Stole that from Mark Steyn, of course, whose piece on the subject is here. See also Steven Hayward, who draws on his Index of Leading Environmental Indicators. He points out that due to population growth, holding CO2 emissions to current levels will mean abandoning basically all household appliances.
If households are to emit no more than their present share of CO2, emissions will have to be reduced to 204 million tons by 2050. But in 2050, there will be another 40 million residential households in the U.S.

Today, the average residence in the U.S. uses about 10,500 kilowatt hours of electricity and emits 11.4 tons of CO2 per year (much more if you are Al Gore or John Edwards and live in a mansion). To stay within the magic number, average household emissions will have to fall to no more than 1.5 tons per year. In our current electricity infrastructure, this would mean using no more than about 2,500 KwH per year. This is not enough juice to run the average hot water heater.

You can forget refrigerators, microwaves, clothes dryers and flat screen TVs. Even a house tricked out with all the latest high-efficiency EnergyStar appliances and compact fluorescent lights won't come close. The same daunting energy math applies to the industrial, commercial and transportation sectors as well. The clear implication is that we shall have to replace virtually the entire fossil fuel electricity infrastructure over the next four decades with CO2-free sources – a multitrillion dollar proposition, if it can be done at all.

Or we could turn our attention to the demand side and try to prevent that coming population growth. Which is what environmentalism (as opposed to conservation) usually turns out to be about anyway. We've already had hints, but wait for it. I sense a new UN global climate/ population initative coming on.