The emperor of Mongolia actually killed so many people - more than 40 million according to researchers - that Earth was cooled. All this killing withdrew about 700 millions of tons of carbon dioxide from the atmosphere and, because of that, the greenhouse effect was lessened. So you have an idea, this amount is equivalent of all the carbon dioxide emitted by cars of the whole world in one year! Some areas of Mongolia were even depopulated by Khan's army, and forests appeared where before there was only pasture or no plants at all.
Specialists claim that this was the first case of artificial global cooling in large scale in the world's history. We can take from that that humans CAN act in large scale to change the climate, and they HAVE done it for thousands of years, not just recently.
So, if you want to cool the planet, there's one way to do it.