New study finds deep-down warming
Heat trapped by greenhouse gases is building up in the oceans at an increasing rate, according to researchers who are trying to get a more detailed understanding of the oceans’ role in the global climate cycle.
After studying data from a variety of sources, the researchers with Lawrence Livermore National Laboratory andthe National Oceanic and Atmospheric Administration 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.
Tracking ocean changes is critical to understanding future warming and sea level rise because oceans absorb 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.
The rise in upper ocean temperatures since the 1970s is well documented. By including measurements from a 19th 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 in 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 highly accurate modern instruments used on a global array of robotic profiling floats (called Argo) that “phone home” the data using satellites, starting around 1999.
This study found that estimates of ocean warming over a range of times and depths agree well with results from the latest generation of climate models,” said LLNL oceanographer Paul Durack. “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,” he added.
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.