LONDON Expanding cities threaten to eat up a swath of land the size of France, Germany and Spain combined in less than 20 years, putting the world under even more environmental pressure, experts said at a climate conference on Tuesday.
Cities are growing to accommodate a rising global population and as countries like China, India and Brazil pursue fast economic growth.
The world's cities are currently on track to occupy an extra 1.5 million square kilometers by 2030 - equivalent to France, Germany and Spain combined - spelling growing greenhouse gas emissions and resource demand, experts said at the "Planet Under Pressure" conference in London.
"The way cities have grown since World War II is neither socially or environmentally sustainable and the environmental cost of ongoing urban sprawl is too great to continue," said Karen Seto, associate professor of the urban environment at Yale University.
"The North American suburb has gone global, and car-dependent urban developments are more and more the norm."
The United Nations sees global population rising to 9 billion people by 2050 from 7 billion now, adding around a million people each week.
Most of the growth is expected to come in urban centers with migration from rural areas potentially adding another 1 billion people to cities. That would increase the total urban population to 6.3 billion people by 2050 from around 3.5 billion today.
Over 70 percent of current carbon dioxide emissions already come from cities. Urban emissions are forecast to grow to 36.5 billion metric tonnes by 2030 if no action is taken, from 25 billion in 2010 and 15 billion in 1990.
Urbanization cannot be stopped, but climate experts argue there is plenty of scope for improving the way cities are planned, developed and run.
"Everything being brought into the city from outside - food, water, products and energy, need to be sourced sustainably. We need to rethink the resource flow to cities," said Sybil Seitzinger, executive director of the international geosphere-biosphere program at the Royal Swedish Academy of Sciences.
New cities offer an opportunity to rethink urban planning while established ones can become more efficient through technology such as time-adjusted toll systems to cut traffic congestion, said Shobhakar Dhakal, executive director of the Tokyo-based Global Carbon Project.
Congestion wastes fuel, time and causes pollution.
It costs world economies an estimated 1 to 3 percent of gross domestic product and costs New York alone around $4 billion a year in lost productivity, experts said.
Utility meters and sensors that monitor power generation network capacity and electricity supply and demand can also help conserve energy.
Urban planners can also target more efficient land use, better building standards and policies to promote public transport over car use.
Some cities have made efforts to improve their green credentials, such as Iceland's capital Reykjavik, which depends on geothermal energy and hydro electricity for its energy needs.
Vancouver in Canada sources 90 percent of its energy from renewable sources like wind, solar and tidal energy and has developed a 100-year sustainability plan.