Counting the Cost of Global Warming

Since the last ice age, when ice enveloped most of the northern continents, the earth has warmed up by about five degrees. Within a century, it is likely to warm by another four or five, because of the greenhouse gases that we are dumping into the atmosphere. This will have immense and mostly harmful effects on the lives of people not yet born. How much should the present generation be prepared to pay to mitigate these harmful effects? How much should we sacrifice for the sake of the future?