Toronto, Feb 13 (IANS): It is technically feasible for cities to slash their greenhouse gas emissions by 70 percent or more in the long-term, says a new study.

Cities need to implement aggressive but practical step wise policy changes to make this happen, said Chris Kennedy, professor of civil engineering from the University of Toronto, and World Bank climate change specialist Lorraine Sugar.

Kennedy and Sugar note that more than half the world's population lives in urban areas and over 70 percent of greenhouse emissions can be attributed to cities, the Canadian Journal of Civil Engineering reports.

"Cities are where people live, where economic activity flourishes. Cities are where local actions can have global impact," said Sugar, according to a Toronto statement.

The study focuses on buildings, energy supply and transportation. Best practices as well as options and opportunities - for example, encouraging electric cars and increasing bicycling infrastructure - are detailed.

The biggest obstacle is the city's building stock, said Kennedy.

Buildings have a lifespan measured in decades, so it takes time to replace older buildings with more energy-efficient ones, he added.