National Geographic: Portugal became the latest country to coast solely on renewable energy this month, going without fossil-fuel power for four days straight. Denmark, Germany, and others recently have declared similar feats. But are these temporary clean-power surges a mark of real change, and if so, how far can that go?
"This is definitely signalling a shift in terms of the energy mix," says Luca De Lorenzo, project manager at Sweden's nonprofit research group Stockholm Environment Institute.
Overall, renewable...