The United States could run almost entirely on clean energy by 2050, with a larger economy, $5 trillion in savings––and no acts of Congress. That’s a vision of the future as seen by Amory Lovins, a sustainability expert who talked about how to reach that goal in a presentation Tuesday at Harvard University in Cambridge, Mass.

Electric vehicles, retrofits, the sharing economy and the rise of clean energy in Europe and China—all these technologies and trends show how a transition from oil, coal and nuclear power is possible, he said.

Lovins is chairman and chief scientist at the Rocky Mountain Institute, a nonprofit think tank and consulting firm that supports renewables and energy efficiency. His talk was based on his 2011 book “Reinventing Fire: Bold Business Solutions for the New Energy Era,“ and a 2012 TED Talk (a global ideas conference) where he presented the book’s findings.

“Long ago, fire made us human, and then fossil fuels made us modern,” Lovins said. “But now we need a new fire that makes us safe, secure, healthy and durable, and that turns out to be feasible, and in fact, to be cheaper than what we are doing.”

The lecture was part of a series of events at Harvard Climate Week. Other speakers will address issues such as climate change and public health, and the role of corporations in climate action.

Lovins doesn’t see himself as an optimist. Instead, he aspires to what he calls “applied hope.” His vision for 2050 shows the U.S. relying on wind, solar, biomass, hydropower and other renewables, along with some natural gas—but a third less than the country consumes today. Throughout his talk, he shared numerous examples of how the world is already turning from fossil fuels and why the trend will continue.

To begin with, there’s tremendous potential in cutting energy waste from buildings, Lovins said. In 2011, for example, the Rocky Mountain Institute helped retrofit the Empire State Building, ultimately reducing energy use by 38 percent by upgrading its lighting, windows and heating units.

Lovins used his own house as another example. Perched in Snowmass, Colo., the building, with its curved, S-shaped walls, is heated entirely through solar power, despite winter temperatures that can reach 47 degrees below zero. Lovins even grows bananas in an atrium covered in energy-efficient windows. Our house is “a net exporter of electricity and bananas,” he said.

On a larger scale, Lovins noted that U.S. electricity use peaked in 2007 and has been dropping ever since. Over the next 40 years, U.S. buildings could triple or quadruple their energy productivity while saving $1.4 trillion, he said.

Lovins also highlighted changes in the automobile industry. Uber and other car-sharing services have reduced the demand for individual car ownership, he said, and some Chinese cities have electric cars in “giant vending machines” that people can rent for about $3 an hour.

Demand is falling even in the U.S., as many young adults no longer want to own a car, he said. When Lovins asked how many members of the audience didn’t have a car, about a third of the 70 or so people raised their hands.

That may be because Cambridge has multiple public transportation systems. The city of 107,000 is also known for other sustainability initiatives. Cambridge officials want to end its contract with energy company TransCanada, which provides electricity to municipal buildings—but is also the company behind the proposed Keystone XL pipeline, a carbon-intensive project that would ship Canadian oil sands from Alberta to Nebraska.

Harvard and the Massachusetts Institute of Technology, its Cambridge neighbor—have robust student movements seeking to divest their institutions’ fossil fuel assets. Over the past few years, 28 universities and colleges have divested or pledged to divest from fossil fuels, along with 41 cities, 72 religious institutions, 30 foundations and hundreds of individuals.

Lovins didn’t mention divestment in his talk, but he did say that some investors are pulling out of the fossil fuel industry in favor of more secure investments. In 2013, Bloomberg LP unveiled the world’s first financial tool to help investors track how climate regulations and falling demand could impact fossil fuel assets.

On the renewable energy front, Lovins said the U.S. has a lot to learn from western Europe, where industrialized, developed nations rely on clean energy for a large share of their electricity. For example, Germany got 27 percent of its electricity from renewables in 2014, while Italy clocked in at 33 percent. And although high renewable use leads to concerns about blackouts, Lovins said those countries have tuned their electrical grids to handle the challenges.

Denmark and Germany have electric grids that are 10 times more reliable than those in the U.S., despite the higher use of renewables, he said.

“Our energy future is not fate but choice,” Lovins concluded. “And that choice is very flexible.”