COPENHAGEN — 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.

Now a question is coming into focus: Can Denmark keep the lights on as it chases that lofty goal?

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.

Their policy is similar to that of neighboring Germany, which has spent tens of billions pursuing wind and solar power, and is likely to hit 30 percent renewable power on the electric grid this year. But Denmark, at the bleeding edge of global climate policy, is in certain ways the more interesting case. The 5.6 million Danes have pushed harder than the Germans, they have gotten further — and they are reaching the point where the problems with the energy transition can no longer be papered over.

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.