If a nuclear war occurs, serious atmospheric consequences will result from the production of large amounts of particulate matter due to the many fires that will start in cities, industry, and particularly in forests, agricultural fields, and oil and gas fields. We estimate that for most of the Northern Hemisphere the average intensity of sunlight reaching the surface of the earth would be reduced by a large factor as a result of light absorption by particles in the submicron diameter size range, and this darkness would persist for as long as the fires burn, which is expected to be many weeks. The screening of sunlight by the fire-produced aerosol over extended periods during the growing season would eliminate much of the food production in the Northern Hemisphere. Marine ecosystems may be particularly sensitive to the loss of sunlight that would result. The fires would also inject large quantities of oxides of nitrogen and reactive hydrocarbons into the atmosphere, creating the potential for a severe photo-chemical smog situation throughout much of the Northern Hemisphere. The production of strong oxidants such as ozone and per-oxyacetylnitrate (PAN) as the aerosol is removed from the atmosphere would also negatively influence food production throughout the Northern Hemisphere during the first year following the war.