One of the great dangers environmentalism faces in reaching for technological solutions to existing pollution problems is that they've got it wrong. Let's face it, environmentalists tend not to do the math. When environmentalists overreach or exaggerate, they damage the credibility of the green movement as a whole.
In the current landscape, megatonnes of Carbon Dioxide have become the conscience of environmentalism, with leaders like Tony Blair championing the danger of climate change while the US administration hires Exxon to redact its scientific reports on the subject. One of the possible solutions to the consumption of carbon fuels put forth by environmentalists is the development of renewable energy: solar (specifically photovoltaic), wind, biomass, and reservoir hydro.
Of these technologies, the only proven one is hydro-electric dams... Wind has made strides in installed capacity, but its volatile power output make it a liability to the grid. It is cost effective when the wind is blowing, but the fickleness of mother nature puts an excessive stress on gas-fired load-following plants and voltage regulation. Currently the fate of wind hangs in the balance. With increased decentralization, better coupling to strong load-following power providers like hydro, and the introduction of utility-scale electrify storage like flow batteries, it will be able to provide a significant reduction in CO2 emissions.
That leaves photovoltaic and biomass as underdeployed, but potentially promising solutions. Unfortunately, I believe both of these technologies will be failures over the next 25 years.
Photovoltaic cells are energy intensive to manufacture. Optimistically, the energy return ratio of PV cells is about 20:1. This means that for every Joule used in the manufacturing process, the solar cell will only return twenty times that over the lifetime of the cell. Of course, most cells aren't installed in an ideal clearness index climate, oriented to the sun. Installations on home roofs are usually installed at the wrong angle, and may not be south facing. A ratio of 10:1 is more common in the literature. Compared to wind at 40:1 and hydro at 100:1 , PV is not economically feasible.
Furthermore, all current PV production is done using silicon recycled from the microchip industry. This reduces the material cost of photovoltaic manufacturing. The current prices of solar electric cells, expensive as they are, are not sustainable if demand increases substantially.
While decentralization has its advantages, I think solar has picked the wrong method for gathering power. Photovoltaic, realistically, should stay off the grid for the time being. Centralized concentrated solar-thermal collectors seem to have much greater potential for providing peaking power to the grid. However, it should be noted, concentrated solar performs poorly in the diffuse lighting conditions of overcast skies. Concentrated solar, like wind, often requires the installation of expensive transmission lines to bring power from the boondocks to urban areas.
Biomass is a net zero carbon emitter, since plants consume CO2 from the atmosphere and then we later burn them to liberate energy. Compared to other renewables, biomass is storable and portable. As such, it can be used for transportation.
Unfortunately, photosynthesis is an inefficient solar cell. The amount of arable land necessary requires a lot of energy to fertilize, plant, irrigate, and harvest. The energy return of ethanol is terrible, typically around 1.1:1 if it is even positive. Turning over huge tracts of land for biodiesel and ethanol production will have to come out of land being used for food production. While the world is currently feeding itself, we can't do both food and biomass production at the same time.
PV subsidies in Germany and California, along with ethanol and biodiesel subsidies in America and Europe are essentially cases of government picking the winners. Given the historical success rate of government investment in technologies.