One way to reduce temperatures from Global Warming and greenhouse gases is to create artificial Pyroclastic Volcanic ash into the atmosphere

However, this can create other problems too. One of the reasons this can be a problem is that the big countries doing this might be okay the way they engineered this. However, downwind of that country it might create too much or too little sunlight other places where the plants won't grow right or it could also affect the rain coming down too. 

Do we know if countries are already doing this? No. I think Countries capable of this like China, Russia, the U.S. and EU would not talk about it if they were doing this and that there would be many weather anomalies.

For example, if some state were being helped by this ( in the U.S.) other states might suffer in various ways either with too much rain or not enough or various kinds of storms. So, if these things were done in secret to help certain areas other areas on earth might suffer. This could be happening now at random around the world to lessen the effects of Global Climate change and Global Warming or could be used in the future whether people are told about this or not. 

However, it is the richer nations who could afford to do this.

What would be the cheapest way to do this likely?

To fill the  the fine pyroclastic glass dust into weather helium or hydrogen balloons and let them go up into the atmosphere where they would eventually explode from the altitude. 

Or you could put vents in the tail sections of passenger jets well behind the jet engines because this fine glass in jet engines could make them non-operational. However, in remote areas outside of Jet lanes of traffic in the sky this could already be happening in order to reduce temperatures worldwide by blocking out the sun to the ground in those areas.

NOTE: it isn't the entire blocking out of the sun but only reducing the heat and light generated by the sun before it hits the earth sort of like artificial clouds to reduce temperatures worldwide. However, it is also true that global Climate change will increase moisture in the air in rain clouds by about 5 times presently on earth. 

So, already there is 5 times the risk of flooding everywhere because warmer air can carry up to 5 or more times the moisture already. So, places where it rains a lot are pretty much sure to flood at least in the floodplains more. So, it might be a good time not to live right on a river around the world which is a problem because most big cities on earth are alongside one river or another. The oceans rising from Ice melt is a problem too worldwide because all the major oceans are connected. So, if Ice melts at Antarctica it affects ocean rise worldwide.

I think it's not IF but WHen this is or will be done. (the reducing of temperatures through pyroclastic ash)

 Volcanic Ash: Tiny, jagged particles of rock and glass erupted into the air, according to the USGS (.gov). Fine ash particles can be carried by winds for vast distances, even circling the Earth. While larger ash falls quickly, the smallest particles can linger in the stratosphere for months, affecting sunlight and cooling the planet.