From the Bullitt Center’s roof, Seattle’s status as the North American crane capital is in full view.

Sixty-five cranes are currently reshaping the Seattle skyline, a building boom that reflects this city’s phenomenal growth spurt.

In US Census data released this year, Seattle was ranked the fastest growing US big city over the past decade, with new residents pouring in to take jobs at companies such as Amazon.com.

But that growth is a worry as the "Emerald City" – nicknamed for its evergreen trees and environmentalist politics – strives to become "carbon neutral" by 2050, meaning it will produce no more climate-changing emissions than it can offset by measures such as planting carbon-absorbing trees.

A building boom unlike any seen since the late 19th century Yukon Gold Rush is making an already challenging goal harder, experts say.

Seattle’s greenhouse gas emissions have fallen 6 percent since 2008, according to 2014 data, the most recently available from the city’s Office of Sustainability and the Environment. Over the same period, the city’s population grew 13 percent.

“We have decoupled growth from emissions,” the city’s chief climate official, Jessica Finn Coven, told the Thomson Reuters Foundation.

But, nonetheless, “we’re not on track to meet our ambitious 2050 goals,” she said.

As old structures fall and new ones are built, heating and lighting them account for a third of the city’s emissions, Ms. Finn Coven said.

Figuring out how to slash emissions further without either scaring off economic development or hampering the pace of housing construction is a challenge, she admits.

But “it’s not rocket science," she said. “We have to stop heating our buildings with fossil fuels.”

A view from the top

Companies leasing office space in the Bullitt Center, a six-story modern glass-and-concrete edifice eight blocks from downtown, are doing their part to help Seattle become carbon neutral.

The Bullitt Center is the world’s largest “living building," one that produces more energy than it consumes thanks to an energy-efficient design that exceeds the highest standards set by LEED, a green building ratings system.

Floor-to-ceiling windows allow ample natural light during working hours. Windows open and close automatically to adjust the temperature, and an energy-sipping heat pump takes advantage of stable temperatures below ground to heat the building in the winter and cool it in the summer.

A cantilevered roof, carpeted in photovoltaic panels, captures enough solar energy in this cloudiest of major US cities to power the needs of an engineering firm, a tech company satellite office, and a home contractor, among others.

Occupants of the building say it's comfortable as well as efficient.

“I love that there’s so much daylight," said Amanda Falkenhagen, a lighting designer with Rushing, an engineering and sustainability consulting firm with an office in the center.

"I was recently in a traditional office building and it made me realize all the things in this building I take for granted – the lightness, airiness, connectedness to the outside,” she said.

Denis Hayes, president of the Bullitt Foundation, an environmental philanthropy serving the Pacific Northwest, noted that often, in the center, "there are no lights on, but nobody complains."

The foundation invested more than $32 million – a third of its $110 million endowment – in 2013 to erect the Bullitt Center, hoping to pave the way for other similar buildings.

“It suddenly takes something that developers thought was impossible and shows that it’s in fact practical,” Mr. Hayes told the Thomson Reuters Foundation in the center’s airy conference room.

The challenges of energy efficiency

The problem is the Bullitt Center still remains one of a kind – despite Hayes saying that if it was still the biggest "living" building in the world five years after being commissioned, "I would consider our efforts a failure."

Seattle hopes to change that through incentives. The Bullitt Center, for instance, benefitted from a pilot initiative that waived some zoning requirements on height and roof design, in order to accommodate its energy-efficient makeup.

In April, Mayor Jenny Durkan reauthorized that program and launched a new pilot that will allow up to 30 buildings to be built higher than existing rules permit if they meet certain energy efficiency standards.

She also directed the city-run utility company, whose electricity comes almost entirely from hydropower, to feed excess energy created by buildings such as the Bullitt Center back into the power grid, and pay the building owners for creating it.

The directive, which allows up to 30 buildings to feed power into Seattle's electrical grid, aims to provide a financial incentive for building owners to make hefty investments in energy efficiency.

Seattle also has put in place one of the United States' strictest codes for energy efficiency in commercial buildings.

It requires most new buildings to achieve an energy use index (EUI) rating in the 40s, according to Susan Wickwire, who directs the Seattle 2030 District, an initiative to encourage real estate developers and building managers to meet tough energy-efficiency standards.

The average Seattle building has a rating between 80 and 90, Hayes said, while the Bullitt Center's is 12.

Wickwire's group tracks the energy performance of 250 buildings out of 4,000 in an eight-square-mile area of the city that includes downtown Seattle.

She thinks the incentive program is a good one, and believes real estate developers and property managers are getting on board with greater energy efficency.

“There is a shift in the DNA of the city to move away from fossil fuels to [greener] electricity and that’s only going to get tighter,” Ms. Wickwire said.

But the overall energy efficiency of her district is dragged down by inefficient buildings constructed in the 1960s and 1970s, she said.

Owners of those buildings are hesitant to pursue energy-efficiency retrofits, she said, because they would also trigger demands for other upgrades, such as improved access for the disabled and better ability to withstand earthquakes.

Overcoming that problem may require further tweaks to city government policy to keep Seattle on target with its climate change goals.

Those goals are ones activists such as Hayes see as key to helping set similar standards nationwide.

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"If you can't do it in Seattle, I don't know how you can do it in Cleveland," he said.

This story was reported by the Thomson Reuters Foundation.