The New York Times Can a city cancel out its greenhouse gas emissions? Copenhagen intends to, and fast. By 2025, this once-grimy industrial city aims to be net carbon neutral, meaning it plans to generate more renewable energy than the dirty energy it consumes. Here’s why it matters to the rest of the world: Half of humanity now lives in cities, and the vast share of planet-warming gases come from cities. The big fixes for climate change need to come from cities too. They are both a problem and a potential source of solutions. The experience of Copenhagen, home to 624,000 people, can show what’s possible, and what’s tough, for other urban governments on a warming planet. The mayor, Frank Jensen, said cities “can change the way we behave, the way we are living, and go more green.” His city has some advantages. It is small, it is rich and its people care a lot about climate change. Mr. Jensen said mayors, more than national politicians, felt the pressure to take action. “We are directly responsible for our cities and our citizens, and they expect us to act,” he said. In the case of Copenhagen, that means changing how people get around, how they heat their homes, and what they do with their trash. The city has already cut its emissions by 42 percent from 2005 levels, mainly by moving away from fossil fuels to generate heat and electricity. Politics, though, is making it hard to go further. A municipal government can only do so much when it doesn’t have the full support of those who run the country. Mr. Jensen, 57, a left-of-center Social Democrat, for instance, has failed to persuade the national government, led by a center-right party, to impose restrictions on diesel-guzzling vehicles in the capital. Transportation accounts for a third of the city’s carbon footprint; it is the largest single sector and it is growing. By contrast, the national government, in a move that its critics say encouraged private car use, has lowered car-registration taxes. The transportation minister, Ole Birk Olesen, said the government wanted to reduce what he called “the over-taxation of cars,” though he added that, ideally, Danes would buy only zero-emissions cars in the coming decades. And so, Copenhagen’s goal to be carbon neutral faces a hurdle that is common around the world: a divide between the interests of people who live in cities and those who live outside. Many opposition politicians and independent analysts say they doubt Copenhagen can meet its 2025 target, and some critics say the plan focuses too much on trying to balance the city’s carbon books rather than change the way people actually live. “We run around in fossil fuel burning cars, we eat a lot of meat, we buy a hell of a lot of clothes,” said Fanny Broholm, a spokeswoman for Alternativet, a left-of-center green party. “The goal is not ambitious enough as it is, and we can’t even reach this goal.” Mr. Jensen, for his part, is bullish on what he calls the capital’s “green transformation.” City officials say this is only the start. A new Metro line, scheduled to open this year, will put the vast majority of the city’s residents within 650 meters, a bit less than half a mile, of a station. Bicycle paths are already three lanes wide on busy routes for the whopping 43 percent of Copenhageners who commute to work and school by bike — even on wet, windy days, which are plentiful. All that wind helps generate the city’s electricity. Buildings are heated, in part, by burning garbage in a new high-tech incinerator — what garbage there is to burn, that is, considering that every apartment building now has eight separate recycling bins. For every unit of fossil fuels it consumes, Copenhagen intends to sell units of renewable energy. The city has invested heavily in wind turbines. In big cities, you have the money and the scale to change thing