Fossil fuel usage will decline as better alternatives become economically viable.
If you add to the cost of fossil fuel the damage they do that time will come much sooner. If my neighbor builds a house by piling all the dirt on my property it will be a lot cheaper for him. If someone burns fossil fuel and warms the planet they don't personally bear the costs. Proper treatment of what economists call "externalities" has to be the job of society in the form of the government. That's what a carbon tax is all about. We solved acid rain at much less costs than anticipated. The miracle of the market really can find the best solution if the costs of externalities are factored in properly. We'll probably never get to zero use of fossil fuels, but we can get to much much less. The pope has done a service by pointing out that it's our moral imperative for the future. Now if only one party would stop saying "we're not scientists" we could make a lot of progress.
After WWII the country believed Gov't worked and was good for people. We believed that the space program was a response to a Russian threat. We have somewhat the same motivations now, expect that a large number of people believe any money spent by the Gov't is bad. We muster much more money now for big machines because OMG it would be terrible if the Chinese had a machine faster than ours. In science we are more motivated to use money to fight competition, not because it will help our society. We are also of course motivated by things people understand, e.g. curing cancer, though strangely not fighting diseases like Ebola which we think is restricted to Africa and which congress did not fund at the levels requested.
There's a real distortion in what we spend and what people think we spend. Polls have been conducted about whether we spend too much or too little on various items in the discretionary budget. They often think for example that many believe we spend too much on foreign aid, and those same people believe we spend more than 10x what we really do on foreign aid.
In other columns and particularly his blog, which usually has much more data (and visualizations of the data etc) http://krugman.blogs.nytimes.c... Krugman is well aware that the US must act before the rest just because we are one of the biggest offenders here with much more CO2 use per capita than others.
"Anyone attempting to generate random numbers by deterministic means is, of course, living in a state of sin." -- John Von Neumann