WASHINGTON – Rising temperatures are widening the racial achievement gap in U.S. schools, new research suggests, offering the latest evidence that the burdens of climate change fall disproportionately on people of color.

In a paper published Monday in the journal Nature Human Behavior, researchers found that students performed worse on standardized tests for every additional day of 80 degrees or higher, even after controlling for other factors. Those effects held across 58 countries, suggesting a fundamental link between heat exposure and reduced learning.

But when the researchers looked specifically at the U.S., using more granular data to break down the effect on test scores by race, they found something surprising: The detrimental impact of heat seemed to affect only Black and Hispanic students.

R. Jisung Park, the paper's lead author and an assistant professor of public policy at the University of California, Los Angeles, said the gap seemed to reflect the fact that minority students are less likely to have air-conditioning at school and at home. Being exposed to higher temperatures appears to take a gradual and cumulative toll on those students' ability to absorb their lessons, he said. "It's like a thousand little cuts to your ability to focus and concentrate and learn," Park said.

The findings are the newest addition to a body of research showing that climate change in general, and rising temperatures in particular, have a greater effect on minorities.

A study published in January found that a history of redlining — the long-discredited policy of marking minority neighborhoods as risky places for banks to lend money — and the underinvestment that goes along with it has left many Black neighborhoods with more paved areas and fewer trees. As a result, those neighborhoods were hotter than their white counterparts, leading to more cases of heat-related illnesses.

In June, research published in JAMA Network Open showed that pregnant women exposed to high temperatures or air pollution are more likely to have children who are premature, underweight or stillborn, and African American mothers and babies are harmed at a much higher rate than the population at large.

The link between heat and education has become an important part of that story.

In an earlier paper published in May, Park and his co-authors, including Joshua Goodman of Boston University, looked at the effects of heat on U.S. high school students. They examined 10 million students who took the PSATs twice and found that students did worse on the test that followed a year of higher temperatures.

They calculated that those effects were greater for minority students and estimated that heat exposure explained "roughly 5% of the racial achievement gap."

But high school students who take the PSATs are not representative of the entire U.S. student population. So in their new paper, Park and Goodman, along with A. Patrick Behrer of Stanford University, examined more than 270 million state-administered test scores for third to eighth-graders between 2009 and 2015.

They found that students who experienced more school days of 80 degrees or hotter in the year before their tests fared worse than their counterparts in the same school districts who took the tests in years with fewer hot days.

But that connection was true only for Black and Hispanic students, and for students with lower family income. For white students as a group, there was no statistically significant effect. (The data did not allow the researchers to look at race and income together, preventing them from determining the effect of heat exposure on test scores for more specific groups such as low-income white students.)

The findings could reflect differences outside school, including less access for minority students to tutoring to augment classroom lessons, Goodman said.

So the researchers separated hotter school days from hotter weekend or summer days. They found that the strongest effect on test scores were linked to higher temperatures on days when students were at school. "The same amount of outdoor heat makes certain classrooms hotter, just because their buildings are lower quality," Goodman said. "Low-income students are in school buildings that have worse HVAC and ventilation systems."

Unequal access to well-funded schools belongs to a long list of racial inequities that magnify the effects of climate change, said Heather McTeer Toney, a senior official at the Environmental Protection Agency under President Barack Obama. That list includes the fact that members of minority groups are more likely to live near toxic waste sites, exposing them to hazardous chemicals during floods, she said, as well as public housing developments that hold heat.

"We could go on and on, talking about different social dynamics that disproportionately impact communities of color," said McTeer Toney, who is now national field director for Moms Clean Air Force, an advocacy group. "For every single one of them, we can make a link to climate."