To borrow a line from science fiction writer William Gibson, the future of green tech is already here, it's just not very evenly distributed.

Today is Earth Day, a good time to consider how the technology meant to preserve our environment and natural resources is progressing. If you consider individual green products, whether it's plug-in cars or home solar panel leasing, the impact on the giant scale of the energy industry is quite small. Hybrids, never mind plug-in hybrids, are less than 2 percent of total sales, and renewable energy is about 10 percent of electricity generation, with most of that from hydropower.

But viewed in aggregate, there's clearly momentum. What's still up for grabs is where the innovations will come from and how quickly they will be deployed at scale, a question with big economic, environmental, and national security implications.

The debate over the pace and scale of green technology played out in front of me a few weeks ago at the Yale Climate and Energy Institute's annual conference. I walked away thinking that, even though there are big hurdles to making the economy cleaner, we'll look back five years from now and be impressed with the amount of change.

In a keynote speech, David Lawrence, the executive vice president of Exploration and Commercial at Shell, gave a big-picture view of world energy, where he said fossil fuels will remain dominant for the next 40 to 50 years. Biofuels and wind are poised to play a more significant role, but the big winner in the future will be natural gas because it's abundant in the U.S. and burns cleaner than coal.

The next day, I moderated a panel on advanced biofuels to discuss the potential of genetically manipulating microorganisms to make fuels directly or improve existing processes. One of the panelists, Flagship Ventures CEO Noubar Afeyan, challenged the notion of a slow, multidecade transition espoused by Lawrence.

A veteran biotech venture capitalist and entrepreneur, Afeyan had seen the pharmaceutical companies 20 years ago say that they would lead the industry in pursuit of novel drugs because they had the means--both the R&D and the sales networks. The reality turned out to be very different, as a number of biotech start-ups entered the market and grew to become public companies.

There's no reason that the same kind of dynamic won't play out in energy, he argued. One example is Joule Unlimited, which projects it can make diesel fuel in bioreactors at competitive prices within a few years. Rather than count on the incumbents to lead on alternative energy, challengers with low-cost products will set the pace, he said.

This will take a while Is it realistic to expect that a lab full of scientists and eager start-up entrepreneurs will upend the hulking energy industry that's developed over decades Well, overnight success is unlikely, but with the right business models and technology, new technologies can make a dent in a slow and steady kind of way.

Certainly, many clean-tech companies have run into a wall trying to scale up their products, for purely business reasons, such as the high costs of building factories or the difficulty of selling to conservative utilities. But by picking their niches, some promising green-tech companies have made real progress, even if the field of venture investors may be narrowing.

A few examples: BrightSource Energy has started construction of a huge solar power plant in Southern California, making it one of the few solar companies started in the past decade to cross over into large-scale operations.

EnerNoc built a demand-response service by applying IT to the power grid, and has helped make efficiency a resource that grid operators increasingly rely on during peak times. Another company to go public is Gevo, which expects to be making by the end of next year specialty chemicals (an easier market to crack into than fuels) from sugar at the same cost as petroleum.

In the realm of scientific discoveries, dozens of companies and academics funded by the ARPA-E program are researching a dizzying array of approaches around cheap storage, biofuels, and repurposing carbon dioxide emissions. A high-profile one is Sun Catalytix, which is designing an "artificial leaf" to make hydrogen fuel from a solar cell soaked in a solution with its special catalyst. Its first market may be community energy systems in India.

In many cases, the successful green-tech companies appear to be going after a relatively narrow market niche. But that's how many technologies will likely first take hold, showing where they work best before more people are willing to try them and banks are willing to invest in them.

Reality check It's not just the energy business, either. Increasingly, corporations in all fields have "green business" efforts, which range from purchasing fleets of electric vehicles to making more eco-friendly packaging. Big companies are not only important customers but they also investing in, and sometimes acquiring, smaller green-tech outfits, bringing capital and distribution. Consumers, obviously, can live a more eco-friendly lifestyle and create demand for eco-friendly products as well.

Even with all the advances on green technology, one has to recognize that it's a long-term transition off of fossil fuels. Energy moves slowly, unlike the IT industry, which has been ruled by Moore's Law.

Battery technology, for example, can be improved, but energy storage and many other fields simply don't benefit from the exponential pace of change that the semiconductor industry has seen over the past 40 years. (Academic Vaclav Smil refers to this overly optimistic thinking as "Moore's Curse.")

Still, one can feel optimistic about green technologies, because so many people have been captured by this idea of making a cleaner economy. It's what's leading university students to start energy clubs and scientists and some businesses to take environmental sustainability seriously.

The other reason we've seen more activity in green technology is government policies. U.S. clean energy industries got a boost from the stimulus, and countries with aggressive clean-energy policies, such as China, Germany, and South Korea, are galloping ahead.

In the U.S., some people seem like they're automatically turned off by anything labeled "green," "clean," or "sustainable," including many politicians. But even while there's a debate over clean-energy policies, you'd think that a commitment to innovating around energy and preserving natural resources would be something many could agree on. That matters on Earth Day, because clean air and water are not something anyone can take for granted.