As the Soviet Union began to collapse, the socialist economist Robert Heilbroner admitted that central planning had failed economically but said we needed “to rethink the meaning of socialism.” Now it was the thing that had to emerge if humanity was to cope with “the one transcendent challenge that faces it within a thinkable timespan.” Heilbroner considered this one thing to be “the ecological burden that economic growth is placing on the environment.” Markets may be better at allocating resources, Heilbroner thought, but only socialism could avoid ecological disaster.

Not long after, however, it became clear that the socialist economies of …