"Estimates of the consequences of human-made and natural disasters are crucial for informing decision making by both public and private actors. The 9/11 attack stands out as a particularly important event whose consequences need to be well understood. This study evaluates the macroeconomic impacts of the 9/11 attack on U.S. real GDP growth and the unemployment rate by examining how forecasts of these variables were revised after the attack occurred. By this approach, the immediate impact of the 9/11 attack was to reduce real GDP growth in 2001 by 0.5%, and to increase the unemployment rate by 0.11% (reduce employment by 598,000 jobs.) Results are robust to controlling for how economic forecasts typically change over the course of the forecasting horizon in normal and recession years. Impacts on 2002 outcomes are more difficult to identify. Forecasted real GDP growth in 2002 fell dramatically immediately after the 9/11 attack but then recovered fully. The recovery in the forecast could have been due to unforeseen responses that mitigated the impact of the attack, but it also could have been due to erroneous forecasting and a poor understanding of how the attack would impact the economy. The forecasted unemployment rate in 2002 rose sharply immediately after the 9/11 attack, but unlike real GDP growth, it never subsequently returned to a pre-9/11 level. Forecasters seemed to have anticipated the 2002 'jobless recovery' early in that year."
United States Department of Homeland Security: http://www.dhs.gov/