Environment News Service: A brown cloud of pollution caused by human activities, three kilometers thick and stretching from the Arabian Peninsula across Asia to the western Pacific Ocean, is darkening cities, speeding the melting of Himalayan glaciers and affecting human health, finds a new report from the United Nations Environment Programme. Atmospheric brown clouds, formed by the burning of fossil fuels, biofuels, wood and plants, absorb sunlight and heat the air, experts write in the study released today …