Airplanes contribute to global warming because their contrails turn into cirrus clouds, which trap heat rising from the Earth. Now there’s a solution: if planes fly 6,000 feet lower, it will reduce contrails by 47%. While we search for solutions to modern problems, scientists have discovered that humans began warming the climate 10,000 years ago, when they became farmers.

Paul Rincon writes in that researchers tried different scenarios on a computer model of modern air traffic, in which planes flew 2,000, 4,000 and 6,000 feet below current levels, and found that flying 6,000 feet lower reduces contrails by almost half.

We haven’t been flying in airplanes for very long, but humans have been warming the climate for a long time?almost 10,000 years, according to researcher William Ruddiman, who says that agriculture has pushed up the temperature by putting greenhouse gases into the atmosphere.

If the Earth had followed its natural cycle, it would have gotten steadily colder over the last 10,000 years, but agriculture balanced this out, keeping the temperature steady. In, Richard Black quotes him as saying, “What should have happened with the natural climate is it should have cooled substantially. And instead, humans just started adding greenhouse gases at a rate which cancelled most, but not all, of that natural cooling; and so it’s a combination of a natural cooling mostly cancelled by a human warming.”

Agriculture increases greenhouse gas emissions several ways. First, trees are cut down that would otherwise absorb carbon dioxide. When wet farming is done?such as with growing rice?methane gases are released. Cattle also release large amounts of methane into the atmosphere. However, global warming was speeded up considerably when man discovered fossil fuels.

Agriculture-based global warming may have led to some of the early great civilizations, like that of Mesopotamia. Farmers in that area depended on rain for 1,000 years, until a drought hit 8,200 years ago, which may have been set off by agriculture-based global warming. Farmers then all moved to the same area, by the Tigris and Euphrates rivers, so they could use the water for irrigation. So many people coming together in the same spot eventually led to the building of cities and to the development of art, culture and science.

Harvey Weiss of Yale University finds it ironic that climate change led to the creation of modern society, while modern society is now changing the climate in ways which threaten its existence.

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