"Eat local." It's one way to reduce human effect on the planet.Eating local means to try to buy and consume foods that are grown in places close to home.However, most of the food sold at supermarkets is not locally grown or produced.Trucks and planes deliver these foods from hundreds or thousands of miles away.During the transportation, greenhouse gases are produced, causing global warming.So the shorter the distance your foods must travel, the less the harm is done to the environment. But how do you get local food if you live in a large city, hundreds of miles away from farms?Environmental health scientist Dickson Despommier and his students came up with the idea of a "vertical farm". A vertical farm is a glass-walled structure that could be built as tall as a skyscraper.Since the garden is built upwards, rather than outwards, it requires much less space than an ordinary farm.The world is quickly running out of room for ordinary farming.Vertical farms could be a key to this situation.Despommier imagines a 30-story building with a greenhouse on every floor.The walls of the building would be clear, to allow crops to get as much sunlight as possible.Depending on a city's water resources, Despommier thinks hydroponic  farming is another method for the vertical farm which needs no soil to grow plants. Despommier says the hydroponic greenhouses would use a system that would use a city's waste water and fill it with nutrition to make the crops grow.If this method works, it would provide food to a city and save millions of tons of water. The idea of a vertical farm has attracted the attention of government officials around the world.Scott Stringer, a government official from New York City, thinks the city is suitable for the vertical farming."Obviously we don't have much land left for us," Stringer said, "But the sky is the limit in Manhattan." Despommier admits that there is still a lot of work to do to make vertical farms a reality."But I think vertical farming is an idea that can work in a big way." he says. Why are people advised to eat local?
Because it means convenience to people. Because it can help people save a lot of money. Because local food has more nutrition. Because it is environmentally friendly.