经济学人下载:天然气是清洁能源吗?

Natural gas is cheap and plentiful. That makes it an appealing alternative to wind and solar power, which are relatively expensive and erratic. And plants powered by gas emit far less carbon dioxide than those powered by dirty coal. Nearly half of America’s electricity is generated by coal, and electricity generation accounts for 40% of America’s carbon-dioxide emissions. In 2009, the most recent year for which data are available, the country’s greenhouse-gas emissions were at their lowest level since 1995. That was partly due to the bad economy, and partly to the increased use of renewables and natural gas.

天然气价格低、储量大，比价格高、稳定性低的风能和太阳能更有优势。如果用天然气发电，它比用煤发电排放的二气化碳量要少很多。美国有近一半的电力来自煤电厂，而美国二氧化碳的40%来自于发电。我们能得到的最新数字是2009年的，这一年温室气体的排放达到了1995年以来的最低点。一部分原因是因为那一段时间经济萧条，另一个原因就是可再生能源以及天然气的使用量增加。

But some question whether natural gas is really as green as all that. For one thing, fracking uses a tremendous amount of water, a severely undervalued resource inland. And the process gives off methane, a potent heat-trapper. A study led by Robert Howarth of Cornell University found that greenhouse-gas emissions over the life cycle of natural-gas production could actually be considerably higher than those of coal per unit of energy provided. Greater care about methane venting would, of course, reduce that problem.

但有人怀疑天然气是否真得就那么清洁环保。一方面，破裂法需要耗用大量的水，水作为一种陆地上的资源，价值被严重低估。另一方面，这种方法还会释放大量的甲烷，一种极易吸热的气体。康奈尔大学的罗伯特??霍华斯领导的一项研究发现，提供等量的能量，所使用的天然气比所用的碳在整个生命周期里所释放的温室气体大很多。当然，谨慎处置甲烷的排放会减少这种问题。

And fracking raises other concerns. In the process, a mix of sand, water and chemicals is pumped deep underground at ferocious pressure. That breaks up shale formations, releasing the gas trapped inside so it can be pumped to the surface. But fracking is almost entirely unregulated, because of a 2005 statutory exemption from the Safe Drinking Water Act. Three members of Congress recently released a report on fracking, saying that oil and gas companies used more than 2,500 fracking products containing 750 chemicals between 2005 and 2009. Some ingredients were benign, such as citric acid and instant coffee. Others, though, were extremely toxic, such as benzene and lead. Only five states require public disclosure of the chemicals used, and only to limited and varying degrees. That is making landowners, locals and environmentalists edgy, particularly in big production states like Colorado, Pennsylvania, and Texas, the nation’s largest natural-gas producer, where an enormous reserve—the Barnett Shale—sits under a big city, Fort Worth.

同时，破裂法还会带来其他方面的问题。在这个过程中，要用巨大的压力把一种沙、水和很多化学品的混和物压入地下。这会破坏页岩层的构造，致使储藏其间的气体被挤出地表。因为开采天然气在2005年得到《安全饮用水法案》的法定豁免，现在破裂法的使用几乎没有任何的监管。最近有三名国会议员就破裂法发表报告，报告指出，2005—2009年间石油和天然气公司使用的2500多种破裂材料含有750种化学物，有些成分是对人体无害，如柠檬酸、速溶咖啡。但也一些含有巨毒，如苯和铅。只有五个州要求他们向公众披露所使用的化学物，但这种披露程度有限，而且披露要求也不统一。这使得土地所有者、当地居民以及环保分子很是不安，特别在科罗拉多、宾夕法尼亚、得克萨斯等一些天然气生产大州。其中得克萨斯天然气产量最大，沃思堡市巴耐特页岩里储藏着大量的天然气。