New research reported by Peter Gleckler of Lawrence Livermore National Labsoratory and colleagues finds that ocean heat content has been rapidly accelerating over the past few decades, with about half of the increase since 1865 occurring in the past 20 years.

Reporting their findings in Nature Climate Change, the researchers found that climate model predictions closely matched observed ocean heat changes. They suggest that slower rates of global warming during part of the past 15 years might be attributable to the increased warming of the deeper oceans.

Image 1
Estimated ocean heat content since 1865. The year 1997 is highlighted at the point where warming was half of the 2015 value. Figure from Gleckler et al 2015.

Oceans are a critical part of the climate system. Although much of the focus has been on the warming of the atmosphere – that is where we all live, after all – the oceans absorb about 100 times more heat from human activities than does the atmosphere.

Water is particularly good at storing heat, and it takes a lot of energy to heat water. Air, on the other hand, heats and cools quickly and stores relatively little heat. As illustrated in the figure below, the earth’s oceans have absorbed more than 90 percent of the heat associated with modern human-caused warming.

Image 2
Figure from Rosamund Pearce at Carbon Brief.

Research generally has focused on the temperature of the shallow oceans to depths of about 700 meters. Until recently, there has been relatively little good observational data of the deeper oceans, and even records for shallow oceans have been spotty and inconsistent.

That situation has changed over the past 15 years with the introduction of the ARGO network, a system of automated buoys that each day dive deep into the ocean, take measurements of the different layers as they rise up to the surface, and send the data to researchers via satellite uplink. The ARGO network has rapidly expanded: more than 3,900 floats now take real-time temperatures of the world’s oceans.

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In their paper, Gleckler and colleagues used both newer ARGO data and older reconstructions of shallow- and deep-ocean temperatures. They compared them to the predictions of the set of climate models used in the most recent Intergovernmental Panel on Climate Change (IPCC) report.

As shown in the illustration below, the average of all the models (called the multi-model mean) generally does a reasonably good job at matching the observed changes in ocean heat content, for both shallow- and deep-ocean layers. For deep oceans, models may actually underestimate the rate of warming seen in the observations.

Image 4
Models and observations for shallow and deep ocean heat content adapted from Gleckler et al 2015. Multi-model mean is represented in black (for shallow oceans) and grey (for deep oceans). Thin black line for shallow oceans represents models adjusted to account for recent volcanic events. OHC stands for Ocean Heat Content and is measured in zettajoules (1022 joules).

For shallow oceans, models show a bit more warming than observations have in recent years. However, models do not incorporate recent volcanic events (they effectively assume no volcanic eruptions since 2000 even as a large number of small eruptions have occurred in the real world). To account for this, a variant of the model runs generated by the authors accounts for recent volcanoes (the thin black line in the figure), which matches the observational record quite well in recent years.

In addition to using more recent observational records, Gleckler and colleagues also examined data from the famous Challenger expedition between 1872 and 1876, which took a number of measurements of shallow ocean temperatures. They compared modern (2004-2010) measurements from the ARGO floats in the same regions that the Challenger made measurements, calculated the difference and heat content, and compared it to the predictions of climate models. They found that the amount of warming that climate models predicted between the 1872-1876 and 2004-2010 periods (29 zettajoules) matches the observations (33 zettajoules) quite well.

These multiple lines of evidence raise scientists’ confidence that the models reasonably assess global ocean heat content since the late 1800s. Their findings also allow Gleckler and his colleagues to determine, based on models and observations, that “half of the total global ocean heat uptake since 1865 has accumulated since 1997 – nearly coincident with the beginning of the observed surface warming hiatus.”

With the seeming “hiatus” in surface warming quickly vanishing in the face of record surface warmth in both 2014 and 2015, and with oceans warming at a rate unprecedented in recorded history, the new research supports other evidence that the warming of the Earth as a result primarily of human emissions of greenhouse gases is continuing apace.

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Zeke Hausfather

Zeke is an energy systems analyst and environmental economist with a strong interest in conservation and efficiency. He was previously the chief scientist at C3, an energy management and efficiency company,...