immediately slow global warming

In the case of rapid action to slow catastrophic climate change, the best alternatives appear to be: methane and soot.

“If the world pays attention and puts resources to it, we will see an effect immediately. I’m talking weeks, at most a few months, not decades or centuries.” —atmospheric physicist Veerabhadran Ramanathan of Scripps

Eliminate methane releases from coal mines—particularly in China—by capturing it and burning it.
Eliminate the venting or accidental release of methane co-produced by oil drilling (and, of course, gas drilling itself), particularly in Africa, the Middle East and Russia.
Capture gas from landfills in the U.S. and China as well as promote recycling and composting of biodegradable trash.
Occasionally aerate flooded rice paddies to prevent the growth of methane-producing microbes.
Stop leaks from natural gas pipelines, particularly in Russia.
Use bio-digesters—vessels in which microbes break down manure into gas—to cut methane from livestock globally.
Update wastewater treatment plants to capture methane.
Filter the soot produced by incomplete combustion of diesel fuel in vehicles, and attempt to eliminate inefficient internal combustion engine vehicles entirely.
Replace indoor cooking and heating fires with clean-burning cookstoves fired either by wood, manure or other biomass or, even better, methane.
Replace traditional brick kilns with more advanced firing methods.
Replace traditional ovens for turning coal to coke with modern technologies.
Ban the open burning of crop stubble and other agricultural waste.