These water molecules have been sitting untouched in the deep Pacific for 700 years

Some 700 years ago, before mankind began pumping carbon into the atmosphere and warming the climate, the Earth chilled in a centuries-long cooling event called the Little Ice Age.

Today, new research finds, the depths of the Pacific still hold memories of this colder time. Just over a mile (2 kilometers) down, the Pacific Ocean is getting a tad cooler as waters that were last at the surface during the Little Ice Age are only just now mixing with deeper, warmer waters.

This eerie echo of temperatures from a past era is important for modern climate scientists because the ocean's capacity to hold heat matters for what happens in the atmosphere and on land, said study researcher Jake Gebbie, a physical oceanographer at Woods Hole Oceanographic Institution in Massachusetts.

"If we're going to understand climate change," Gebbie told Live Science, "it's all about trying to study where heat and carbon move around the Earth system." [The Reality of Climate Change: 10 Myths Busted]

Diving deep

Gebbie and his colleague at Harvard University, Peter Huybers, had previously found that the deep waters of the Pacific are very old indeed. Below about 1.5 miles (2.5 km) beneath the surface, the waters of the deep Pacific last saw the surface around 1,000 years ago, the researchers reported in 2012. What this means, Gebbie said, is that you should be able to detect hints of what the past ocean surface was like by examining the ocean's deep waters.

The problem is that it's hard to study the bottom half of the ocean, Gebbie said. Since 2002, an international consortium called the Argo Program has used floating instruments to measure temperature, salinity and other ocean features around the globe; those instruments, however, don't go below 1.2 miles (2 km). The last global deep survey was something called the World Ocean Circulation Experiment in the 1990s, Gebbie said.

Using data from that survey, Gebbie and Huybers trained a computer model to mimic the ocean's modern-day circulation patterns. To look at historical patterns, though, they needed some real-world data for comparison's sake. Luckily, they had it in the first-ever modern oceanographic survey: That of the HMS Challenger in the mid-1870s.

New life for old data

The HMS Challenger was a British survey vessel that traveled 70,000 nautical miles (130,000 km) for an expedition between 1872 and 1876. The crew of the Challenger periodically dropped thermometers on ropes down to below 1.2 miles (2 km). Gebbie and Huybers had to correct this data slightly, since the pressures in the deep ocean can compress the mercury in an old-style thermometer, skewing the measurements. [In Photos: Ocean Hidden Beneath Earth's Surface]

Those corrections revealed that over the last 125 years, the Atlantic Ocean has warmed at all depths, while the Pacific shows a cooling trend over the 20th century starting between 1.1 and 1.6 miles (1.8 and 2.6 km) deep, the researchers reported in the Jan. 4 issue of the journal Science.

The precise amount of cooling isn't yet clear, but it is small, the researchers found, probably between 0.036 degrees and 0.144 degrees Fahrenheit (0.02 degrees and 0.08 degrees Celsius). Those numbers are preliminary, Gebbie said, and the researchers plan to take a closer look at the data to make them more precise.

Still, the temperature difference between the waters of the Atlantic and Pacific makes sense, Gebbie said. The Atlantic Ocean waters mix more readily than those of the Pacific. This is partially because cold, dense waters enter the Atlantic from both the South and North polar regions, Gebbie said. These waters sink to the bottom rather quickly, forcing rapid churning. The Pacific is bigger and isn't replenished from the north at all, so its deep waters hang out near the bottom for longer.

That means that old climate patterns hang out longer, too. In this case, Gebbie said, the cooling trend is caused by the mixing of old surface waters from two distinct periods. The first is the Medieval Warm Period, a balmy period between about A.D. 950 and 1250. At more than a mile (2 km) deep, waters that were at the surface during the Medieval Warm Period are now being replaced by cooler waters from the Little Ice Age.

All of this is vastly overshadowed by modern-day warming, however, Gebbie said. The difference in ocean-surface temperatures from the Medieval Warm Period to the Little Ice Age was about 0.72 degrees F (0.4 degrees C) over 900 years, he said. For comparison, sea-surface temperatures have gone up 1.5 degrees F (0.8 degrees C) since 1901, according to National Oceanic and Atmospheric Administration (NOAA) data. Climate scientists centuries in the future won't be able to see any hints of the Medieval Warm Period or the Little Ice Age in Pacific data, Gebbie said; it will have all been wiped out by the effects of 20th-century warming.

Nevertheless, the findings are important for today. Taking the deep ocean into account will help climate modelers develop better estimates for future climate change, Gebbie said.

"If you really want to get to the bottom of longer-term climate trends, decades and longer," he said, "you can't ignore the deep ocean."

Originally published on Live Science.