The U.N. climate report released Monday presents a major leap forward in predicting how geoengineering to limit global warming might affect the planet, although scientists said the greatest hurdle remains deciding whether to use the controversial methods. Geoengineering involves large-scale interventions that shift the climate, generally with an aim of cooling the earth. The United Nations panel addressed two types of geoengineering in the report - solar radiation management and greenhouse gas removal. Solar radiation management techniques generally control how much sunlight is reflected back out into space. For example, humans could spray sulfate aerosols - tiny reflective particles - into the stratosphere ... to reflect more sunlight back into space, which lowers global temperatures. But sulfate aerosols have the side effect of also lowering average precipitation. While some regions could gain in an artificially cooler world, others could suffer by, for example, no longer having conditions to grow crops.