Can renewable energy make a dent in fossil fuels?

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4.2 billion.

That's how many rooftops you'd have to cover with solar panels to displace a cubic mile of oil (CMO), a measure of energy consumption, according to Ripudaman Malhotra, who oversees research on fossil fuels at SRI International. The electricity captured in those hypothetical solar panels in a year (2.1 kilowatts each) would roughly equal the energy in a CMO. The world consumes a little over 1 CMO of oil a year right now and about 3 CMOs of energy from all sources.

Put another way, we'd need to equip 250,000 roofs a day with solar panels for the next 50 years to have enough photovoltaic infrastructure to provide the world with a CMO's worth of solar-generated electricity for a year. We're nowhere close to that pace.

But don't blame the solar industry. You'd also have to erect a 900-megawatt nuclear power plant every week for 50 years to get enough plants (2,500) to produce the same energy in a year to equal a CMO. Wind power? You need 3 million for a CMO, or 1,200 a week planted in the ground over the next 50 years. Demand for power also continues to escalate with economic development in the emerging world.

"In 30 years we will need six CMOs, so where are we going to get that?" Malhotra said. "I'm trying to communicate the scale of the problem."

The CMO is a figure you might begin to hear more as utilities and governments map out their renewable energy strategies. SRI's Hew Crane came up with the term as a way to normalize all the different measurements (kilowatt-hours, BTUs, million barrels of oil equivalents, cubic feet of gas, etc.) in the energy business.

It's also a big enough measure to suit the global energy market without saddling everyone with a train of zeros. Many of these stats and a far lengthier discussion of the issue will be found in a book coming from Oxford University Press by Crane, Malhotra, and Ed Kinderman called A Cubic Mile of Oil.

And judging by some of the stats Malhotra gave me, the book will alarm policy makers, environmentalists, and pretty much anyone else interested in weaning ourselves from fossil fuels. (To be honest, one of the truly great things about this job is getting the bejeezus scared out of you on a regular basis. One day, China is plunging into a water crisis. The next day, doctors report seeing malaria spreading to new regions because of climate change.)

One of the more compelling aspects of Malhotra's research is how it highlights the amount of energy, particularly in the form of fossil fuels, that the world consumes. Oil provided about one-third of worldwide energy (1.06 CMO) in 2006 followed by coal (0.81) and natural gas (0.61). Together, the three fossil fuels accounted for 2.48 CMOs of the 3 CMOs consumed that year.

The figures drop quickly after that. The fourth largest source of energy is biomass, mostly in the form of burning wood. Biomass, however, only provide 0.19 CMOs, while hydroelectric and nuclear provided, respectively, 0.17 and .015 CMOs.

Wind and solar accounted for less than 0.005 CMOs.

The minuscule size of renewables, unfortunately, also means progress will come slowly. Some more comparisons: A large hydroelectric dam can generate about 18 gigawatts of power a year. To get an annual CMO from new hydroelectric dams, you'd need to build the equivalent of 200 Three Gorges Dams. There aren't that many available rivers in the world left to dam up. Solar thermal? 7,700 plants, or 150 a year for 50 years, required for an annual CMO. One plant went up last year, and it was the first in over 15 years. In his calculations, Malhotra takes into account the fact that solar, wind, hydroelectric, and even nuclear plants don't operate at optimal conditions 24-7; in other words, he has baked in real-world assumptions.

If consumers worldwide could replace 1 billion incandescent bulbs with compact fluorescents, it would save only 0.01 CMOs in a year.

"What is truly humbling is that we aren't going to make any impact on CO2 emission levels for the next 20 to 30 years," Malhotra said. Much of the growth for energy demand will come from emerging markets. Still, North Americans will continue to consume far more energy per person than people in China and India, according to SRI's figures.

Ultimately, the world will likely have to continue to burn fossil fuels and buy time with nuclear power and carbon capture technology, particularly capture technology that can pre-treat and clean fossil fuels before they get burned. GreatPoint Energy, GreenFuel Technologies, and others are looking at capture technology, but the whole field is in the embryonic state.

If there's a bright spot here, it's that the world has a lot of fossil fuel, he claimed, so we won't be plunged into darkness yet. Oil reserves come to around 46 CMOs, while natural gas reserves total 42 CMOs. There are 121 CMOs of coal out there. These numbers all go up when difficult-to-extract energy such as tar sands are added.

"It's been 30 years of (oil) reserves for the last 50 years," he joked. "It's like your pantry. Do you look at it and say 'Oh, no. I'm going to run out of flour in two weeks'? You go out and buy more."

Correction: This story misstated the title of the upcoming book from Oxford University Press. It is called A Cubic Mile of Oil.