The global transition from fossil fuels to renewable energy sources is widely regarded (0) AS one of the defining challenges of the twenty-first century. Wind, solar, and hydroelectric power have all seen significant growth in recent years, 1 part because of advances in technology and falling production costs. In several countries, renewable sources now account 2 a substantial proportion of the total energy supply. The shift away from fossil fuels is 3 from complete, however, and many nations remain heavily dependent on coal, oil, and natural gas. One of the main obstacles is the intermittent nature of renewable energy — the sun does not always shine, 4 does the wind always blow. This makes it difficult to guarantee a stable supply of electricity 5 all times. Battery storage technology is improving rapidly, but it has not 6 reached the point where it can fully compensate for these fluctuations. Experts believe that a combination of different renewable sources, along 7 investment in smarter energy grids, will be essential if we are to meet our climate targets. The question is 8 longer whether we should act, but how quickly we can do so.