Copenhagen, the Danish capital city, has determined that, on a net basis, it will stop adding climate changing gases to the atmosphere by 2025.

The decision came in an August 2012 city council vote that orders a switch from coal to scrap wood and other sustainable forms of biomass for heat and power. And Copenhagen has also begun to cool its largest buildings using an offshore resource, cold seawater.

Copenhagen already gets much of its electricity from wind power, but cooling buildings sea water is a good way to dramatically reduce usage of electricity for cooling.

Two pipes, each nearly a yard in diameter, buried 20 feet underground re-circulate water three-quarters of a mile from Copenhagen’s harbor. Originally built to cool off the old power plants' generators, today the system provides cold water to the new district cooling plant.

"If we didn’t have these pipes we didn’t have this project because it would be so expensive for us to put in new pipes in the ground," said engineer Jan Hoge. "And although they’re more than a 100 years old, they look fantastic. We just have to reline them and now we use the seawater from the harbor to cool down our chiller units."

The project, which started in 2009, is now providing cooling at very low cost, said environmental journalist Justin Gerdes, who recently examined the project.

"They estimate that doing cooling in this way reduces carbon emissions by nearly 70 percent and electricity consumption by around 80 percent," he said.

For now, the project focuses on large buildings — hotels, office buildings, data centers — mostly in the city center. For the moment, though, there are no plans for expanding the expanding the program to residences.

Copenhagen has a great deal of wind power — in fact more than it needs to power the city. That's actually key to the city's plan to go carbon-neutral by 2025.

Because of all of the cars that will still run on fossil fuels, Copenhagen is counting on exporting its excess wind power to other areas to make up for the carbon emissions vehicles produce within the city limits.

But there are other plans as well — including a bike superhighway.

"Most Americans if they have heard anything about Copenhagen, they know it's a bike city. It's one of the world's best bike cities," Gerdes said. "And they've been remarkably successful — 36 percent of trips in Copenhagen to school or work are made by bike."

But that's not enough. They realize they need to do more to entice residents, and more importantly, people commuting in from the suburbs, to take bikes.

"So they're building these so-called bike superhighways. The first of them opened in April of last year, and it connects a suburb north of Copenhagen to the city center," Gerdes said.

Gerdes said the effort to halt climate change will be won or lost in the cities.

"Cities are responsible for around two-thirds of both energy consumption and carbon emissions," he said. "So if we can follow the example of some of these forward-thinking cities like Copenhagen ... especially ones in developing countries can learn from that example, and we can start putting a downward trajectory on emissions."