We know that carbon dioxide's transparency to radiation is dependent upon the wavelength of the radiation. That is what makes it a greenhouse gas. Like the glass of a greenhouse, carbon dioxide traps heat from the sun's radiation. Much of the radiation from the sun is in the visible spectrum, and in the ultraviolet. Radiation from warm Earthly sources lies more in the infra-red, which carbon dioxide does not so readily transmit. So the Earth gets warmer than a naked stone or iron sphere would. This is a good thing, but enough is enough.
Fossil carbon is not a renewable resource.
Not all of the oxygen in the atmosphere is being renewed, either!
A small minority of geophysicists reckon that some of the methane has a cosmic origin, like that in the giant gas planets.
But if it does, there is no corresponding quantity of free oxygen.
The Carboniferous era lasted from 354 to 290 million years ago. In other words, by general scientific and geological opinion, nearly all of the oxygen in the atmosphere and all of the fossil carbon on and within the Earth took 64 million years to be made by photosynthesis, flooding, and geologic processes.
Humankind have managed in a few centuries to exhaust a significant amount of this reserve. Since the oxidation of carbon creates carbon dioxide (and costs oxygen), it is implausible to deny that global warming is not in significant part human-induced.
You must remember that solar energy is the basis of all the ancient traditional energy technologies. So the trouble with most of the alternative energy sources, also known as sustainable and renewable energy, is that they're not new. Well, photovoltaic and solar powered heat engines are perhaps an exception, We'll come back to them.
Al Gore's website, which is correct about the perils of global warming, gives a figure for the rate at which the Sun supplies Earth with energy.
The oceans receive more of it than Earth's land mass, obviously.
It is large compared with the total energy consumption rate even of industrial humans. But he does not go into the difficulties of harnessing it.
Hurricanes are a good example — a hurricane is an immense heat engine, which transfers heat from the ocean surface and sends it to the upper atmosphere and into space. In the process, the thermal energy is quite efficiently turned into mechanical and electrical energy, called winds and thunderstorms, and has been known to wreak immense destruction. Heat energy was being released at the eye of Hurricane Andrew at 5000 times the power of the nuclear power plant over which it passed. It is a feat of immense meteorological skill even to predict, with a lead time of a few days, where this destruction will strike, to an accuracy of several counties. Harnessing this immense energy source is out of the question.
Nuclear power is widely ignored and even excoriated by most "renewable energy" enthusiasts but is in fact renewable and sustainable, and is new enough that it depends upon a phenomenon unknown to the great physicist Lord Kelvin. The technology to do it renewably, safely, and with negligible nuclear waste, has existed in the USA since before the catastrophe at Chernobyl.
Wave power is the energy given to the oceans by the winds over them. Unlike tidal power, its origin is solar.
Of wave power it is said by its proponents "The total potential off the coast of the United States is 252 million megawatt hours a year." That is about 28.75 Gigawatt-years, and would perhaps suffice to replace 29 or 30 1000-MW coal plants. But existing nuclear power, if doubled, would replace 90 to 100 such power plants, without any disturbance to sea birds, sea mammals, or fish.
Hydro-power, wind power, and biofuels were not new at the time of the Roman and even the Persian empires.
Ocean transportation by wind power was the basis of the British Empire when Britannia ruled the waves, up to the middle of the eighteenth century. But see the website wind energy the truth by a Dutch engineer.
Water wheels drove many of the most power-intensive industries, like milling.
Biofuels primarily were the food that powered horses, oxen, camels, elephants, peasants, serfs, and even slaves. Illumination came from the fat of domestic animals, from olive trees, wax from the labors of inordinate numbers of bees, and then there was whale oil. The last is perhaps the most horridly wasteful of all. Huge intelligent mammals were slaughtered for their fat. Even the meat was simply thrown back into the sea.
Forests are theoretically a renewable resource. The trouble is, that at the rate of human consumption of wood for heating or for transportation energy, a forest does not give you a sustainable resource. The natives even of Aku-Aku (Easter Island to Europeans) denuded their forests to the point of reducing the island's productivity catastrophically.