UPDATE (November 14, 2018): This study has been retracted and the data are now being re-considered after the discovery of a mathematical error. While it appears that the oceans have warmed more than previously thought, this could be as little as 10% or as much as 70%. These margins of error are too large to determine exactly how much our oceans have warmed and/or how inaccurate the United Nations report published earlier this year.
The oceans have long been considered our planet's heat sponge - a 2014 report from the Intergovernmental Panel on Climate Change (IPCC) stated that the oceans had absorbed 93% of the excess heat that greenhouse gases have trapped within the Earth's atmosphere. However, a recent study shows that the world's oceans have absorbed 60% more heat over the past 25 years than initially thought.
According to the lead author, Dr. Laure Resplandy, "If the ocean was only 30 feet [9 meters] deep, our data show that it would have warmed by 6.5 °C [11.7 °F] every decade since 1991. In comparison, the estimate of the last IPCC assessment report would correspond to a warming of only 4 degrees Celsius (7.2 degrees Fahrenheit) every decade."
Dr. Resplandy believes that this recent development in our ability to more precisely measure the ocean's capacity for heat absorption is due to improvements in technology. Ocean heat content measurements were formerly made using Argo Floats, instruments that measure several environmental parameters at different ocean depths. However, for this study, the researchers used a more precise approach and measured gaseous oxygen and carbon dioxide released from the ocean between 1991 and 2016; as the oceans warmed, they lost their ability to retain these gases, causing carbon dioxide and oxygen to escape into earth's atmosphere.
Additionally, continuous losses in ocean oxygen will make marine habitats less hospitable for ocean life. "Warming-driven deoxygenation can create suffocating conditions for marine biodiversity, cause ocean animals to grow smaller, and can reduce the productivity of lucrative fisheries and fish farms," says Dr. Douglas McCauley, assistant professor at UC Santa Barbara and director of the Benioff Ocean Initiative.
Last month's IPCC report suggested that we have just over a decade to dramatically attenuate greenhouse gas emissions. The proposed target, 45% below 2010 levels by 2030, was based on the previous IPCC estimates. This new finding that the oceans are even closer to their maximum ability to buffer our planet's rising temperatures accelerates the need to dramatically decrease our emissions. Still, if we capped emissions productions tomorrow, warming in our oceans and on this planet would still occur for several more years.
“When you stop the greenhouse gases, the ocean continues to warm for like another two decades, and so everything continues to warm,” said co-author Ralph Keeling. “Extra warming in the pipeline means it’s harder to stay below the climate targets.”