https://www.forbes.com/sites/trevornace/2018/12/05/harvard-scientists-begin-experiment-to-block-out-the-sun/

Harvard Scientists Begin Experiment To Block Out The Sun

A group of Harvard scientists plans to tackle climate change through geoengineering by blocking out the sun. The concept of artificially reflecting sunlight has been around for decades, yet this will be the first real attempt at controlling Earth's temperature through solar engineering.

The project, called Stratospheric Controlled Perturbation Experiment (SCoPEx), will spend $3 million to test their models by launching a steerable balloon in the southwest US 20 kilometers into the stratosphere. Once the balloon is in place, it will release small particles of calcium carbonate. Plans are in place to begin the launch as early as the spring of 2019.

The basis around this experiment is from studying the effects of large volcanic eruptions on the planet's temperature. In 1991, Mount Pinatubo in the Philippines erupted spectacularly, releasing 20 million tonnes of sulfur dioxide into the stratosphere. The sulfur dioxide created a blanket around Earth's stratosphere, cooling the entire planet by 0.5 °C for around a year and a half.