I don't think human psychology will allow a united approach to global warming. Too man people are too stupid, to egoistic or just in such a tight spot they can not afford being "climate-friendly".

So I wonder if there are Mega-Projects available to stop global warming?

Some coming to mind:

Reflector mirror at Lagrange1 between Sun and Earth - even a 1.000.000km² mirror from ultrathin film would weigh 1000 tonnes at most.

Giant Air Scrubers and I mean giant. They would dwarf the pyramids and remove pullutants and CO² from the athmosphere.

In addition I think a originator principle should become common: Any nation not really trying to act clean should simply be burdened with massive tariff measures. Using modern technology it shouldn't be too hard to find nations who polute the ocean unnececerrally.

Edit, it became reality:

USA CO² Scrubber german

Project Cypress english