As the world marks Earth Day on Friday, we're taking a look at the biggest source of greenhouse gas emissions in the city. And it's not cars. NY1 Manhattan Reporter Michael Scotto has the story.

Inside 345 Park Avenue, mechanical systems dating back to the 1960s still hum on the 34th floor.

But a modern computer system, building officials say, has helped to not only make those systems run more efficiently, but reduce the emissions blamed for climate change by some 40 percent over the last few years.

"As we have the brain and the heart, then the central nervous system — which is a series of sensors that goes throughout the building, both wired and wireless, bringing data back," said John Gilbert of Rudin Management.

The system uses algorithms based on occupancy and time of day to determine just how much electricity the building needs to use.

It's all part of an effort to reduce greenhouse gas emissions produced by buildings, which are responsible for 70 percent of the city's carbon dioxide pollution.

In midtown, building managers packed into a meeting called by a real estate industry trade group to learn how their systems can run more efficiently.

"Go green to save green, which is absolutely the bottom line," said John Banks the president of the Real Estate Board of New York.

Over the last decade, greenhouse gas emissions in the city have dropped by about 12 percent. De Blasio is pledging to cut them further: 80 percent by the year 2050.

There are incentives as well as some requirements for the owners of big buildings to green their properties.

But environmentalists say it's going to take some time for these policies to make a difference.

"Are we on a path to reduce greenhouse gas emissions 80 percent by 2050? Not even close," said Russell Unger of the Urban Green Council. "But the fact that we are reducing energy use — reducing greenhouse gas emissions at a time when there's economic, population, and square footage growth — that's amazing."

The hope is that as the city reduces its carbon footprint, the rest of the world goes green as well.