The New Socialism
Socialists have long wanted to impose extra taxes, control the economy and redistribute wealth to the third world on the basis of alleged wrongs done to the third worlds by industrialized nations. The "climate change" movement offers them all of this.
This explains why leftists almost universally have signed up to it.
UPDATE: One reader argues that a key difference between classical socialism and the new enviro-socialism is that classical socialism at least claimed to want to improve people's life, while the new enviro-socialism demands that people lower their standard of living. That is true for many or even most enviro-socialists, but quite a few of them is in fact arguing (especially here in Sweden) that the new "carbon free" energy will ultimately deliver higher productivity.
Update II: Another reader gives the following sarcastic review of enviro-socialism:
The enviro-socialists are effecting a Kuhnian paradigm shift in the socialist model. This revolution in socialist science derives from their discovery of an essential aspect of socialism that Marx and his intellectual heir could not have known until recently.
Marx thought that communism would bring about universal material prosperity, but a century of experience demonstrates that socialism results in widespread material poverty. In the mid-20th century, the Frankfurt School explained the failure of the proletariat to embrace Marxism due to a “false consciousness” brought about largely by the material success it enjoys under capitalism. However, this explanation hardly inspired the workers to throw off their chains.
The contribution of the enviro-socialists is that they discovered that widespread poverty isn’t a bug in socialism; it’s a feature.