What do the bright lights all over in Tokyo, dazzling Las vegas visible from space, and a dimly glowing incandescent lamp in a dark village have in common? Energy. This intangible object is at the center of all human actvity, however primitive or advanced it may be. The source of the energy can be as varied as the activity itself : while a leisurely walk down a park draws its energy from our own bodies, a particle physics experiment in a particle colider gobbles up several megawatts of electrical energy supplied by perhaps a nulcear reactor. Whatever be the nature of the work, energy is central to our civilization, and following the trend since ancient times, our energy needs will only grow even though the usage becomes more efficient.
The most popular renewable source of energy is Solar Energy. While it really is admirable how this principle imitates life itself, in that life, too, draws its energy ultimately from the Sun, it may not be as rosy a picture if Solar energy were to become as popular as fossil fuels are today. The energy that should have reflected off the earth's surface will now be absorbed into the photoelectric cells and be converted into electricity. Finally, the energy that should have heated the earth, will now be converted into some form of mechanical energy. A portion will be converted into heat, and some will escape as light. Therefore, even though energy is indestructible, the ecological balance will not be maintained completely with this source.
The same story presents itself again when we consider different sources like tidal energy, wind energy etc. That said, I would like to emphasize that I am not a prophet of doom saying that mankind should give up all that it has learned and go back to its primitive existence. Any forward march of civilization will consume energy, and that energy will have to come at the expense of the delicate ecological balance of Nature. A somewhat more pleasant way of looking at things would be if we consider mankind's technological advancement as much a part of the evolution of the planet as the metamorphosis of a caterpillar into a butterfly. If this is done with minimum damage to the ecological balance, it would be an added bonus. Life is delicate, but it is also very resilient. Given time, it always finds a foothold, and regains a lost one.
Then where does that leave us with the problem of a new source of energy? Well, the solution is staring us right in the face. Solar energy, tidal energy, and their cousins are just the order of the day. The problem is the volume of consumption. If we would drive a concerted effort towards reducing the human population., not only would it solve the energy crisis effectively, it would reduce the requirement of other resources such as food and land. All beings consume energy in some form of other. On some level, a beaver's lair is not very different from the skyscrapers of modern cities. The difference lies in the number of lairs built by the beaver. We must learn from Nature at every step. We started our scientific journey by learning from Nature, but somewhere we started trying to master her. We still have a lot to learn from Her.