Even the pitch black, nearly freezing waters at the bottom of the ocean – far from where humans live and burn fossil fuels – are slowly warming, according to a study of a decade of hourly measurements.

The temperatures are rising quicker than previously thought, as recorded at stations at four different depths in the Atlantic Ocean off the coast of Uruguay. Between 2009 and 2019, the water there at points between 1,360m (4,462ft) and 4,757m deep warmed by 0.02-0.04C.

The change may seem minuscule, but it is significant.

“If you think about how large the deep ocean is, it’s an enormous amount of heat,” said Christopher Meinen, an oceanographer at the US National Oceanic and Atmospheric Administration (NOAA) and lead author of the study, published in the journal Geophysical Research Letters.

While the general consensus has been that the deep ocean is warming, scientists have had to rely on a snapshot of data collected every 10 years from research vessels. Climate models have found that high levels of climate pollution by the end of the century will penetrate deeper in the ocean, threatening deep sea creatures.

Roughly 90% of the heat absorbed by the Earth goes into the oceans. Although they warm slowly, the heat makes water molecules expand, contributing to sea-level rise. It also intensifies hurricanes.

By comparison, the global land and ocean surface temperatures are heating up much faster. In 2009, they were 0.56C higher than the long-term average. By 2019, they were 0.95C higher, according to NOAA data.

Meinen, who spoke for himself and not on behalf of the government agency, said the new findings are consistent with human-caused climate change. However, more research is needed to make definitive conclusions because there is not enough historical data on the deep ocean, which has not been studied as much as Earth’s atmosphere.

“We didn’t expect that you would see hour-to-hour and day-to-day variations down that deep,” Meinen said. “There are processes in the deep ocean that are making things change rapidly, and we don’t really know what those processes are yet.”

The research data came from a package of instruments scientists had been using for years to study ocean currents. After reading a study from the University of Rhode Island, the team realised the thick glass sphere weighted down by barbell plates also included a temperature sensor that was built into its pressure sensor.

The scientists also concluded that deep ocean temperatures must be taken at least yearly to understand long-term trends. They hope the study will prompt others to examine their own temperature data from similar instruments.

A better system for observing the ocean – including the deep ocean – could help scientists forecast seasonal weather so farmers can better choose which crops to plant, Meinen said.