Forgetsolarand wind power, biofuels, and all the other renewable energy projects in the world. For that matter, forget about making the transportation and industrial sectors carbon-free.

None of that will make a whit of difference in combating climate change without a major greening of the way we construct and renovate our buildings.

That was the message that Marc Porat, former Apple Computer employee and General Magic founder turned green building entrepreneur, delivered Thursday at Greentech Media's Green Building Summit.

"It is all about the built environment," Porat told an audience in Menlo Park, Calif. "No matter what we do with transportation and industry, it doesn't matter."

Granted, as chairman of three green building startups – green window and drywall maker Serious Materials, energy-efficient brick and cement maker CalStar Products and "net-zero energy" building designer Zeta Communities – Porat has a vested interest in pushing the idea that his chosen industry is the one needing the lion's share of sustainability-focused investment (see Here Comes the Green Brick and Green Builder Zeta Communities Gets Funding).

And Porat didn't dismiss the contributions that renewable energy and efficiency in transportation and industry will make toward reducing the globe's greenhouse-gas emissions. It's only that their contributions will pale in comparison to making building more efficient, he said.

That's because the 300 billion square feet of commercial and residential real estate in the United States makes up just over half the country's total energy use in materials, construction and operations, and about 55 percent of its greenhouse-gas emissions, he said.

And that built environment is in flux, he said. In the next two and a half decades, about 50 billion square feet will be demolished, another 150 billion square feet built and a further 150 square feet renovated, he said.

"By the end of 2035... we've touched about 75 percent of it," he said – and that represents $40 trillion in new construction and $15 trillion in renovation, he said.

That presents an opportunity and an imperative for the green building community to get its ideas and practices adopted as fast and as broadly into "this massively heterogeneous, complex, resistant and conservative" construction industry, he said (see A Green Building Market Overview).

And proving that green building works in the United States will be critical to making changes in the countries that are likely to have an even greater effect on global climate change – China and India.

China's built environment takes up about 37 percent of the country's energy use, since China's export-oriented industrial base has been the main driver of energy demand growth there, he said. Still, the nation of 1.3 billion people is expected to add about 350 million new urban residences and another 250 million in the countryside by 2020, equaling 300 billion square feet of real estate.

"And China stands in as a great surrogate for India," in terms of development policies and practices the world's second most populous country is likely to adopt, he said.

China is overwhelmingly dependent on coal for its energy, but its coal reserves may peak in production by the middle of the 2030s, he said.

Thus, helping the country halve its projected energy demand by that time through green building could spell the difference between prosperity and desperation for its people, he said - not to mention the potential for it government to "project force" abroad in search of energy sources.

But pitching green building by predicting global catastrophe if it doesn't happen isn't the way to get it adopted on a mass scale, he conceded. Instead, "We have to start by making money."

The economic downturn is actually a blessing for the green building community, Porat said. Changes of the magnitude he's suggesting are "impossible" in boom times, he said, but the recession "gives us a couple of years, intellectually and commercially."

That's a strategy that Serious Materials, for one, would appear to be putting into practice by taking over bankrupt factories to make its green materials (see Serious Materials Shopping for Acquisitions in Bankruptcy Court).