If the exploitation of fossil fuels becomes economically prohibitive in 50 to 100 years, our grandchildren and our 4th generation descendants will have to find new ways to power their cities and industries. In addition, they will have to try to reverse the effects of global warming from the combustion products of fossil fuels that we are using today.
It is unlikely that industrialized nations will give up the use of fossil fuels any time soon because they are so convenient and so cheap. In spite of the Kyoto Protocol and similar agreements to reduce greenhouse gas emissions, the use of fossil fuels will only stop when these fuels cannot be economically extracted from the Earth. BP’s “Statistical Review of World Energy” published in mid 2014 says that the world has in reserves 892 billion tons of coal, 186 trillion cubic meters of natural gas, and 1688 billion barrels of crude oil. At current rates of usage, the oil and gas will be exhausted in 55 years, and the coal will last 113 years.
Hydroelectric power will not be a viable option in the future because global warming will reduce the glacier ice in the mountains which is the source of the water in the rivers. Similarly, the experiences with Chernobyl and Fukushima show us that we cannot build nuclear power plants that guarantee the safety of our environment. One accident can make the surface of the Earth uninhabitable for hundreds or thousands of years, and no satisfactory solution has been found for the problem of disposing of nuclear waste that remains radioactive for millennia. The zone contaminated by the Fukushima disaster is roughly 310 sq miles (800 sq km). Radioactive cesium-137 has a half-life of 30 years, and since it takes about 10 half-lives for any radionuclide to decrease to a tolerable radiation level, the Fukushima exclusion zone will be closed to human habitation and farming for at least 300 years. The Fukushima nuclear reactors are still leaking radioactive material, and if one of the heavily damaged reactors should collapse, additional radiation would be released and much of Japan could become uninhabitable.
This only leaves geothermal, tidal, wind and solar energy as the safest and most reliable power sources for the future. Geothermal energy is used extensively in Iceland, but places which are not in volcanic areas would have to dig very deep to tap the heat in the crust of the Earth. The use of energy from tides and marine currents may only be practical in coastal areas. Similarly, the use of wind energy may only be feasible in areas with constant winds.
Solar energy appears to be the most abundant and widely available clean energy source, and it can be harvested through photovoltaic cells and through biofuels. Biofuels require irrigation, and that is a problem when our supply of fresh water is limited. In a world where there is much hunger, it is a perversion to use corn or cane sugar to fuel our machinery. Farmland should not be used for fuel production because the population of the Earth will only increase and therefore more land will be required for food production. Only cellulose from grasses, or the inedible parts of plants should be used for biofuels. Perhaps algal aquaculture on the surface of the ocean or harvesting the algal blooms that pollute the oceans might be a source of biofuels. If photovoltaic cells could be produced by processes that do not cause pollution, the roofs of our buildings could be covered with photovoltaic cells that could help us meet many of our daily energy needs. Germany is already making substantial progress in the use of solar energy in houses and factories.
 Steven Starr, Costs and Consequences of the Fukushima Daiichi Disaster.