Nearly half of the increases in ocean temperature between 1865-2015 occurred in just the past 20 years, a rate which is steadily getting quicker, a new study published Monday reveals.

Deep underwater, below 700 meters, the ocean holds 35 percent of the world's heat associated with greenhouse gases—an increase from the 20 percent it had absorbed just two decades ago, according to the study, published in the journal Nature Climate Change. Little is known about temperature measurement at that depth, which is partially what makes the findings so distressing, the researchers said.

"In recent decades the ocean has continued to warm substantially, and with time the warming signal is reaching deeper into the ocean," said lead author Peter Gleckler, a scientist with Lawrence Livermore National Laboratory (LLNL).

"The takeaway is that the rate, at which the global ocean is absorbing excess heat, has rapidly increased—so that in more recent times since 1997, it has absorbed as much heat as it took over 100 [years] to absorb," Gleckler said. "That is alarming."

Gregory Johnson, a researcher with the National Oceanic and Atmospheric Administration (NOAA), added, "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."

Overall, the ocean absorbs more than 90 percent of the world's heat associated with greenhouse gases. Quantifying oceanic temperatures is paramount to understanding the climate crisis and projecting how fast the planet will warm and sea levels will rise in the future, the researchers said.

Indeed, as Gleckler told the Guardian on Monday, "When we discuss global warming, the most familiar way we do that is talk about temperature changes on the surface—but it’s clear that the oceans are doing the bulk of the work in terms of absorbing the heat in the system. And if we want to really understand how much heat is being trapped, we can’t just look at the upper ocean anymore, we need to look deeper."

The researchers analyzed an array of ocean temperature observations and models dating back to the 1870s, including data from a fleet of 3,000 robotic floats known as Argo.

New "Deep Argo" floats that measure temperatures from the seabed found that even the bottom levels of the ocean have warmed in recent decades.

As oceans heat up, they set the stage for extreme weather events and related phenomena, such as "terrifying" tropical storms, the historic coral bleaching currently underway throughout the globe, widespread species loss, or the "changing contours" of the U.S. fishing industry. These, in turn, threaten to bring devastating consequences for the food security and livelihoods of millions of people within 85 years.