Global dimming

Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth’s surface. Global dimming effect varies by location, but worldwide it has been estimated to be of the order of a 4% reduction over the three decades from 1960–1990.
Global dimming is a result of industrial pollution that produced sunlight-blocking aerosols. Because of the rapid rate of industrialization of China, India, and other Asian countries in the last few decades, there is still considerable global dimming today. Aerosols and other particulates absorb solar energy and reflect sunlight back into space rather than falling in to earth. The pollutants can also become nuclei for cloud droplets. Water droplets in clouds coalesce around the particles. Increased pollution causes more particulates and thereby creates clouds consisting of a greater number of smaller droplets (that is, the same amount of water is spread over more droplets). The smaller droplets make clouds more reflective, so that more incoming sunlight is reflected back into space and less reaches the earth’s surface. In models, these smaller droplets also decrease rainfall.Clouds intercept both heat from the sun and heat radiated from the Earth. Their effects are complex and vary in time, location, and altitude. Usually during the daytime the interception of sunlight predominates, giving a cooling effect; however, at night the re-radiation of heat to the Earth slows the Earth’s heat loss.
Some scientists now consider that the effects of global dimming have masked the effect of global warming to some extent and that resolving global dimming may therefore lead to increases in predictions of future temperature rise. According to Beate Liepert, “We lived in a global warming plus a global dimming world and now we are taking out global dimming. So we end up with the global warming world, which will be much worse than we thought it will be, much hotter.” The magnitude of this masking effect is one of the central problems in climate change with significant implications for future climate changes and policy responses to global warming.
Some scientists have suggested using aerosols to stave off the effects of global warming as an emergency geoengineering measure. In 1974, Mikhail Budyko suggested that if global warming became a problem, the planet could be cooled by burning sulfur in the stratosphere, which would create a haze. An increase in planetary albedo of just 0.5 percent is sufficient to halve the effect of a CO2 doubling.
The simplest solution would be to simply emit more sulfates, which would end up in troposphere – the lowest part of the atmosphere. If this was done, Earth would still face many problems, such as:
Using sulfates causes environmental problems such as acid rain
Using carbon black causes human health problems
Dimming causes ecological problems such as changes in evaporation and rainfall patterns
Droughts and/or increased rainfall cause problems for agriculture
Aerosol has a relatively short lifetime
The solution actually advocated is transporting sulfates into the next higher layer of the atmosphere – stratosphere. Aerosols in the stratosphere last years instead of weeks – so only a relatively smaller (though still large) amount of sulfate emissions would be necessary, and side effects would be less. This would require developing an efficient way to transport large amounts of gases into stratosphere, many of which have been proposed though none are known to be effective or economically viable.