Living in dense cities makes for certain efficiencies: being able to walk or take mass transit to work, living in buildings with (at least potentially) efficient HVAC systems, and more. That's why cities have been lauded in recent years for their (relatively) low environmental impact. But it seems at least one aspect of city life has an environmental effect felt at extreme distances from the cities themselves: waste heat. All those tightly packed sources of heat, from cars to banks of AC units, result in temperature changes not just directly (and locally) but by affecting weather systems surrounding the source city. From the article: "The released heat is changing temperatures in areas more than 1,000 miles away (1609 kilometers). It is warming parts of North America by about 1 degree Fahrenheit (0.6 degrees Celsius) and northern Asia by as much as 1.8 degrees Fahrenheit (1 degree Celsius), while cooling areas of Europe by a similar amount, scientists report in the journal Nature Climate Change. The released heat (dubbed waste heat), it seems, is changing atmospheric circulation, including jet streams — powerful narrow currents of wind that blow from west to east and north to south in the upper atmosphere. This impact on regional temperatures may explain a climate puzzle of sorts: why some areas are having warmer winters than predicted by climate models, the researchers said. In turn, the results suggest this phenomenon should be accounted for in models forecasting global warming."