Ocean warming doubles in recent decades – ‘With time the warming signal is reaching deeper into the ocean’

Desdemona Despair

18 January 2016 (NOAA) – Lawrence Livermore National Laboratory scientists, working with National Oceanic and Atmospheric Administration and university colleagues, have found that half of the global ocean heat content increase since 1865 has occurred over the past two decades.

“In recent decades the ocean has continued to warm substantially, and with time the warming signal is reaching deeper into the ocean,” said LLNL scientist Peter Gleckler, lead author of a paper published in the journal Nature Climate Change.

Changes in ocean heat storage are important because the ocean absorbs more than 90 percent of the Earth’s excess heat increase that is associated with global warming. The observed ocean and atmosphere warming is a result of continuing greenhouse gas emissions. Quantifying how much heat is accumulating in the Earth system is critical to improving the understanding of climate change already under way and to better assess how much more to expect in decades and centuries to come. It is vital to improving projections of how much and how fast the Earth will warm and seas rise in the future.

Increases in upper ocean temperatures since the 1970s are well documented and associated with greenhouse gas emissions. By including measurements from a 19^th century oceanographic expedition and recent changes in the deeper ocean, the study indicates that half of the accumulated heat during the industrial era has occurred in recent decades, with about a third residing in the deeper oceans.

The team analyzed a diverse set of ocean temperature observations and a large suite of climate models. Scientists have measured ocean temperatures in a variety of ways over time, from lowering pairs of minimum-maximum thermometers to different depths on lines dangled overboard during the H.M.S. Challenger 1872-1876 expedition, to use of highly accurate modern instruments placed on robotic profiling floats (called Argo) that “phone home” the data using satellites, starting around 1999.

This study found that ocean warming estimates over a range of times and depths agree well with results from the latest generation of climate models, building confidence that the climate models are providing useful information. “The year-round, global distribution of ocean temperature data collected by Argo has been key in improving our estimates of ocean warming and assessing climate models,” notes LLNL oceanographer Paul Durack.

While Argo only samples the upper half of the ocean volume, pilot arrays of new “Deep Argo” floats that sample to the ocean floor are being deployed. This vast ocean volume in the deeper half is only measured infrequently by research vessels. Those deep data also show warming, even in the bottom layers of the ocean in recent decades. “Given the importance of the ocean warming signal for understanding our changing climate, it is high time to measure the global ocean systematically from the surface to the ocean floor,” said NOAA oceanographer Gregory Johnson.

Other authors include NOAA climate modeler Ronald Stouffer and Penn State Climate Scientist Chris Forest. The study was conducted as part of the Climate Research Program at Lawrence Livermore National Laboratory through the Program for Climate Model Diagnosis and Intercomparison, which is funded by the Department of Energy’s Regional and Global Climate Modeling Program. The NOAA contribution was supported by NOAA’s Office of Oceanic and Atmospheric Research. Work at Penn State was partially supported by the Department of Energy, Office of Science, Office of Biological and Environmental Research and by the National Science Foundation through the Network for Sustainable Climate Risk Management (SCRiM).

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Monica Allen, director of public affairs for NOAA Oceanic and Atmospheric Research, at 301-734-1123 or by email at [email protected]

Anne M. Stark, public affairs for Lawrence Livermore National Laboratory, at 925- 422-9799 or by email at [email protected]

Ocean warming doubles in recent decades

ABSTRACT: Formal detection and attribution studies have used observations and climate models to identify an anthropogenic warming signature in the upper (0–700 m) ocean^{1, 2, 3, 4}. Recently, as a result of the so-called surface warming hiatus, there has been considerable interest in global ocean heat content (OHC) changes in the deeper ocean, including natural and anthropogenically forced changes identified in observational^{5, 6, 7}, modelling^{8, 9} and data re-analysis^{10, 11} studies. Here, we examine OHC changes in the context of the Earth’s global energy budget since early in the industrial era (circa 1865–2015) for a range of depths. We rely on OHC change estimates from a diverse collection of measurement systems including data from the nineteenth-century Challenger expedition¹², a multi-decadal record of ship-based in situ mostly upper-ocean measurements, the more recent near-global Argo floats profiling to intermediate (2,000 m) depths¹³, and full-depth repeated transoceanic sections⁵. We show that the multi-model mean constructed from the current generation of historically forced climate models is consistent with the OHC changes from this diverse collection of observational systems. Our model-based analysis suggests that nearly half of the industrial-era increases in global OHC have occurred in recent decades, with over a third of the accumulated heat occurring below 700 m and steadily rising.

Industrial-era global ocean heat uptake doubles in recent decades