In the long term, depletion of easily recoverable petroleum resources will make presently uneconomic automotive energy sources competitive. Natural gas, which can be used directly or converted to methanol, is projected to outlast petroleum. The even greater energy storehouse of solid feedstocks like coal can be transformed into liquid automotive fuel. Depletion of fossil fuels is not foreseen before the latter half of the 21st century. Potentially significant non-fossil replacements include both biofuels, and nuclear and solar electricity. Electricity can either power battery-electric vehicles or be used to produce electrolytic hydrogen for combustion engines or fuel cells. Combustion engines, which are being improved continually, remain technically viable into the foreseeable future. Battery-electric vehicles, which are being developed aggressively because of their freedom from local emissions, are not necessarily pollution-free on regional and global scales. Some of their current shortcomings can be overcome by electric/combustion-engine hybrids. Fuel cells for general-purpose automotive use are still quite immature and await further development to establish their role in the spectrum of automotive power. The timing for these many options to impact the automotive field depends on evolving technology and economics, and may be further influenced by government policy.
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