Little Ice Age is still causing the bottom of the Pacific Ocean to get colder, claim researchers

Water on the surface of the Pacific Ocean may take several hundred years to circulate down to its bottom

Two researchers claim to have solved the mystery of why the bottom of the Pacific Ocean is getting colder when other oceans on Earth are warming up.

According to the researchers, the reason is most likely due to the continuing after-effects of the Little Ice Age, which ended about 150 years ago.

But, how is that possible?

Jake Gebbie from the Woods Hole Oceanographic Institution (WHOI) and Peter Huybers from Harvard University claim that to arrive at their answer they needed to dig through about 150 years of data.

They say that the water on the surface of the Pacific Ocean may take several hundred years to circulate down to its bottom because it is replenished only from the south.

These waters are so old and haven't been near the surface in so long, they still 'remember' what was going on hundreds of years ago

In the 1870s, a group of scientists used a ship named the HMS Challenger to explore the world's oceans and to measure ocean temperatures up to a depth of about two kilometres. Gebbie and Huybers used this dataset, along with modern-day recordings of ocean temperatures from the Argo Programme, to create a computer model of water circulation in the Pacific Ocean over the past 150 years.

The results obtained from the model revealed that the water of the Pacific Ocean at depths of 1.8 to 2.6 kilometres cooled over the course of the 20th century, and the drop in temperature was between 0.02 and 0.08 degrees Celsius.

According to scientists, our planet was experiencing a "Little Ice Age" a few hundred years ago. This Little Ice Age started in roughly 1300 AD and continued until 1870 AD or so. During that period, the water at Pacific's surface got chilled because of the colder conditions on Earth. The surface water then took more than hundred years to reach the ocean's depths.

"These waters are so old and haven't been near the surface in so long, they still 'remember' what was going on hundreds of years ago when Europe experienced some of its coldest winters in history," said Jake Gebbie, a physical oceanographer at WHOI and lead author of the study.

The results also suggest that some oceanic layers - due to slow circulation - could be preserving the conditions of the past, and scientists can glean information about those conditions by looking deeper into the oceans.

"These findings increase the impetus for understanding the causes of the Medieval Warm Period and Little Ice Age as a way for better understanding modern warming trends." said Peter Huybers Professor of Earth and Planetary Sciences at Harvard University and co-author of the paper.

The findings of the study are published in journal Science.