Abnormally hot days could have consequences for everyday thinking, a new study from the Yale School of Public Health warns.
After analyzing data from eight years of cognitive tests administered to teenagers and adolescents across China and then matching the test results to local weather data at the exact time of the test administration, researchers discovered that high-temperature days were associated with reduced scores on math tests. The lower test scores represented a learning deficit equivalent to as much as a quarter of a year of education, the researchers said.
A pre-proof version of the study manuscript can be found in the Journal of the Association of Environmental and Resource Economists.
The findings are believed to be the first to identify a potential association between extremely warm weather events and low-stakes cognitive ability in people aged 10 to 99 years, the researchers said. Previous studies have found similar trends among children taking high-stakes exams in warm weather, while there has been little evidence for young adults, middle-aged, and older adults.
I think the way we try to uncover the cognition and decision-making costs will get us closer to the real economic cost of high temperatures.
Xi Chen, associate professor of public health
The researchers defined abnormal hot weather events as a daily mean temperature exceeding 32 degrees Celsius or 89.6 degrees Fahrenheit. Low-stakes cognitive activities include routine exams given periodically to assess a person’s knowledge of a subject or straightforward cognitive assessments given to the public. High-stakes cognitive activities may involve critical decision making or larger tests with critical importance such as scholastic aptitude tests that could determine whether a student progresses to another grade or higher level of schooling like college. Exposed to heat waves, the lower test scores were identified only in math tests in the study and not in other subject areas.
Still, with the frequency of heat waves increasing due to climate change, the findings raise a potential concern about diminished thinking capacity in hot climates and the subsequent economic costs that may manifest in the future as a result, said Xi Chen, an associate professor of public health (health policy) at the Yale School of Public Health and the paper’s corresponding author.
“I think the way we try to uncover the cognition and decision-making costs will get us closer to the real economic cost of high temperatures,” Chen said.
The research team relied on data from a battery of exams given to young people in their homes across China. The tests use mathematics and word-recognition questions of increasing difficulty to assess respondents’ knowledge and skills. The data included information on when and where the tests took place, along with pertinent details about the test takers' housing arrangements. The researchers then cross-referenced those results with weather data provided by the Chinese government, while controlling for the daily average temperature.
The researchers employed an array of control measures to avoid confounding variables and bias. They used statistics to show that the cognitive deficits could not be explained away by the test takers being impatient, uncooperative, or more rushed because of the uncomfortable heat, or that air pollution or weather factors other than mean temperature could better account for the results. “We tried to do our best to rule out other channels,” Chen said.
The findings revealed that a test day average temperature above 32 degrees Celsius (89.6 F) — compared to a day around 22-24 degrees Celsius (71.6 - 75.2 F) — reduced math test scores by 0.066 standard deviations, the equivalent of just under a quarter of a year of schooling.
That deficit is significant, Chen said, especially when learning inequities have already been an issue among different age brackets in China and many other nations, and when the overall level of education in some vulnerable communities has tended to be low.
Distinguishing hotter areas from cooler areas of China according to the county-level average summer temperature in history, the test score reductions were most acute among individuals living in cooler areas. Those who lived in hotter areas experienced less of a deficit when the temperature rose significantly. This suggests that those individuals were acclimated to a warmer climate and could therefore perform better on the tests. Still, even individuals in warmer areas felt the negative effects of abnormally high temperatures to some degree, the researcher said.
People who took tests in air-conditioned locations fared better on their scores but were still impacted by abnormally hot days, the study found. Air conditioning reduced the harmful cognitive effects of abnormally hot weather by 36.6 percent, the study found, but it did not eliminate them. “It’s still very helpful,” Chen said of the temperature-controlled environment.
The researchers acknowledged limitations to their analysis. Because the tests were administered during the summer months, the researchers could not determine the effect of abnormal cold spells on cognitive ability. Even though the tests covered both math and verbal skills, the researchers only found a statistically significant heat-related impact on the math results. And it remains to be seen if the same cognitive deficits appear in areas other than China.
Still, the findings present potentially worrisome questions about the quality of routine arithmetic decisions – from calculating bills to choosing retirement plans – as the planet warms in the future.
“Every day, we make all kinds of decisions, and some decisions are really critical, determining the rest of our lives, and some are less high-stakes but still determine our well-being,” Chen said. “Once we can see that even day-to-day decision-making is affected, then we can see that the cost of climate change is absolutely beyond what we have understood.”