Global ocean basins heat up

Saturday 1st July 2017
New analysis of ocean basins

New analysis, published in the journal "Climate Dynamics" confirms that although there is some uncertainty in the distribution among Earth’s ocean basins, there is no question that taken as a whole oceans are heating up rapidly.

The paper, “Consensuses and discrepancies of basin-scale ocean heat content changes in different ocean analyses”, combines the results obtained by three different ocean temperature measurements made by three different groups. It turns out that regardless of whose data was used or where the data was gathered, the oceans are warming. The research was a collaboration of the Institute of Meteorology and Oceanography, PLA University of Science and Technology, Nanjing, China, the International Centre for Climate and Environment Sciences, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, the University of St. Thomas, St. Pau, lUSA and LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing.

The most important measurement of global warming shows up in the oceans, but changes in the ocean climate can only be measured accurately using many sensors spread out across the globe taking measurements from the ocean surface to the deepest water, and over a long time period so that long-term trends can be identified.

Removing the biases
The thrust of the research centered on removing the biases from the different data analyses, biases that can distort results and give rise to inaccurate conclusions. Start point was the discovery of inconsistent global/basin OHC (ocean heat content) in different ocean subsurface temperature analyses, especially in recent studies related to the slowdown in global surface temperature rise. Because this finding challenges the reliability of the analyses it was decided to undertake a more comprehensive inter-comparison, the subjects being the OHC changes in ocean analyses by Ishii (Japan), EN4 (UK Met Office) and IAP (International Association of Prosecutors).
A comparison would investigate the uncertainty in OHC in four major ocean basins ranging from decade to multi-decade scales. Significantly, all these analyses show an increase of OHC since 1970 in each ocean basin revealing a robust warming, although the warming rates are not identical.

The paper describes the three most important factors that affect ocean-temperature accuracy. First, sensors can have biases and these biases can change over time. An example of bias was identified in the 1940s. Then, many ocean temperature measurements were made by gathering water in buckets. Then the US navy started making measurements using sensors at engine intake ports. It turns out that bucket measurements are slightly cooler than measurements made using hull sensors, which are closer to the ship’s engine. During World War II an apparent rise in temperature occurred because the British Navy cut back on its measurements (using buckets) and the US Navy expanded its measurements (using hull sensors); But this warming was an artefact of the change. Postwar previous service was resumed and the ocean temperatures seemed to fall a little. Again, this was an artefact. Other such biases arose as oceanographers updated their measuring equipment. To get the true rate of ocean temperature change, these biases have to be removed.

Other sources of uncertainty are related to the uneven spread of sensors around the oceans and in time. Some sensors, which are dropped from cargo ships, are densely located along major shipping routes, while sensors dropped from research vessels are confined to specific locations across the globe. Current measurements rely heavily on the Argo fleet, which has approximately 3800 autonomous devices spread out more or less uniformly across the ocean, but these only entered service in 2005. Prior to that, temperatures measurements were not uniform in the oceans, and analysts have to map the oceans by interpolate among the temperatures they do have. This can represent a bias. Finally, temperatures are usually referenced to a baseline climatology, so it is essential to know what and when baseline climatology is.

Significant warming
In the study, the analysts found that each ocean basin has warmed significantly, but there are some differences among the three groups. For instance, at 300-700 metre depths in the Pacific and Southern oceans, significant differences are exhibited amongst the three groups. But regardless of the method and who does the measurements, or when and where they were made, the oceans are warming up.

Warming versus slowdown
The paper’s lead author,  Dr Gonjgie Wang (right) characterised the importance of the study: “Our study confirms again a robust global ocean warming since 1970. However, there is substantial uncertainty in decadal scale ocean heat redistribution, which explains the contradictory results related to the ocean heat changes during the “slowdown” of global warming in recent decade. Therefore, we recommend a comprehensive evaluation in the future for the existing ocean subsurface temperature datasets. Further, an improved ocean observation network is required to monitor the ocean change: extending the observations in the boundary currents systems and deep oceans (below 2000-m) besides maintaining the Argo network.”

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