On the hottest of summer days, officials at DFW International Airport do something that at first might seem counterintuitive -- they turn off the air conditioners.

In their place, six million gallons of super-cooled water is pumped through pipes in the terminals to keep passengers comfortable.

The high tech solution, along with more prosaic steps like purchasing its electricity from renewable sources, has helped DFW reduce its net emissions to the point where it was recognized last month as the first airport in North America, and one of only 26 in the world, to achieve carbon neutral status.

"This started at the airport probably 15 or 20 years ago. I think it's fair to say it started because we saw an opportunity to be smarter with how we ran the airport from an energy perspective," said airport CEO Sean Donohue, who took over in 2013. "Over the last couple of years, it became apparent we were really on the path to carbon neutrality. We said to ourselves, 'What do we need to do to actually attain (that)?'"

It was no easy task for a 24/7 operation that spans five terminals across 17,000 acres, nearly 2,000 flights per day and is home to 60,000 airport, airline and other employees.

The path to carbon neutrality can be traced back as far as the late 1990s, when the airport began switching its buses over to compressed natural gas fuel, a process that was finished in 2011.

Over the years, the airport has taken various sustainability-minded steps like retooling its central energy plant to be more efficient and adopting a process known as "continuous commissioning," which constantly adjusts heating and cooling systems in the terminals to account for the time of day and passenger flows.

The net effect has been an 18 percent reduction in carbon emissions, despite a 15 percent increase in passenger traffic, according to figures from the airport and the Airport Carbon Accreditation Program.

Altogether, the airport says it has reduced its carbon emissions by 31,000 metric tons since 2010, with more than half of the savings coming from the switch to purchasing only renewable wind energy. At the same time, the airport's annual energy bill has fallen from $32 million in 2006 to just under $18 million in the most recent fiscal year.

"This has been a win win," Donohue said. "We've reduced our carbon footprint, but we've also reduced our costs. As a business person, that really resonates."

The airport still produces 146,000 metric tons of carbon emissions annually, which it offsets with renewable energy certificates and carbon offsets from emissions reduction projects at Texas landfills and wastewater treatment plants.

The takeaway, according to the airport's vice president of environmental affairs Robert Horton, is that there's no single solution to reaching carbon neutral status.

"About 70 percent of our (carbon) footprint comes from electricity use. The other 30 percent comes from energy to heat facilities, as well as fuels we use for fleet vehicles. That's why a lot of initiatives centered around efficiency" he said. "We took a balanced approach focused on all the different components."

While the issue of sustainability has become a growing topic of conversation in the airport industry, there's still work to be done, Donohue said.

Only 20 North American airports participate in the Airport Carbon Accreditation Program, an independent system that tracks airport emissions and recognized DFW for its carbon neutral accomplishment. Globally, 156 airports take part, representing about a third of airline passenger traffic.

There's also the issue of airline emissions, which globally represent about 2 percent of all human-induced carbon emissions. Although the airport can't directly affect those emissions, Donohue said his organization has partnered with American Airlines to purchase renewable energy together, helping lower costs for both.

Donohue and Horton said putting DFW Airport on the path to carbon neutrality is key for enabling future growth as passenger volumes increase and eventually a new terminal is built.

Although they're proud the airport was the first to reach the milestone in North America, they said it can't afford to slow down now.

"If we stand still on this, within a couple years it won't be an achievement anymore," Donohue said.

Future steps include engaging rank and file employees more directly in sustainability initiatives, with an emphasis on sustainable decision-making that looks beyond strictly financial measures. The airport is also looking at new types of natural gas with even lower emissions for its fleet of buses and designing new building additions to have no net effect on the airport's emissions.

Later this month, Donohue will travel to the Netherlands where he will join the CEO of the Amsterdam's Schiphol Airport in becoming the first signees of an international sustainable airport pledge.

"You have to commit to it. It can't be one of these fads where you say 'Let's give it a shot' and then leadership changes or you go through a challenging financial period and you say, 'We can't keep investing in this,'" Donohue said. "That's to me the biggest challenge."

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