The burning of the remaining quantities of fossil fuels creates a shortage of plastics, fertilizers and other petroleum-derived substances. Eventually, the resulting global warming raises ocean levels, inundating all coastal cities. Concern for the environment vanishes as everyone does whatever it takes to maintain their accustomed lifestyle. The poor nations become poorer. The rich nations jealously guard their rapidly dwindling wealth.
International tensions rise.
Some nations, frustrated with the inequities and convinced that they have nothing left to lose, try to use nuclear blackmail to get their way. There are so many nuclear devices left over from the excesses of the Cold War that it is not difficult for anyone to get their hands on them. Some are home-made from the tons of fissionable material which resulted from the expansion of the nuclear industry when fossil fuels ran out. Entire capitals vanish instantly in blazes of atomic fire. Some scientific experts point to the moon and the asteroids as possible sources of the materials now so desperately needed on Earth, and wonder about harnessing the sun's energy in space, but to no avail.