Global warming has come to Tokyo with a vengeance: While the average global temperature has increased by 1 degree Fahrenheit in the last century, the average temperature in the Japanese capital has risen by more than five times that. Like most large cities, Tokyo is an island of heat. Concrete, cars, and rooftops absorb sunlight all day and discharge it at night, preventing the city from cooling; vertical buildings block breezes; and omnipresent air conditioners cool interior environments but further warm the outside air. The heat has brought with it increased smog and new, subtropical species, including dengue-fever-bearing mosquitoes. But the city has generated at least one plan for cooling off: Last year, it passed a law mandating that all new medium-sized buildings dedicate at least 20 percent of roof space to a garden. The city has also established a variety of tax incentives to encourage rooftop gardens, which lower temperatures and increase greenery to absorb carbon dioxide and cleanse the air.