Denmark, a tiny country on the northern fringe of Europe, is pursuing the world's most ambitious policy against climate change. It aims to end the burning of fossil fuels in any form by 2050 - not just in electricity production, as some other countries hope to do, but in transportation as well. Lest anyone consider such a sweeping transition to be impossible in principle, the Danes beg to differ. They essentially invented the modern wind-power industry, and have pursued it more avidly than any country. They are above 40 percent renewable power on their electric grid, aiming toward 50 percent by 2020. The political consensus here to keep pushing is all but unanimous. The trouble, if it can be called that, is that renewable power sources like wind and solar cost nothing to run, once installed. That is potentially a huge benefit in the long run. But [it] can render conventional power plants, operating on gas or coal or uranium, uneconomical to run. Yet those plants are needed to supply backup power for times when the wind is not blowing and the sun is not shining. So the trick now is to get the market redesign right. If Denmark can figure out a proper design for the electric market, it has another big task to meet its 2050 goal: squeezing the fossil fuels out of transportation.