Like many in the renewable energy industry, I'm an optimist. I also believe that by setting big goals – things like a 20% wind penetration by 2030 or even Al Gore's 100% renewable electricity by 2020 target – we help drive the public discourse about our energy future toward renewables.

Surely that's a good thing. But are we being disingenuous when we set such goals?

There are endless theoretical studies out there showing how we can digitize our entire grid, put solar on every rooftop or switch our fuel system to natural gas and biofuels. Pick your favorite technology and you'll find a study supporting its rapid mass-adoption.

While these studies “prove” such a transition can be done in a short period of time, they often neglect the historical context of previous transitions.

Switching from one energy source to another takes a long time. And many energy experts would say that this latest transition from fossil resources to renewables is no different. Although it might feel like a new, urgent time, a broader historical look shows that it's very similar to all other previous energy transitions.

“It's taken between 50 and 70 years for a resource to reach a large penetration. When you look at the money, the infrastructure, the regulation, the technologies, it takes many decades for any fuel source to make a large impact,” says Vaclav Smil, a distinguished professor at the University of Manitoba who writes extensively on this exact topic. ::continue::

Smil's expertise spans across many different areas, including energy, environmental issues, population growth, and economics. He's written 19 books on the history of energy, food, politics and global catastrophes.

His most recent book, Energy Transitions, examines the characteristics of major energy shifts in modern times. While each shift took place within a different socio-economic context, they all had one thing in common – they were very slow.

After reading much of his work, I spoke with him about whether this transition is different, especially given new macro-drivers like climate change, globalization and the internet. His answer was sobering.

“There's no doubt that it's different today,” he says. “But the often-neglected factor is how much energy we need to substitute compared with previous transitions. It's far greater than at any other time.”

In the late 1800's, when the combustion of coal overtook biomass, total global energy use was about 500 million tons of oil equivalent. In 2010, it would take nearly 4.5 billion tons of oil equivalent to replace 50% of global fossil energy consumption with renewables. Renewables also typically have a much lower EROI than coal and oil (although the gap is closing). That means it will take a lot more power plants and fuel production facilities to get the same amount of energy.

A more distributed energy infrastructure based on renewables is not only doable, it's desirable. Even the traditional energy players support these new sources of energy. But many of those same oil, coal and gas companies have made infrastructure investments that will take decades to pay off. It would be silly to think they'd abandon those assets outright.

These factors make the transition to renewables long and meticulous, not immediate and revolutionary, says Smil. After all, non-hydro renewables still only make up around 2.5% of global electricity consumption. It took 50 years for natural gas to reach 10% of the global energy mix from the time of early commercial production, and it took the nuclear industry 27 years to reach that level.

“We should absolutely be working toward integrating renewables. The more the better. But let's be realistic, this is a multi-decade approach and it can not be accomplished in one or even two decades,” says Smil.

Depending on how you look at it, this argument could either be refreshing or depressing. I personally find it to be both.

The need for a mass penetration of cleaner, renewable sources of energy is clear. But while big goals like “100% renewable by 2020” sound good, they don't really give the enormity of the task much justice.

With that said, not many things so large just “happen.” It will take an incredible about of drive to bring renewables to scale. And setting the bar high – even sometimes unrealistically high – is an important part of creating the needed momentum.