Over the weekend, the National Weather Service issued an excessive heat warning across a huge swath of the country, putting 132 million people under a heat alert . This warning is only issued when a heat index of at least 105°F is expected for more than three hours per day on two consecutive days or when the heat index is expected to rise above 115°F for any length of time. Recently in Iowa, the heat index reached 131°F , a level normally found only along the Red Sea in the Middle East. Scientists warn that these types of events could become much more common in the future, thanks to climate change.
The heat index takes into account both temperature and humidity in an attempt to determine how hot it feels to people and how dangerous the heat level could be to human health. So that old weather expression – it’s not the heat, it’s the humidity – gets it half right; it’s really the combination that matters.
Heat index values are affected by climate change in two ways – first by raising temperatures and second by increasing the amount of moisture in the atmosphere. These two effects increase the risk of high humidity heat waves like the one we have just experienced. Moreover, because atmospheric moisture traps heat near the ground, nighttime temperatures are higher during humid heat waves compared to dry heat waves. This increases the risk of health problems since people without access to air conditioning are unable to cool down at night and recover from the heat of the day. According to the U.S. Global Change Research Program, the last three to four decades have seen an increasing trend in high-humidity heat waves  with extremely high nighttime temperatures, and this change is consistent with what scientists expect from climate change.
In all 1,966 daily high temperature records, with 66 of those also being all time records, were broken between July 1 and July 23. In addition, 4,376 daily high minimum temperature records were broken. Of those, 158 also surpassed all-time high minimum records. On July 22, Newark, New Jersey, and Dulles, Virginia, set all time record-high temperatures  at 108°F and 105°F respectively with heat index values hovering around 115°F.
Heat waves are not just uncomfortable, they bring significant public health risks. The CDC recommends drinking extra water, avoiding alcohol or sugary drinks, and taking cold showers to reduce the risk of heat related illnesses. Those who are most at risk, such as the elderly, should be checked on twice per day.
The 1995 Chicago heat wave killed 700 people , mostly vulnerable elderly residents. Since then, Chicago authorities have adapted by setting up cooling centers around the city and encouraging residents to go to air conditioned public buildings. In addition, any time temperatures rise above 90°F, city workers check on the frail and elderly to ensure that their needs are met. These public health programs are an excellent example of the type of adaptive responses we need to take now to reduce the costs (and deaths) from our changing climate. The recent heat wave is being blamed for 34 deaths , but this is a 95 percent reduction in heat-related deaths compared to 1995. This demonstrates the importance of risk management and adaptation in dealing with climate change.
The C2ES maintains a weather events map  that highlights the types of events that are consistent with what scientists say we should expect from increasing greenhouse gas concentrations. 2011 has been an extreme year already, with floods, wildfire, and drought already making an appearance on the map.
Dan Huber is Science & Policy Fellow