The bottom of the Pacific Ocean is cooling and scientists believe that it is caused by the Little Ice Age that took place hundreds of years ago.
On the surface, the water temperature has been responding to global warming caused by the emission of excess greenhouse gases. A new study, however, revealed that the deep ocean is lagging several centuries behind.
The study, authored by researchers from the Woods Holes Oceanographic Institution and Harvard University, was published in the journal Science.
Deep Within The Pacific Ocean
The ocean has a long memory. Previous studies have revealed that it takes a long time for the water on the surface of the Pacific Ocean to circulate down to its lowest depths.
The process takes a few hundred years, which means that the surface water during the Little Ice Age about 700 years ago is only making its way to the bottom of the Pacific Ocean. Researchers believe that studying the water temperature in the deep ocean might offer new information about the environment from the 1300s to 1870s.
"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," explained Jake Gebbie, a physical oceanographer at the WHOI and lead author of the study.
The researchers used the data recorded by scientists onboard the HMS Challenger in the 1870s and compared them to modern observations by the World Ocean Circulation Experiment. They found that as expected, the most parts of the global ocean are warming due to climate change. However, the deep Pacific is still cooling.
They predict that over the course of the 20th century, the depths of the Pacific Ocean at around 1.8 to 2.6 kilometers cooled between 0.02 and 0.08 degree Celsius. Prior to the Little Ice Age was a period called the Medieval Warm Period that caused the deep Pacific to warm before the current cooling.
The Future Of Earth's Oceans
The researchers explained that their model changes the understanding of how much heat the ocean has absorbed in the last century. They hope that their findings will help provide better insight into modern warming trends.
"Part of the heat needed to bring the ocean into equilibrium with an atmosphere having more greenhouse gases was apparently already present in the deep Pacific," added Peter Huybers, co-author of the study and professor of Earth and Planetary Sciences at Harvard.
The researchers, however, said that the cooling of the Pacific Ocean due to the Little Ice Age hundreds of years ago will not override the increase in global temperature.