Energy

Energy for millions of generations / billions of years could come from several sources.

The sun is expected to maintain roughly the same output for at least hundreds of million of years, probably several billion years. As long as the earth maintains its orbit and weather patterns, the sun would deposit many thousand times more energy on the earth's surface than is consumed by current civilizations. It could be harnessed directly or indirectly via biomass, wind or water.

Tidal forces caused by the moon are also expected to change little in strength over millions of years and could also serve as energy for long periods. The moon moves slowly away from the earth, its distance would be expected to increase by about 10% in a billion years due to tidal forces.

Geothermal energy from nuclear decays of radioactive elements in the earth is also expected to be only slowly reduced over the course of billions of years.

If it ever becomes feasible, energy from nuclear fusion might last for millions of years at current usage if lithium can be won from the sea or billions of years if fusion using only deuterium would become possible. Harnessing nuclear fusion for energy consumption with currently conceivable technologies seems considerably more expensive than collecting energy from the sun.

Energy from nuclear fission has no role for civilizations lasting for millions of generations, as for energy production viable fuels would only last for a few tens of generations even with the best technologies. Such fuels would better be saved for applications where their use becomes indispensable. E.g., travel to other solar systems, if possible, must be expected to require very high energy density fuel sources. Using fission fuels up now because they are relatively somewhat cheaper than lasting sources seems irresponsible in the light of reduced options of future generations.

Fossil fuel sources such as coal, oil and gas and their variants seem limited in quantity and their combustion products, mainly CO2, seem to significantly alter the atmosphere, changing living conditions. If civilization continues their use will be a very brief event in history. Continued consumption despite of the knowledge of the limits and effects will be seen as irresponsible and must be stopped both to maintain viable living conditions in the short term and options for future generations in the long term.