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Climate Change

Is the Deep Pacific Cooling?

Source: Gebbie, G., and P. Huybers (2019), The Little Ice Age and 20th-century deep Pacific cooling. Science, 363, 70-74. doi:10.1126/science.aar8413.

Water is transported around the world’s oceans via a current system called the meridional overturning circulation (MOC), often referred to as the great ocean conveyer belt. Figure 1 below shows the approximate path of this conveyer belt–red indicates transport near the surface and blue represents deep transport. Along with water, these currents carry and redistribute heat, salt, and gases throughout the world’s oceans. Therefore, the overturning circulation plays an important role in regulating the global climate system.

One of the MOC pathways consists of water sinking in the Antarctic and then flowing northward into the Pacific at great depths before eventually rising up to the surface and flowing into the Indian Ocean. This particular pathway takes a long time to travel–the deep water of the north Pacific left the surface hundreds of years ago. Still, the water carries some properties of the Antarctic surface waters it came from. In a recent study published in Science, Jake Gebbie from Woods Hole Oceanographic Institution and Peter Huybers from Harvard University investigated how the long memory of the waters in the deep Pacific changes our understanding of global ocean heat uptake.

Figure 1: Schematic of the global overturning circulation (Robert Simmon via Wikimedia Commons)

Observations from the past several decades show that the ocean is currently warming in response to increased atmospheric carbon dioxide concentrations. Because oceanography is a relatively new field, we don’t have many observations from hundreds of years ago (when deep Pacific waters resided at the surface near Antarctica). As a result, scientists have to infer past temperature trends using other methods. For example, the ratio of oxygen isotopes, which are different forms of oxygen atoms with varying numbers of neutrons, in ancient ice cores can tell scientists about the air temperatures at the time of formation.

In this study, the researchers used an ocean circulation model and ice core records going back more than 2000 years to simulate the response of the deep ocean to changes in surface temperatures from 0 AD to the present.

Figure 2: HMS Challenger, which conducted the first global oceanographic research cruise in the 1870s (NOAA Archive via Wikimedia Commons).

As opposed to observed surface warming for large parts of the global ocean, the results suggest that the deep Pacific is still adjusting to a period of widespread cooling from about 1300 to 1850 AD known as the Little Ice Age.

In addition to analyzing circulation model output, the researchers also compared modern observations with data from the first global oceanographic research cruise by the HMS Challenger in the 1870s (see Figure 2). Differences in ocean temperatures between the 19th century and today support the idea that the current deep Pacific is still cooling in response to the Little Ice Age.

Why does it matter if the deep Pacific has been losing heat since 1750 AD while the rest of the ocean has been warming? It means that climate models that do not account for this cooling may overestimate present day ocean heat uptake, having broader implications for how much our planet could actually heat up in the next few centuries. Accurately quantifying ocean heat gains and losses (called a heat budget) is important given the key role of the ocean in regulating the earth’s climate. These results suggest that the ocean’s long memory must be considered in assessing the global ocean heat budget.


One Response to “Is the Deep Pacific Cooling?”

  1. Great Experience ever, Thanks for sharing this brilliant article with us . I love these ideas! I’m glad you list the various brilliant experiment.Thanks for sharing this article with us.

    Posted by Natural Ice Melt | March 7, 2019, 11:29 pm

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