"At the moment a lot of cities don't know what they're emitting so it's very difficult to set targets," Reddy said.

More important than emissions targets, he said, is that cities draft action plans, outlining specific goals needed to reduce emissions, like switching a certain percentage of the public transit system to alternative fuels.

London Mayor Ken Livingstone's Climate Action Plan calls for cutting the city's CO2 emissions by 60 percent in 2025, compared to 1990 levels. However, planners acknowledge the cuts are not realistic unless the government introduces a system of carbon pricing.

Barcelona, Spain's second biggest city, has, since 2006, required all new and renovated buildings to install solar panels to supply at least 60 percent of the energy needed to heat water.

The project has been emulated by dozens of Spanish cities and inspired national legislation with similar, though less stringent, requirements, said Angels Codina Relat of the Barcelona Energy Agency.

It's not only in Europe that cities are taking action on climate change.

Several US cities including Austin, Texas; Portland, Ore.; and Seattle have launched programs to cut greenhouse gas emissions. Bogota, the capital of Colombia, has reduced emissions with the TransMilenio municipal bus system and an extensive network of bicycle paths.

In Vaxjo, (pronounced VECK-shur), the vast majority of emissions cuts have been achieved at the heating and power plant, which replaced oil with wood chips from local sawmills as its main source of fuel. Ashes from the furnace are returned to the forest as nutrients.

"This is the best fir in Sweden," said plant manager Ulf Johnsson, scooping up a fistful of wood chips from a giant heap outside the factory.

He had just led Michael Wood, the US Ambassador to Sweden, on a guided tour of the facility, which is considered state of the art. Not only does it generate electricity, but the water that is warmed up in the process of cooling the plant is used to heat homes and offices in Vaxjo.

Every week, foreign visitors arrive to see Vaxjo's environmental campaign. Last year, a delegation of 10 energy officials from the Democratic People's Republic of Korea got a tour.

A similar but much larger system is in place in Copenhagen, Denmark's capital, where waste heat from incineration and combined heat and power plants is pumped through a purpose-built 800-mile network of pipes to 97 percent of city.