Our planet is getting alarmingly warmer, on this the science is clear. So when scientists find that a major part of our planet is actually getting colder, it creates something of a conundrum.

That's the case with the very deep layers of the Pacific Ocean. While the oceans are getting warmer in general, including the upper layers of the Pacific, the bottom of the largest ocean in the world is actually cooling down. How is this possible?

Now researchers with the Woods Hole Oceanographic Institution and Harvard University have finally unlocked the mystery, but it took digging through around 150 years of data to solve it, reports Phys.org.

Back in the 1870s, the HMS Challenger — a three-masted wooden sailing ship originally designed as a British warship — was used for the first modern scientific expedition to explore the world's oceans and seafloors. Part of this ship's mission was to record temperatures down to a depth of two kilometers, a remarkable and unprecedented dataset to have access to. Using this, along with modern-day recordings of deep ocean temperatures, researchers were able to model the circulation of water in the Pacific Ocean over the past century and a half.

A time capsule in the ocean depths

What they found was quite remarkable. It turns out that the Pacific Ocean's water can take hundreds of years to circulate down to its lowest depths. The lower layers are therefore time capsules, of sorts, about the conditions near the surface hundreds of years ago.

And what was the climate like a few hundred years ago? Earth was experiencing what has been called the "Little Ice Age," a cold streak that lasted from roughly 1300 until 1870 or so. Researchers therefore surmise that the reason the deep Pacific waters are getting colder is because these are the same waters that were among the top layers during the Little Ice Age. They were chilled hundreds of years ago, and they've been sinking into the ocean's depths, ever-so-sluggishly, ever since.

The findings could also have profound implications about our ability to study climate conditions from hundreds of years ago, even from times for which we don't have complete datasets. Different ocean layers in the Pacific are, in some ways, like tree rings or ice core samples. Due to slow circulation, oceanic layers preserve the conditions of the past, and we can glean new knowledge about the past merely by looking deeper into the ocean.

It's a reminder about the length of the timescales that many of Earth's systems operate on. It's also a reminder that reversing the effects of global warming will require long timescales too, and that there's no quick fix for our modern climate woes.