The present-day climate left was effectively born, in the United States, with the November 2018 Sunrise Movement sit-in. At the time, hardly anyone on the planet had heard of Greta Thunberg, who had just begun striking outside Swedish Parliament — a lonely, socially awkward 15-year-old holding up a single sign. Not four years later, her existential rhetoric is routinely echoed by presidents and prime ministers and C.E.O.s and secretaries general, and more than 80 percent of the world’s economic activity and emissions are now, theoretically, governed by net-zero pledges pointing the way to a carbon-neutral future in just decades.