In the depths of the Pacific Ocean, where the water runs cold and cool, temperatures continue to drop – and it's all because of a significant cooling off period that began in the 16th century.
Although the surface temperatures of the ocean are increasing, it seems that the message did not reach the waters at the bottom of the Pacific. They are behind some centuries behind the rest of the world when it comes to temperature patterns, according to new research.
"The weather varies on all time scales," said Earth scientist and planetary scientist Peter Huybers of the Harvard School of Engineering and Applied Sciences John A. Paulson.
"Some regional patterns of warming and cooling, such as the Little Ice Age and the Medieval Warm Period, are well known. Our goal was to develop a model of how the interior properties of the ocean respond to changes in surface climate."
The Little Ice Age, as it is known, is a period of several centuries of cooling that took place between 1550 and 1850 AD, after the Medieval Warm Period.
However, despite the name, it was not really an ice age, as it seems to have occurred regionally, rather than globally – though exactly what caused this is still unknown.
But the regions affected were large and numerous and obviously included the ocean.
And the circulation of the Pacific Ocean takes a long time. Once the water descends, it is isolated from the atmosphere (and its temperature) for an estimated 8 to 14 centuries.
This means that those waters that were on the surface during the Little Ice Age may still be cooling down as they descend, following behind the warmer waters of the Medieval Warm Period.
"These waters are so ancient and not near the surface for so long, they still remember what was happening hundreds of years ago when Europe experienced some of its coldest winters in history," said the physical oceanographer Jake Gebbie of Woods. Oceanographic Institution of Hole.
"If the surface of the ocean was generally cooling for most of the last millennium, the most isolated parts of the ocean from modern warming may still be cooling."
The team used data and computer modeling to determine the deep circulation of the Pacific since the 1870s.
During the 1990s, the World Ocean Circulation Experiment did a series of Pacific temperature readings down to a depth of 2 km (1.24 miles).
And between 1872 and 1876, scientists on board HMS Challenger repeatedly plunged a thermometer into the world's oceans to make more than 5,000 temperature readings, also at depths of about 2 kilometers.
"We have analyzed these historical data from outliers and considered a variety of corrections associated with pressure effects on the thermometer and the lengthening of the hemp rope used to lower the thermometers," Huybers said.
These corrected and corrected data were then compared with the most recent WOCE data.
Gebbie and Huybers found, as expected, a warming trend closer to the surface – but at depths between 1.8 and 2.6 kilometers (1.1 to 1.6 miles), the ocean cooled.
Not by a large amount – just between 0.02 and 0.08 degrees Celsius – but compared to global surface warming trends, it is an intriguing result.
And that has implications for how the planet is warming today – showing that previous climate changes on the surface may still be affecting the current rate of climate change.
This is because earlier estimates of how much heat was absorbed by the Earth in the 20th century are based on an equilibrium ocean at the beginning of the Industrial Revolution.
But this finding indicates that the Earth could have absorbed up to 30% less heat than these estimates.
"Part of the heat needed to put the ocean in balance with an atmosphere with more greenhouse gases is apparently already present in the deep Pacific," Huybers said.
"These findings increase the impetus to understand the causes of the Medieval Warm Period and the Little Ice Age as a way to better understand modern warming trends."
The research was published in the journal Science.