In response to the global warming problem, there has been a recent renewed call for geoengineering "solutions" involving injecting particles into the stratosphere or blocking sunlight with satellites between the Sun and Earth. While simple calculations have shown that such measures would cool the planet on a global average basis, volcanic eruptions, the closest analogs we have to these proposed schemes, also produce large regional climate changes in temperature and precipitation. In this talk, I describe different proposed geoengineering designs, and then show climate model calculations that evaluate both their efficacy and their possible adverse consequences. We simulated the climate response to both tropical and Arctic stratospheric injection of sulfate aerosol precursors using a comprehensive atmosphere-ocean general circulation model, the National Aeronautics and Space Administration Goddard Institute for Space Studies ModelE. We simulate the injection of SO₂ and the model converts it to sulfate aerosols, transports them and removes them through dry and wet deposition, and calculates the climate response to the radiative forcing from the aerosols. We conducted simulations of future climate with the Intergovernmental Panel on Climate Change A1B business-as-usual scenario both with and without geoengineering, and compare the results. We found that if there were a way to continuously inject SO₂ into the lower stratosphere, it would produce global cooling. Tropical SO₂ injection would produce sustained cooling over most of the world, with more cooling over continents. Arctic SO₂ injection would not just cool the Arctic. Both tropical and Arctic SO₂ injection would disrupt the Asian and African summer monsoons, reducing precipitation to the food supply for billions of people. These regional climate anomalies are but one of many reasons why geoengineering may be a bad idea. I will also delineate 20 reasons why geoengineering is a bad idea. Global efforts to mitigate anthropogenic greenhouse gas and aerosol emissions and to adapt to climate change are a much better way to channel our resources to address anthropogenic global warming.