A Strong Greenhouse Gas
1.Methane is a colorless, odorless gas; it is also a potent greenhouse gas, and once released into the atmosphere, it absorbs heat radiating from Earth’s surface. That’s why methane is a major contributor to the planet’s increasing temperature rise—or global warming. Molecule for molecule, methane’s heat-trapping power in the atmosphere is 21 times stronger than carbon dioxide, the most abundant greenhouse gas.
2.With 13 billion cows belching almost constantly around the world (100 million in the U.S. alone), it’s no surprise that methane released by livestock is one of the chief global sources of the gas. Other prime methane sources: petroleum, drilling, coal mining, solid-waste landfills and wetlands.
3.Greenhouse gases like methane and carbon dioxide make up only a small part of Earth’s atmosphere, which is 78 percent nitrogen and nearly 21 percent oxygen. And without greenhouse gases to trap the sun’s heat and warm the planet, life was we know it couldn’t exist. But in the last 200 years, human activity that requires burning oil, natural gas, and coal for energy has magnified the greenhouse effect.
4.Atmospheric concentrations of methane have more than doubled in the last two centuries. Blame for this often focuses on big industries and gas-guzzling vehicles. But agriculture plays a major role, too. In the past 40 years alone, the global cattle population has doubled.
5.Cows munch mostly grasses and hay—yet they grow big and hefty. Why? Because of the rumen. The rumen holds 160 liters of food and billions of microbes. These microscopic bacteria and protozoa break down cellulose and fiber into digestible nutrients. A cow couldn’t live without its microbes. As the microbes digest cellulose, they release methane. The process occurs in all animals with a rumen (cows, sheep, and goats, for example), and it makes them very gassy. It’s part of their normal digestion process. When they chew their cud, they regurgitate some food to rechew it, and all this gas comes out. the average cow expels 600 liters of methane a day.
6.That’s why we say livestock gas is also a major factor of causing the global warming.
词汇：
methane 甲烷 munch 用力咀嚼
odorless 无味的 hefty 健壮的
potent 强有力的 rumen 瘤胃
belch 打嗝 protozoa 原生动物(复数)
livestock 家畜，牲畜 cellulose 纤维素
landfill 埋填的废渣 nutrient 有营养的
magnify 扩大 cud 反刍的食物
guzzle 大量消耗(燃料等) regurgitate 反刍
