New Data Suggests Global Ocean Warming Rates May Have Been Severely Underestimated

Kim Seng, 'Atlantic Ocean Wave Rushing onto Beach Rocks' via Flickr. CC BY-NC-ND 2.0

It’s been known for some time that our oceans bear the brunt of climate change. In fact, it’s estimated that Earth’s oceans store more than 90% of the warming associated with greenhouse gases. That means that building an accurate picture of how our oceans are currently changing and how that compares with the past is pivotal to understanding the pace of global warming.

While scientists have been gathering data on ocean temperatures and sea levels for some time, new research has suggested that the rate of warming in the upper ocean has been vastly underestimated. However, it is not entirely doom and gloom as another study found that the deep ocean has barely changed since 2005, although they readily admit that their estimate has a large uncertainty. The two studies appear in Nature Climate Change.

Our oceans and climate are intimately connected; the global ocean plays a huge part in mitigating climate change because it serves as both a heat and carbon sink, meaning it contributes just as much to the global climate system as the atmosphere. While an abundance of cargo ships and well-funded projects in the Northern Hemisphere have meant that this half has been well studied for decades, the south has been neglected in comparison.

To address this issue, researchers from the Lawrence Livermore National Laboratory in California used a combination of methods to investigate warming in the top 700 meters of the ocean since 1970. Because water expands as it heats up, they used sea level changes as measured by satellite data as a proxy for warming where direct measurements were not available. They then combined this data with model simulations and recently gathered temperature measurements.

The team found that models and observed changes generally matched up quite closely in recent years, but there was a big discrepancy prior to when more direct temperature data started pouring in from a network of buoys called the Argo floats, which were installed in 1999. Although this inaccuracy was specific to the Southern Hemisphere, it was large enough to mean that global upper-ocean warming rates could have been underestimated by as much as 55%.

But it’s not just the Southern Hemisphere that has been overlooked; the deep ocean, below depths of 2,000 meters, is trickier to study than shallower depths and has thus been excluded from many studies. To gain a more comprehensive picture, Caltech scientists used the data collected from the Argo floats to calculate how much heat has been absorbed by the top half of the ocean, or approximately the top two kilometers (1,240 miles). By subtracting that from the total warming rates deduced from sea level data, they were able to estimate heat change in the deep ocean. Although they found that the bottom half has remained unchanged, lead author Dr. William Llovel admits that their estimate had a lot of uncertainty.

The take-home message from these two studies is that if we want to build a more accurate picture of global climate change, we need to shrink these uncertainties which will require more measurements. This could be achieved with the recent installation of more buoys that are capable of taking ocean measurements as deep as 6,000 meters below the surface. 

[Via Nature Climate Change, Nature Climate Change, Science, New Scientist and BBC News]

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