport, (such as biodiesel), these carry their own problems. But there are currently no suitable alternative fuels for sea and air transport. For ships, the only alternative so far used is nuclear powered engines, which carry other risks. There is no replacement fuel at all for use in aircraft jet engines.
Of all the fossil fuels, oil is likely to be the one that will become scarce first or become too expensive to use.