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Climate Change Driving Increase in Black Flag Days at 100 U.S. Military Installations

US Military and Extreme Heat

An Army Ranger trainee completes a 12-mile march at Fort Benning in Georgia while wearing heat sensors under his uniform to measure his core temperature and heart rate. (Brock Stoneham/NBC News)

By Marc Kodack

Elevated temperatures from climate change will adversely affect the health of military personnel. The increased heat that will occur over the next 30 years will affect multiple installations in the U.S. On average, these installations will experience additional days of high heat conditions according to research by the Union of Concerned Scientists (UCS).

UCS examined different climate scenarios to calculate the number of hot days for 169 installations in the U.S. across the Services. They found that without any reductions in global CO2 levels by 2050, the average installation will experience 33 additional days with a heat index above 100 degrees Fahrenheit. According to the report:

Historically, only nine major military installations in the US have experienced 30 or more days per year with a heat index above 1000F. By midcentury, with no action to reduce emissions, 100 installation would experience such conditions.

Of the top 10 installations across all Services that would experience the largest increase in days with a heat index above 1000F, five occur in Florida, two in Mississippi, two in Texas, and one in Louisiana. For example, the top installation, Homestead Air Reserve Base, Florida, historically has had 13 days with a heat index above 1000F. By 2050, it may experience 115 days.

For the eight installations where basic training occurs across the Services, by 2050: “they are projected to experience an average of an additional six weeks per year with a heat index above 1000F.” The implications for training are that it will be interrupted or curtailed to avoid heat illness among personnel.

The Navy,  Marines, and Air Force use the WetBulb Global Temperature (WBGT) rather than the heat index which is calculated in the shade with light wind conditions for air temperature and relative humidity. When the heat index is calculated in direct sunlight, heat index values can increase by up to 150F. The WBGT combines temperature, humidity, wind speed, sun angle, and cloud cover in direct sunlight to determine if people can safely work outside in hot, humid conditions. When black flag conditions exist, all physical training and strenuous exercise is suspended. The Army also uses the WGBT, but when risk/flag categories 4 (red flag) or 5 (black flag) occur all outdoor or training activities should be avoided. While the heat index and WGBT incorporate different variables, the UCS research considers them equivalent.

UCS provides a top 10 list for each individual Service of the installations that will be most affected by an increased number of days where the heat index will be above 1000F. By contrast, in the Department of Defense’s report to Congress on the effects of climate change to DoD installations, days where the heat index is above a certain temperature is not one of the climate change-related events that was considered for any installation. However, a few installations in DoD’s individual Service lists, also appear in the UCS top 10 lists, including the following:

  • Naval Submarine Base Kings Bay, GA; expected to experience 73 days compared to a historic 12 days
  • Norfolk Naval Shipyard, VA, assuming this is Naval Station Norfolk in DoD’s list; expected to experience 42 days compared to a historic 9 days
  • Patrick Air Force Base, FL; expected to experience 72 days compared to a historic 4 days
  • Lackland Air Force Base, TX; listed by DoD as part of Joint Base San Antonio; expected to experience 105 days compared to a historic 36 days
  • Fort Hood, TX; expected to experience 90 days compared to a historic 25 days

The research recommendations include updating heat-related Service guidelines to include forecasts of increased heat; all personnel, but particularly leaders, should be aware of the health consequences of heat related illness and how it can be prevented during physical exertion including training; innovation in equipment that can cool personnel during strenuous activities should be explored, and families living on installations should also be protected from extreme heat.

As future forecasted temperatures rise into levels that can be unhealthy for military personnel when they are outdoors exercising or training, the military will be challenged to keep these personnel safe. Not least as  active component military personnel already experience heat exhaustion and potentially life-threatening heat stroke. If climate change increases the number of days when heat conditions prevent any training, maintaining readiness may become problematic if black flag days overwhelm the scheduled number of days available for a unit to train. Alternative locations may not be available depending on the scale of the training required. Training under realistic condition may become more difficult which can lead to higher risks in personnel and equipment availability and preparedness. Thus, military ocliptions to meet national security threats will require innovative strategies that reduce readiness risks from climate change to acceptable levels.

See two of our previous blog articles on the subject from Marc Kodack and Christine Parthemore. Also see our briefer from Mariah Furtek on heat implications for military aircraft, as well as Chris Gaulin’s excellent article in our Climate and Security Fellowship Program’s “Risk Briefers” publication, titled “Incorporating Heat Stress as a Risk to Military Readiness.”

For policy recommendations to the DoD (and other U.S. government agencies) for addressing climate change risks writ large, see our Climate Security Plan for America.


  1. Chris Gaulin says:

    Great stuff! I believe there was also a short briefer from one of last years CSAG Fellows on this in the end-of-program publication…

  2. Caitlin Werrell and Francesco Femia says:

    Thanks Chris! We’ve updated the article (see the closing) to include a reference to your great article “Incorporating Heat Stress as a Risk to Military Readiness.”

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