By 2042 the Chinese were desperate. The Vietnamese, Indonesians and Bangladeshis were beyond desperate. Sea level rise was drowning their river deltas and low lying coasts, and half the time the rice crop that wasn't lost to flooding was failing because of the intense heat.
Yet the counties of higher latitudes, which weren't suffering nearly as much from the changes, were still paralyzed by their domestic politics. Many people in Europe and North America were ready to try some geo-engineering to stop the warming, but the Greens in every country bitterly opposed it. They thought it was too dangerous. It was a distraction from the real job of getting the emissions down, and it was tampering with nature. The governments feared eco-terrorist attacks.
So China and the tropical countries decided to act by themselves. China supplied the balloons, and Vietnam and Indonesia provided the launching sites. There were over a hundred of them, each releasing up to a dozen balloons a day, each balloon bearing a load of several tons of sulpher up into the stratosphere. Within a couple of months, the sulphate particles were dispersed all around the world in the stratosphere, where they would remain for several years, reflecting some of the incoming sunlight back into space.
There was outrage in western countries when the tropical countries started putting sulpher dioxide into the stratosphere, but with China backing them, they were safe from western military action. And for a while, the project went well. The goal was to drop the average global temperature by two and a half degrees, all the way back down to what it used to be in 2010 in order to stabilize the ice sheets.
Everything was coming along nicely until Mount Pinatubo erupted again, a bigger eruption this time, enough to drop the temperature another two degrees. The Vietnamese and Indonesians stopped sending up the balloons immediately, but the combination of two degrees cooling from the volcano and two and half degrees from the geo-engineering, added up to a sudden cooling of four and a half degrees. Many countries had switched to more heat tolerant plants to withstand the warming, so there were crop failures worldwide, and hundreds of millions of people starved to death.
The temperature went back up within two years, but several things had changed irrevocably. Many more states had failed in the chaos of the famine, and one third of the human race lived and died in places where there was effectively no government. Geo-engineering became a completely taboo subject in the west, and several major western countries let it be known that they would use nuclear weapons against any country that tried it again, even China.
There are, as yet, no alternative ways to avoid runaway warming.