There are two good reasons to be very excited about the latest breakthrough in nuclear fusion technology announced this week. Not only does the generation of net positive energy in a fusion reaction represent an outstanding scientific achievement, it also opens up the tantalising possibility of cheap, green and abundant energy that might put the environmentally ruinous fossil fuel industry out of business.

But there are also two good reasons not to be overly excited by the news. Even with a favourable wind, it will take at least 20 years of development and massive injections of capital before fusion energy can ever become commercially viable at scale. Unfortunately, that means it is not likely to arrive quickly enough to solve the imminent threat of the climate emergency. If so, the question arises: is the technology worth backing at all if alternative clean energy sources, such as solar, wind and next-generation nuclear fission, are already proven and available?

There was no shortage of excitement on Tuesday when Jennifer Granholm, the US energy secretary, confirmed the experimental breakthrough, first reported in the FT. For the first time in history, scientists at the Lawrence Livermore National Laboratory in California had succeeded in generating more energy in a fusion reaction than was consumed by the laser beams that triggered it. “Simply put, this is one of the most impressive scientific feats of the 21st century,” Granholm said.