Tesla Motors' bid for SolarCity gives a glimpse of an emissions-free future for the transportation sector: Electric cars that can charge their batteries with clean power from home solar arrays.

The potential is tremendous. But, when it comes to solving climate change, we also need to reckon with an inconvenient truth: There are more than 1.2 billion gas-powered vehicles on the world's roads right now, and we can't realistically replace enough of them with solar-fed Teslas in time to keep global temperatures in a safe range.