A mobile application launched in dozens of U.S. and Canadian cities on Monday measures the planet-warming greenhouse gas emissions of inner-city travel, its creators said, letting concerned commuters map their so-called carbon footprints. Mapping app Cowlines can suggest the most efficient route as well which uses the least fuel, combining modes of transport such as bicycling and walking, within cities, its Vancouver, Canada-based creators said. Some two-thirds of the world's population is expected to settle in urban areas by 2050, according to the United Nations. The trend presents an environmental challenge, given that the world's cities account for the bulk of greenhouse gas emissions. Not only will the app measure a trip's emissions and suggest alternatives, it will provide the data to cities and urban planners working on systems from…