While the use of renewable energy is booming, the boom started from a very low point. We're only now reaching the stage where renewable power is providing a substantial fraction of the energy used in some developed economies. Pushing things further and faster would require a lot of resources as, per unit of electricity produced, renewable power equipment takes more material than fossil fuel plants. Plus, at least initially, a lot of the manufacturing will be powered by fossil fuel plants.

How does all this balance out? An international team of researchers has looked at the material demands and pollution that would result from a push to get the globe to 40 percent renewables by the middle of the century. The analysis finds that despite the increased materials and energy demands, a push like this would result in a dramatic reduction in pollution. And for the most part, the material demands could be met, with the possible exception of copper.

The work involved what's called a life cycle analysis, which tracks the material and energy demands of items from the production of the raw materials through to obsolescence and recycling. Normally, these studies are done with static assumptions; take the life cycle of copper in 2011, for example, and use that figure for the entire analysis. In this case, however, being able to shift values over the course of the study period was essential. For example, as more renewable energy is produced, the pollution associated with producing new equipment will go down, as less of it will be provided by fossil fuels. Meanwhile, demand for raw materials could shift mining to sources that result in higher environmental damage.

The analysis updates these life cycle factors as things shift. For raw materials—the authors track aluminum, copper, nickel, iron and steel, metallurgical grade silicon, flat glass, zinc, and clinker—the analysis assumes that production increases in line with industry plans and expectations (something the authors term "optimistic-realistic"). For renewable power, they consider concentrating solar power, photovoltaics, wind power, and hydropower. Natural gas and coal plants, both with carbon capture and storage, were used for fossil fuel production. Nuclear and biofuels weren't considered, because there's a lot of controversy at the moment about what their life cycle actually looks like.

All this was fed into two models. Both assumed a doubling of electricity demand by mid-century; one assumed a mix of generating sources similar to the one we have today, while the second pushed things to 40 percent renewables. (The high-renewable option is the International Energy Agency's Blue Map energy scenario.)

The analysis found that, as expected, renewable energy production is resource intensive: "Wind and solar power plants tend to require more bulk materials (namely, iron, copper, aluminum, and cement) than coal- and gas-based electricity per unit of generation." In fact, obtaining raw materials accounts from anywhere between 20 and 50 percent of the cost of building renewable generating capacity; it's less than two percent for fossil fuels. But the environmental impact of these raw materials is dwarfed by the impact of fuel production and burning in fossil fuel plants.

Wind and gas generation have a small footprint in terms of land use—wind because the land itself can largely be used for other purposes. Solar, in contrast, has a huge footprint unless it's mounted on rooftops. Coal also uses a lot of land but only because of the mining of fuel.

Carbon capture and storage does cut down on carbon emissions, but it has a cost. Since it eats into the power output of the plant, more fossil fuels need to be burned to produce the same amount of electricity. As a result, just about all other forms of pollution go up as a result of its use. In fact, under the business-as-usual scenario, even the building of new, efficient coal plants isn't enough to keep various forms of pollution from doubling by the middle of the century. For the heavy renewables scenario, most pollution numbers are roughly 50 percent lower.

So by most measures, the high-renewable scenario seems to work as expected, even when the full life cycle of the generating hardware is considered. Should we dive in?

The analysis suggests we might not be able to. We'd need 90 percent of the global production of iron in 2011 just to build out renewables at the required pace (iron mostly going to wind and concentrated solar production). We'd need 150 percent of 2011's aluminum production and a staggering 200 percent of its copper (mostly used in photovoltaics). Could we do this? Probably, but it would be tough for copper. The researchers suggest that it might require tapping into lower-quality ores, which might have additional environmental impacts.

The overall picture painted by the analysis is somewhat mixed. Renewable energy is resource intensive, but obtaining those resources doesn't do much to undercut the improvements in pollution issues that the resulting power will bring. However, building sufficient renewables to have a large impact will greatly strain our existing supply of raw materials. In the end, the report highlights a factor that isn't even considered—the need for greater efficiency. If we increase efficiency, we wouldn't have to actually double our electricity supply by 2050.

PNAS, 2014. DOI: 10.1073/pnas.1312753111 (About DOIs).