Turning up the global thermostat will sap U.S. workers’ productivity, which could mean tens of billions of dollars a year in economic losses by the end of this century as the climate continues to warm, researchers predict. For every degree above 59 degrees Fahrenheit (15 degrees Celsius), average economic output in American counties dipped by nearly 1 percent, according to a report released Monday from the National Bureau of Economic Research, a Massachusetts-based research center whose work helps economists determine when the country is in a recession.

The authors of the report balanced daily temperatures in the U.S. over 40 years against economic data in counties across the country. What they found was that Americans’ income fell by about $5 on days with temperatures around 77 degrees Fahrenheit, compared with days that averaged in the high 50s. Although most U.S. workers spend their 9-to-5 jobs in climate-controlled environments, they still step outdoors during work hours, a routine that in warm weather raises body temperature and makes people tire faster. The effect can last for some time even after workers return to their air-conditioned workplaces.  

"Hot temperatures are very bad for the economy," Tatyana Deryugina, a professor of finance at the University of Illinois and the study’s co-author, told the Associated Press. Workers’ output peaked when daily temperatures hovered between 54 and 59 degrees, according to Deryugina’s colleague Solomon Hsiang of the University of California, Berkeley.

This year -- like many recent ones -- is shaping up to be the warmest year yet again on the books since accurate record-keeping began 123 years ago. Climatologists expect a chart-buster year if the final months rank among the top five warmest months of recorded time. That is likely to be the case, according to scientists. “If we continue a consistent departure from average for the rest of 2014, we will edge out 2010 as the warmest year on record,” Jake Crouch, a climatologist with the National Oceanic and Atmospheric Administration’s National Climatic Data Center, said during a press conference in September.

Thermometer readings around the globe have been steadily rising since 1975 at a rate of about .27 degrees to .36 degrees Fahrenheit per year, according to NASA. Overall, Earth has warmed about 1.4 degrees Fahrenheit over the past 12 decades. Most warming has occurred on land, which absorbs heat faster than oceans. Ocean temperatures have increased about .18 degrees Fahrenheit.  

The warming is not distributed at all equally. “In the past decade, land temperature changes are 50 percent greater in the United States than ocean temperature changes; two to three times greater in Eurasia; and three to four times greater in the Arctic and the Antarctic Peninsula,” the space administration reported. The Arctic Ocean has seen the most surface warming of any ocean, followed by the Indian and Western Pacific. By 2100, climatologists predict average global temperatures to increase by as much as 11.5 degrees Fahrenheit, depending on whether countries can rein in greenhouse gas emissions.

Previous research has shown that workers in several sectors of the economy are less productive when the weather warms. Labor-intensive corners of the workforce averaged about a 2.4 percent decrease in output for every degree Celsius. After the catastrophic March 2011 earthquake-tsunami in Japan and Fukushima reactor meltdown, most of the country’s nuclear power was taken offline. The government required offices to keep their indoor temperatures at a tropical 82.4 degrees Fahrenheit. In that heat, Japanese workers accomplished, on average, 30 minutes less work, according to research by Shin-ichi Tanabe of Waseda University in Tokyo. Symptoms associated with higher office temperatures include headache and lethargy, the New York Times reported in 2012. Other studies have linked higher humidity, a familiar part of summer in many parts of the world, with lowered concentration and increased drowsiness.

Hsiang described the link between warmer weather and sloth as “not like a hurricane. ... This is more like a story of deaths by a thousand cuts,” he told the AP.

Scientists say global warming means more extreme weather events, including powerful tropical storms, coastal flooding caused by rising sea levels, greater spread of some infectious diseases and species extinction. In the U.S., the economic toll of climate change from effects like flooding in the South and increased storm surges in the Northeast could cost the economy billions of dollars in damages and lost opportunity, according to a 2013 report from the Federal National Climate Assessment.