When and where did green technology begin? It depends on your frame of reference. Some point to the invention of silicon PV cells at Bell Labs in 1954. Others note that car makers produced hybrids and electric cars in the early part of the 20th century. But you can go even further back: Egyptian architecture took advantage of passive cooling, a technique now making a comeback in modern design. Roman emperors had snow hauled to the palace in Rome, a first-century precursor of thermal mass cooling and storage, a concept now touted by Calmac and Ice Energy.

For the 40th anniversary of Earth Day this week, we'd like to salute 40 pioneers who helped crack thorny scientific problems, devise new business models, or come up with policies that paved the way for the world to adopt renewable energy and/or use our planetary resources more wisely. They are listed by category and approximately in chronological order.

SOLAR

1. Archimedes. In the third century B.C., he proposed setting ships on fire by transferring solar heat with bronze shields. The archaeological evidence is scant that the Athenians actually adopted this method, but modern-day tests show it could have worked. Hydrokinetic power owes him a debt, too. He did his best thinking in the bathtub.

2. Edmond Becquerel. Back in the middle of the 19th century, France ruled greentech. In 1839, Becquerel discovered the photovoltaic cell while experimenting with an electrolyte cell. In that instant, the solar module industry was born.

3. August Mouchet and Robert Stirling. In the early decades of the 1800s, Stirling invented the engine that bears his name. In 1860, Mouchet, a mathematician, proposed a solar-powered steam engine, which becomes a forerunner of parabolic solar thermal systems. The two chief schools for solar thermal technology emerge here. (Hat tip to Clarence Kemp for devising the first solar thermal water heater in 1891 in Baltimore.)

4. Calvin Fuller, Daryl Chapin and Gerald Pearson. The trio created the first silicon photovoltaic cell at Bell Labs in 1954. It was only 4 percent efficient, but Bell raised the figure to 11 percent soon after.  Today, the maximum efficiency for silicon PV is around 29 percent with a realistic limit at 25 percent. SunPower will soon be at 23 percent.

5. Arnold Goldman. Back in the 1980s and 1990s, the first solar thermal plants began to go online in California; one of the biggest producers was Goldman's company Luz. However, when California decided to let the property tax exemptions for solar thermal plants expire, Luz went under. But Goldman kept in contact with his engineers and researchers. And when conditions improved, Goldman helped found Luz II, otherwise known as BrightSource Energy. Solar thermal from a wide variety of vendors will account for gigawatts of power in North Africa and North America in a few years. Part hard-nosed businessman, part social philosopher, Goldman embodies the "do good, but profitably" attitude that characterizes the reborn green industry.

6. Harold McMaster. If you are outside of the solar industry, you're probably scratching your head. If you are in solar, you might be scratching your head, too. McMaster has long been a master of quiet understatement. An expert in tempered glass, McMaster founded First Solar in 1986. The company struggled for years until it gravitated toward cadmium telluride, a material that other solar manufacturers eschewed. VCs didn't invest in First Solar: members of the Walton family supported it.

First Solar came out with its first cad-tel panels in 2002, zoomed toward its IPO in 2005, and became the largest solar panel maker in the world a few years later.

7. Zhengrong Shi. After picking up a PhD at the University of South Wales in Australia, Shi founded Suntech Power Holdings in September 2001. Based in Wuxi, Suntech was once just a blip on the radar, but the company grew fast. It went public in 2005 and now jostles with First Solar for the top spot in the solar market. Marking China's entry into solar, Suntech also wants to take on SunPower in high efficiency panels.

8. Hermann Scheer. The German parliamentarian helped design Germany's renewable program. Without him, solar may never have exploded.

TRANSPORTATION AND OIL

9. Rudolf Diesel. Diesel came up with an engine design that remains far more efficient than the gas-burners that populate American roads. Electric cars are coming and hybrids are gaining in popularity, but they might only constitute 25 percent of cars on the road by 2020. Volkswagen, Audi and others continue to put research funds into diesel and diesel-like technology for gas cars. Trains and big trucks will continue to rely on diesel, as well.

Diesel also proved that the fuel used to power his namesake engines needn't be dirty -- he ran his first engines with peanut oil.

10. Henry Ford. While he's known primarily for master-minding mass production and promoting the combustion engine, Ford experimented with electric cars in the 1920s. During World War II, he looked into employing soy materials in cars.

"I see the time coming when a farmer will not only raise raw materials for industry, but will do the initial processing on his farm. He will stand on both his feet -- one foot on soil for his livelihood; the other in industry for the cash he needs," Ford said.

These ideas didn't pan out, but Ford historically has become more willing than many other major auto makers to experiment with alternatives. The company will start to come out with a series of all-electrics and plug-ins later this year and also plans to expand its use of its energy-efficient EcoBoost engines.

11. M. King Hubbert. The patron saint of research analysts. They scoffed in 1949 when geophysicist Hubbert said oil production would peak. Then it did, right on time, in Texas' Permian Basin in the 1970s. The Saudi oil embargo followed. Hubbert's body of work includes several of the most important scientific papers addressing energy to be published in the last century. It also marked the point when the possibility of declining supplies had to be considered.

12. Abdullah Tariki. The charismatic co-founder of OPEC remains one of the most important anti-heroes in green. Without his handiwork, the Saudi oil embargo could not have occurred. And without that, countries like Denmark and Japan would not have decided to set efficiency standards and encourage wind, biomass and solar manufacturing. In a way, OPEC was to green what the Black Death was to the Renaissance: a scary incident that forced contemporaries to rethink their priorities. Interestingly, Abu Dhabi, the capital of the OPEC charter member United Arab Emirates, created the Masdar Foundation to build a renewable industry in the Middle East. 

13. Dave Hermance. Hermance didn't invent the hybrid -- Ferdinand Porsche crafted the first one in 1902. As a Toyota employee, he didn't invent the Prius, either. However, he paved the way for the car's ultimate popularity in the U.S. The then-surprising sales of the Prius reawakened the dream of electronic cars. Tesla Motors, the GM Volt, and the Nissan Leaf all followed in its wake. Hermance was also a frank and forthcoming industry spokesman who was more than willing to explain why plug-ins and all-electrics a few years ago didn't make sense yet. He died in a plane crash in 2006.

14. Aquatic Species Program. Kicked off by President Jimmy Carter in 1978, ASP was the first program to delve into the question of whether algae, that very oily, highly prolific, and not particularly tasty organism, could be converted into fuel. Some of the problems algae startups experience today -- controlling growth, increasing yield, eliminating water -- ASP encountered as well, but the program helped establish algae as the most promising feedstock for biofuels.

GRID

15. Thomas Edison. Edison invented the modern electric industry. General Electric, light bulbs, power plants, even the concept of a grid all came from him. Interestingly, one of Edison's favorite ideas, direct current, fell out of favor early on.  The rise of solar panels and batteries and the need for greater efficiency, however, is prompting Panasonic, Sharp and others to look at ways to incorporate DC into homes, data centers and office buildings.

16. Nikola Tesla. The anti-Edison. Besides differing on AC/DC, Tesla also championed electrode-less lights, a concept now making a comeback, as well as wireless charging.

17. M. Stanley Whittingham. A chemistry professor at the State University of New York Binghamton, Whittingham led a team at Exxon that resulted in the first lithium ion battery. Whittingham's titanium sulfide battery was not a hit -- Sony actually had the first commercially successful lithium ion battery when it came out with its lithium cobalt model in the early 1990s. Still, it opened the door to batteries that greatly advanced storage for portable devices, and soon cars and the grid.

18. Ken Lay. The personification of the law of unintended consequences. Enron under Lay (and later under chief executive stooge Jeff Skilling) engaged in nefarious trading schemes that brought California's grid to its knees in early 2001. The high prices and blackouts prompted the Golden State to move toward renewable energy.

Subsequently, 2001 became a pivotal year for green startups. Suntech Power Holdings, A123 Systems, EnerNoc, CPower, Konarka, Greenfuel Technologies, Akeena Solar and Bloom Energy were all founded that year and SunPower came back from the brink of death after an injection of cash from T.J. Rodgers. In the same year, General Electric approached the collapsed Enron about buying its wind division. GE bought it, got into wind, and now leads the market.

19. Geoffrey Ballard. A visionary and in part a tragic figure, too. Ballard founded Ballard Fuel Cells amid great fanfare in the 1990s. Profits have eluded the company, just as profits have eluded the many companies that have been spawned by Ballard alumni, but the dream of fuel cells remains alive. Bloom Energy, Panasonic, and Ceres Power may ultimately transform it into a full-fledged industry. You can put Stan Ovshinsky, the ovonics promoter, in the same category as many other inventors who could sketch a great future but not quite reap the rewards of bringing the vision to fruition.

20. David Brewster and Tim Healy. Analysts continue to debate whether the term "smart grid" was coined by Andres Carvallo or Massoud Amin. Actually, the concept of the smart grid existed before the term itself was coined. Back in 2001, Brewster and Healy formed EnerNoc to provide demand response services to utilities to curb peak power, and demand response is really the ultimate goal of the smart grid. EnerNoc didn't invent the category -- competitor Comverge is actually older -- but it is the largest company in the space, with over 3 gigawatts of power under its control, and has perhaps done the most to popularize the smart grid concept. (Like many other early energy startups, EnerNoc had a lot of difficulty getting VCs to listen to them at first.)

We'd also like to give a hat tip here to Echelon. Power line networking was invented with the idea of linking up TVs and PCs, but Italian utility Enel exploited it for one of the earliest and still largest smart grids in the world -- 30 million meters and growing. Apple is looking at power line models for its home energy system. No coincidence there: Mike Markkula -- an early investor, employee number three, and the former chairman of Apple -- founded Echelon.

ELECTRONICS

21.  Shuji Nakamura. Nakamura accomplished something in the early '90s many considered impossible at the time: a white light LED. The invention transformed Nichia from a small Japanese manufacturer to a global industrialist. Nakamura also later became a folk hero among salarymen in Japan after suing Nichia for a fair share of the financial rewards.

Nakamura now teaches at the University of California Santa Barbara and is involved in two start-ups called Kaai and Soraa.

22. Dave Ditzel. A former Sun Microsystems chip designer, Ditzel founded Transmeta in the '90s with the idea of producing low-powered, Intel-compatible processors. Ditzel's theory was that the increasing power budgets of modern chips would sap batteries in notebooks and cause servers to melt.

Transmeta crashed miserably. Intel, Advanced Micro Devices and others, however, picked up the low-power mantra from Transmeta. Along the way, electricity prices climbed and the industry shifted to emphasizing utility budgets instead of meltdowns. Energy efficiency is now the top priority in the IT field. Final irony: Ditzel now works for his old nemesis Intel.

23. Fujio Masuoka. While working at Toshiba in the '80s, Masuoka came up with the first chips that could retain data even after their host computer got turned off. Even today, chip makers consider flash something of a miracle. (The secret: the memory bits on flash chips are encased in silicon dioxide, or glass. Once electrons get shoved in, they stay there for years). Flash effectively has made cell phones, MP3 players and other devices functional by reducing weight and battery life. Flash will next colonize the data center through companies like Sandforce and drastically reduce the power required for electronic commerce.

24. Mendel Rosenblum. The father of virtualization. Like Ditzel, Rosenblum wasn't initially primarily concerned with power savings. He wanted to figure out a way to get more work out of each server. A founder of VMware, he still teaches at Stanford, the university near Foothill Junior College. Virtualization is now a standard for green data centers. Runner-up: Marc Beniof, who pioneered the virtualization-dependent software-as-a-service concept.

ALTERNATIVE ENERGY

25. Lord Kelvin. In the 1830s, Lord Kelvin began to record ground temperatures. He discovered that a few feet below the surface, they remain relatively constant throughout the year. Then, in the 1940s, Robert Webber tinkered with the concept and came up with the first geothermal heat pumps.

Throughout most of the U.S., the temperature of the ground about five feet below the surface remains roughly constant throughout the year:  45 degrees to 50 degrees Fahrenheit in the northern parts of the country and 50 degrees to 70 degrees in the south. Pipes that bring that air inside during the summer provide cooling (as well as heating in the winter). More ground cooling could help solve the heinous air conditioning dilemma.

26. Charles Brush and Peder Hansen. Who? Brush built what is considered the first automatic wind turbine for generating electricity. The turbine, built in 1888 in Ohio, had a 50-foot diameter and 144 blades. The industry standard is now three.

Hansen, meanwhile, founded Vestjysk-Stalteknik A/S, or Vestas for short, in 1945. The company entered the wind industry in the '70s and provided it key support. The company also played an instrumental role in turning Denmark into a leader in alternative energy.

27. Stephen Salter. Back in the '70s, Stephen Salter at the University of Edinburgh theorized that waves, particularly the rippers off the coast of Scotland, could be harnessed to generate power. Out of that idea came the Duck, a wooden paddle-like device for harvesting wave power. Margaret Thatcher tried to kill the wave program in the '80s, but the University characterized wave research as an important technology for national defense. Falklands-minded Thatcher let the program -- and Salter's test tank -- survive.

The University of Edinburgh remains a center for wave and tidal research, and Scotland still hopes to generate power, and exports, from the sea. The original tank has been rebuilt a few times, but you can still see some of Saltyre's original equipment in this here video.

28. Enrico Fermi. In the '40s, Fermi conducted the first fission experiments, and in the process discovered a third form of primary energy. The other two are solar (wind, fossil fuels, wave, and biomass are all indirect forms of solar energy) and gravity (geothermal, tidal, hydro). Fission generates nuclear waste, but nuclear power plants do not emit carbon dioxide. Experts ranging from UC Berkeley's Dan Kammen to MIT's Ernie Moniz assert that nuclear remains a necessary component of the future grid.

Fusion, which could eliminate the waste problem, continues to hold promise, but it's still experimental.

SCIENTISTS AND POLICY MAKERS

29. The Presidential Trifecta. Theodore Roosevelt established national parks and made conservation an official government policy. Richard Nixon passed the Clean Air Act, the Endangered Species Act and other environmentally friendly legislation. Carter pushed to get the U.S. off foreign oil, helped establish the scientific backbone for the U.S. alternatives industry and ensured that cardigan sweaters would never, ever be a fashion statement again. All three would also have trouble getting elected by their own parties.

30. Roger Revelle and Hans Suess. Revelle, at Scripps, and Suess, at the USGS, wrote a paper in 1957 that demonstrated that the chemistry of the ocean puts a limit on how much carbon dioxide the seas can absorb. It opened the debate on global warming and mitigation.

31. Art Rosenfeld. Huge! Back in the 1970s, Rosenfeld, a physicist at Lawrence Berkeley lab (and Fermi's last grad student), determined that the power consumption in California and the nation would outstrip our ability to produce it. He kicked off a massive effort to get the state to pass efficiency regulations. Appliance makers fought vigorously, but California instituted Title 24 anyway.

"They all claimed it was the (expletive) end of civilization as we knew it," he told me in 2006. "Autos were getting 14 miles a gallon. Energy efficiency wasn't part of the American ethic whatsoever."

The result? Per capita power consumption has remained relatively flat in California but nearly doubled in the rest of the country. The results can only partly be attributed to the "Rosenfeld Effect." Still, the impact has been huge. Modern refrigerators consume half or less than that of fridges back in the '70s, hold more food and cost less when adjusted for inflation. His work has likely offset hundreds of billions in energy savings.

Until recently, Rosenfeld served on the California Energy Commission and recently published a paper on the efficiency gains possible through white roofs.

32. Steven Chu. Back in 2004, people would ask Steve Chu why he left a cushy job as in-house Nobel winner at wealthy Stanford to run Lawrence Berkeley Lab and grapple with all the budget issues that come with running a state-run organization. To do something about the energy situation, he'd reply. Getting BP to contribute $500 million to UC Berkeley and the University of Illinois for the biosciences institute became one of his first accomplishments. As the Secretary of Energy, he's been handing out checks ever since.

33. William McDonough. This prominent architect, along with chemist Michael Braungart, encouraged manufacturers to redesign products for efficiency and recyclability. But more importantly, recycling and renewability have gone from being acts of puritanical punishment to style statements. Face it: if bamboo fixtures didn't look cool, consumers wouldn't snap them up at Target.

34. Alan Salzman. One of the earliest venture capitalists in green. Salzman's VantagePoint Venture Partners has plunked money into Adura Technologies (lights), Tendril Networks (home networks), Ze-Gen, Bridgelux, Solazyme Better Place and Tesla Motors.

To date, VCs have had a checkered history in green and VantagePoint has participated in none of the few exits in green that have occurred. I can't even find a decent acquisition in there. Nonetheless, the billions washed down on green companies will one day likely sprout and the variety and selection in VantagePoint's portfolio bodes well. If you have to put a VC on the list, Salzman deserves the nod.

35. Al Gore. If he could have been as natural and self-assured on the campaign trail as he has been on the lecture circuit, he may never have had to resort to the "I used to be the next President of the United States" joke.

36. Intergovernmental Panel on Climate Change. 976 pages and only a few minor quibbles. The thousands of scientists behind the watershed report firmly established climate change as a scientific reality.

37. Arnold Schwarzenegger. It's fantastic. The star of Kindergarten Cop has been one of the most pivotal figures in helping turn the debate around green tech from being an environmental issue to an economic one. You can't go to a plant opening in California without running into him talking about chobs, chobs, chobs and chobs.

FOOD AND WATER

38. Sidney Loeb and Srinivasa Sourirajan. The UCLA scientists demonstrated seawater purification through reverse osmosis in 1949. The duo also invented reverse osmosis desalination at UCLA in the 1940s. Although reverse osmosis plants have to date primarily been built in Israel and the Middle East, expect to see more of them in Australia, South Africa, the Southwest and other dry and arid areas. One of the big complaints about reverse osmosis -- the high energy demands of the process -- are being ameliorated by companies like Energy Recovery.

Statkraft in Norway has begun to experiment with techniques for using osmosis membranes for generating power at the seashore.

39. Simcha Blass. At a farmer's request, Blass investigated why a tree grew in dry soil. The cause: a broken pipe. The experience prodded him to invent drip irrigation. The industry of water conservation, slowly but surely, had begun.

40. Norman Borlaug. The most controversial choice on the list. The plant scientist pioneered the techniques for breeding drought-resistant wheat and other crops. Mexico, India and other countries managed to become self-sufficient through the findings of his research, which has also been credited with helping to avert mass starvation and riots.

Critics argue that the so-called green revolution in agriculture led to excessive use of nitrogen-based fertilizer and overpopulation. Many are trying to solve the fertilizer problem with biopesticides. Overpopulation? Borlaug argued that it was more likely caused by social forces rather than food supplies. VCs have recently begun to turn their attention to agriculture.

Shyam Mehta and Eric Wesoff contributed to this article.