The oceans are warming up alongside the rest of the planet, reaching record temperatures not seen in thousands of years. Two recent studies found global warming is causing temperature changes in the surface of the ocean, as well as the freezing layers at its bottom.

A team of researchers from universities in the US and Canada found that the Atlantic Ocean has reached its hottest temperature in 2,900 years. They looked at the “multidecadal sea surface temperature variability” (AMV), which are naturally-occurring warm and cool phases that can last up to 40 years at a time.

The study looked at temperature data as well as ice and sediment cores, drilled from ice sheets and the sea bed. This allowed scientists to sift through layers of microbes and chemical clues to reconstruct the past. The samples were taken from South Sawtooth Lake, an 80-meter-deep lake in the Canadian Arctic.

The researchers looked at titanium in the sediment cores to discover the story of the past 2,900 years in Atlantic Ocean temperatures, revealing variability from decade to decade. They found warm conditions from 100BC to AD420, followed by the period with the longest and most persistent drop in temperatures in the Little Ice Age.

“Temperatures have steadily increased since the 15th-century minimum; the rate and magnitude of warming over the last few centuries are unprecedented in the entire record, leading to the last decade which was the warmest” in the past 2,900 years, the scientists wrote.

They also compared other evidence from across the North Atlantic, which backed their findings at the Sawtooth Lake. They observed that other sediment cores collected on Iceland’s coast gave clues about what was happening over a shorter time period, roughly the past two centuries.

They found that Turborotalita quinqueloba, minuscule, shelled, single-celled organisms which like cold waters, have “been declining at an accelerating pace during the past century and reached unprecedented low values in the last decade”. This led them to conclude that the recent Atlantic warming is “unparalleled” in at least 2,900 years.

The study was published in the journal PNAS

Warming at the bottom of the ocean

Another study found that the freezing waters at the bottom of the ocean are also warming up, with temperatures rising quicker than previously thought. Between 2009 and 2019, water temperatures at four different depths in the Atlantic Ocean warmed by 0.002-0.04ºC. This might seem minuscule, but it’s actually quite significant.

“If you think about how large the deep ocean is, it’s an enormous amount of heat,” Christopher Meinen, an oceanographer at the US National Oceanic and Atmospheric Administration (NOAA) and lead author of the study, told The Guardian. He said the findings are consistent with human-caused climate change.

The data was obtained from a set of instruments scientists had been using for years to study ocean currents. The scientists concluded that deep ocean temperatures need to be taken at least yearly to understand long-term trends. They hope the study will prompt others to examine their own temperature data from existing instruments. A better system for observing the ocean could help scientists forecast seasonal weather, Meinen said.

The study was published in the journal Geophysical Research Letters.