All too often , a choice that seems sustainable（可持续的）turns out on closer examination to be problematic. Probably the best example is the rush to produce ethanol（乙醇）for fuel from corn . Corn is a renewable resource —you can harvest it and grow more, almost limitlessly . So replacing gas with corn ethanol seems like a great idea .
One might get a bit more energy out of the ethanol than that used to make it, which could still make ethanol more sustainable than gas generally, but that’s not the end of the problem. Using corn to make ethanol means less corn is left to feed animals and people , which drives up the cost of food . That result leads to turning the fallow land — including, in some cases, rain forest in places such as Brazil — into farmland, which in turn gives off lots of carbon dioxide（CO2）into the air. Finally, over many years, the energy benefit from burning ethanol would make up for the forest loss. But by then, climate change would have progressed so far that it might not help.
You cannot really declare any practice “sustainable” until you have done a complete lift-cycle analysis of its environmental（环境的）costs. Even then, technology and public policy keep developing, and that development can lead to unforeseen and undesired results. The admirable goal of living sustainable requires plenty of thought on an ongoing basis.

我们经常发现，开始我们觉得找到了一个“可持续发展”的选择，后来仔细思考一下，却发现它存在问题。最好的例证很可能就是大家一窝蜂地去用玉米造乙醇作燃料。玉米是可以再生的----你把它种下去，可以收获更多，几乎是无限次地种，收。所以，用玉米造的乙醇来替代汽油看起来真是一个非常棒的点子。

乙醇产生的能量比生产它所需的能量要多一些，而且可以保持乙醇比汽油有更强的再生能力，但是问题到这里还没有完。用玉米制造酒精，意味着可以用来喂养动物和人的玉米就少了，因此会促使食物价格上涨。结果导致将休闲地------有时甚至包括，有些地方，如巴西的，热带雨林-----被用来种粮食，结果就是向大气中释放出大量的二氧化碳。最终，多年以后，燃酒精获得的效益只能用来补偿森林的损失。可是到了那时候，气候的改变可能已经到了无法挽回的地步。

如果你没有完全地分析过产品整个寿命周期的环境成本，你永远不可能真正地宣称某种做法是“可持续”的。即便你做过分析，因为技术与公众政策一直在进步，技术的进步也可能会导致未预见的和不期望的后果。可持续发展这个远大的目标要想实现也以发展为基础进行大量的思考。