Amidst talk of eco-cities, a Chicago architectural firm has put together a plan to reduce carbon emissions by 80 percent within Chicago’s Loop. How to accomplish this: retrofit older buildings rather than building a lot of new, green buildings.

The architects break what they call the Central Loop into four types of buildings: heritage buildings (1880-1945), which are clad in heat-absorbing masonry and have operable windows; midcentury modern buildings (1945-75), which hog energy due to their vast expanses of glass and heavy reliance on air conditioning; post-energy-crisis buildings (1975-2000), which show greater energy-efficiency but are burdened by an unanticipated rise in computer use; and energy-conscious buildings (2000-present), which continue to improve efficiency but are in relatively short supply.

That brings us to the heart of the matter: The key to cutting pollution isn’t building new green buildings. There simply aren’t enough of them to make a difference. The only way to lower our carbon footprint is to make the buildings we already have more energy-efficient.

That’s possible, as evidenced by the recent transformation of the Merchandise Mart, the massive yet graceful Art Deco commercial and trade show building along the Chicago River. At 4.2 million gross square feet, it’s one of the world’s largest buildings. By taking a variety of steps — from installing energy-saving water pumps to promoting eco-friendly products to the building’s tenants — the Mart cut its overall energy consumption by 21 percent from 2006 to 2010, executives there say.

I wonder how this plan would be received by businesses and building owners. While they suggest energy costs will decrease in the long run and rents may increase, such retrofitting could be costly in the short-term and there could be some anxiety about doing these things in the middle of a tough real estate and business market.

And how much would the City of Chicago really get behind this? Mayor Daley has drawn plaudits in the past for promoting ideas like rooftop gardens but these are limited in number. The City itself faces significant financial troubles in the coming years and I imagine issues like jobs, pensions, crime and the number of police in the streets, will dominate conversations for a while.

I would enjoy seeing their charts or models to see which particular buildings in the Loop use more or less energy. The picture that leads this report on the plan probably shows carbon emissions or energy use by building.