The Little Ice Age lasted for centuries during the last millennium and cooled the surface of the oceans. No question about that. Now a paper published Thursday in Science has discovered that, as predicted by models, while the deep Atlantic is warming in keeping with the global climate trend, the black depths of the Pacific Ocean are still cooling.

The model created by Geoffrey Gebbie of the Woods Hole Oceanographic Institution and Peter Huybers of Harvard University has unavoidable, long-term implications for a world in the grip of anthropogenic global warming.

It is an oceanographic tenet that the response time of the deep Pacific to the surface is centuries longer than the Atlantic response. “The Pacific has a slower response because its deep water is only replenished from the south and because it is a large basin,” Gebbie explains. “The Atlantic has a faster response because its deep water is replenished from both the north and south and the basin is smaller.”

So, differences between the Atlantic and Pacific can indicate past climate change, says Gebbie.

Everybody has been arguing about the Little Ice Age for some 700 years, since it began. It was not an ice age, to be clear, but paleoceanographic data indicate that many places around the globe, including Europe, were colder than today by about half a degree Celsius on average. Anyway, for several hundred years up to 1850, winters were more severe than today and the surfaces of the oceans cooled.

So, almost 200 years after the Little Ice Age, the deep Pacific is still reacting to it. In the deep Atlantic, on the other hand, the signal of Atlantic cooling from the Little Ice Age still exists, but has been overwhelmed by modern warming, suggest Gebbie and Huybers.

Open gallery view The Atlantic: Its depths circulate faster than the Pacific Ocean's. Credit: Michael Dwyer / AP

There could be alternative explanations for the oceans’ behavior, Gebbie stresses, but their interpretation is an inference based on our empirically derived model and reconstructions of past surface temperature.

In an aside, some of the historic measurements they used were from the HMS Challenger, a three-masted wooden British warship that was used to carry out the first modern scientific expedition of the global ocean and seafloor. During the expedition from 1872 to 1876, thermometers were lowered into the ocean depths by hemp rope and more than 5,000 temperature measurements were manually recorded in the ship logs, says Gebbie.

They corrected the Challenger’s data for things like hemp stretch, and compared its data to modern observations from the World Ocean Circulation Experiment of the 1990s.

Back to the point: As expected, they found warming in most of the world ocean depths. But in the Pacific they found cooling at a depth of about 2 kilometers (1.2 miles), as their model predicted.

On the upside, the Pacific is currently providing a ‘brake” on global warming by absorbing this heat, Gebbie says: “The trade-off is the oceans are slowing global warming now, but the heat will eventually resurface from the ocean.”

Getting warmer, warmer…

There is also no question that anthropogenic climate change is warming ocean surfaces and even the groundwater.

Open gallery view The Pacific Ocean off Anchorage, Alaska Credit: Gil Cohen-Magen

Because of how water circulates in the vast oceans, the oceanographers had suspected that temperature anomalies like the Little Ice Age might leave long-term traces in the overturning deep Pacific. The still-cooling water they have observed is just such a “memory,” they believe.

Their paper presents a model of the behavior of the “creeping waters,” not categorical facts. But they’re confident in it and if they’re right, one implication of their model is that even if we suddenly decarbonize and stop changing the climate, we could be contending with the depths of the Pacific Ocean continuing to warm at least for decades, possibly hundreds of years.

There is heat already in the pipeline that will not come into contact with the Pacific depths for decades or centuries, Gebbie says.

Could the oceans react different to heating than to cooling? Maybe. “Most oceanographers believe that the response to heating is more likely to be trapped at the surface, while the response to cooling can more easily penetrate to great depths,” Gebbie says. “The reason has to do with the density of seawater. Waters that are cooled at the surface by winter winds and storms can make it dense enough to sink. Sea ice formation is also an important contributing factor to making water dense in the southern oceans on account of rejecting salt into the water.

“These dense waters are then carried by a turbulent flow running most intensely around Antarctica and along the western boundaries of the Atlantic and Pacific, eventually connecting deep water that is primarily formed in the North Atlantic and Antarctic oceans with the deep Pacific.

“If water is warmed, it becomes less dense, and thus has a harder time sinking,” he sums up.

Getting colder

The model Gebbie and Huybers built – based on massive amounts of observations and data, including the water surface temperature over the last 2,000 years – concludes that cooling in the deep Pacific today is due to the slow, persisting introduction of cold water that was last on the surface during the Little Ice Age.

“An important simplifying assumption we made is that the ocean circulation and mixing did not change, only the surface temperature,” Gebbie clarifies.

So, they found warming in most parts of the global ocean, as would be expected due to the warming planet over the 20th century, but cooling in the deep Pacific at around 2 kilometers depth, he says.

Could the warming of the deep oceans affect marine life? Apparently not, and certainly not in the short run: The deep warming indicated by this study is small relative to surface warming, Gebbie says.

Comparing measurements in the 1990s with those of the HMS Challenger in the 1870s, at about 2 kilometers depth, the Pacific has cooled by about 0.1 degrees Celsius over 120 years, Gebbie says. “While this temperature change is small, it equates to a large amount of energy because of the great size of the Pacific,” he points out.

That isn’t likely to impact marine life. But warming is greater at the surface, which could be another story.