Ironically, only unrestrained nuclear war could possibly save us now.

1. It would move untold gigatonnes of dust into the atmosphere, cutting down on solar radiation in the short term
2. it would destroy high-tech fossil fuel consumption and most human-caused CO2 production in the short to medium term
3. said dust would slowly fall out of the atmosphere over the next decade, most into the oceans, releasing phytoplankton from their limiting environmental factors (mainly a lack of iron)
4. even with reduced sunlight, phytoplankton populations would explode, sucking significant CO2 out of the atmosphere
5. an extended nuclear winter would produce thin ice sheets across most of the northern hemisphere, dramatically increasing the planet’s albedo once the atmosphere clears up, reflecting most incoming radiation back out and (hopefully) maintaining lower temperatures
6. lower temperatures planet-wide would produce a much wetter climate, with much more snowfall and more precipitation in arid areas, encouraging increased carbon sequestration by plants.
7. human populations would crash massively in the first year or three, but - especially in the southern hemisphere - would remain present in relative technological sophistication. We could conceivably stabilize in the very low billion level or high hundreds of millions, with the technological knowledge to rebuild a high-tech civilization without the extensive use of fossil fuels.