So it's possible to ask: how successful has activism around global warming been? This has two sub-questions: 1) is the world on track towards decarbonization, 2) if so, did activism bring this about or speed the process up. As far as I can tell, the answers are equivocally yes to the first and no to the second.
In a sense there were always two basic models of how decarbonization might happen. The first is the activist, or political model, in which people respond to science by organizing themselves and effectively demanding political change. The second is the technocratic, or techno-optimist, one in which experts respond to science by investing more and more money into development of renewable power sources so that they become cheaper than fossil, after which fossil gets replaced by the planners who actually control critical infrastructure. We appear to be on track for the second: renewable power is now cheaper than coal without subsidies and without even pricing in coal's externalities. Once it gets even more cheap, and with another round of battery development, I think it's on track to replace gasoline in cars as well.
How did this happen? Part of it is physics and engineering: it turned out to be technically possible. Part of it is that activists were never able to overcome resistance by elites and by national populations for whom this never became a core political issue. Neither one of these was inevitable. The history of this is waiting to be written, but I suspect that important turning points are going to be:
1. Formation of the IPCC. For experts to respond to science, science has to be very well founded. The IPCC reports are pretty much inarguable, scientifically.
2. Poor elite resistance to subsidies. It's easy for elites to stop an industry from being shut down, but it's difficult for them to prevent subsidies for new industries from being added. The machinery of local interest, political set-asides and so on has purposefully been made easy to run because it normally favors elites, and ways of stopping it were made difficult. This resulted in the early round of funding for renewable power.
3. China's investment in solar panels. To make new technologies cheap you have to ramp up production. This was done by Chinese state fiat -- as with almost all energy infrastructure, the market really had little to do with it. The Chinese state had the capital to do this and the ability to take speculative risks that, in actuality, capitalist multinationals are almost never willing to take. Someone who knows more than I do will have to figure out whether this was primarily due to industrial policy / support for national industry, as a way of combating Chinese coal air pollution, as an actual way to address this problem, or whatever.
4. Possibly, the Paris Agreements. Not that they actually agreed to do anything definitive, but they agreed on something more important: that the science was settled and the problem had to be addressed. It was pretty much the death knell of international denialism.
Why did activism, broadly speaking, fail? Part of it was industry support for denialism, and the concomitant tribal adoption of it as a position of the right wing in the U.S. But this is a huge, international problem, and the left in the neoliberal era really didn't have an international presence. There was no organization that was critical to people's lives for them to accrete around on this issue.
Look at what is happening now in the U.S.: both one of the more conservative countries on the planet and one of the most influential. We just had an election in which global warming policy was one of the clearest differences between the candidates, and it wasn't important. And right now if the left is unified around anything, it's unified in support of the DAPL protests, but this protest is highly fragile -- I recognize the current state of it from the Occupy days. Once people who will support a protest are all in, that defines the boundary of who will respond to a protest being quashed. If the state fully comes in and destroys the protest, there will not be an uprising of additional support from people angered by the police action, there will only be resistance from the people who are already supporters. More generally, Obama's neoliberal era resulted in a lot of incremental, executive-power advances with no popular organization backing them, and when Trump takes power, there are no effective barriers to them being reversed, whether it's lowering car fleet mileage requirements, using the Clean Air Act, or starting up Keystone XL again. There are also ongoing efforts at "inside game" activism: pressuring corporations to make changes, divestment focussed on energy companies, and so on, which seem to me to have some effect but not yet a large one.
Activism is, of course, ongoing. It can't be dropped: the final results of how much warming we get are highly dependent on how soon decarbonization happens and the decisions made in the next decade. If activism can speed that up at all, it has to be done. And protests like #NoDAPL are local and have very important local effects that can't be abandoned. But both the elites and the general public have made activism around this quite difficult, and I think that it's likely that it's going to be one of those things that had to be tried but that in the end didn't have a critical effect.