The effect of climate change on the world's oceans has been understudied, a recent report from the International Union for the Conservation of Nature (IUCN) finds, and it is far worse than many scientists and politicians had previously thought.

"We all know the oceans sustain this planet," said Inger Andersen, IUCN's director general, to National Geographic, "yet we are making the oceans sick."

"Ocean warming is one of this generation's greatest hidden challenges—and one for which we are completely unprepared."

—Inger Andersen, IUCN

The oceans have been shielding us from the worst effects of climate change, according to "Explaining ocean warming: causes, scale, effects and consequences" (pdf), by absorbing 93 percent of the excess heat produced by human activity and trapped in the atmosphere by the greenhouse effect since the 1970s—and humans have produced so much extra heat that without the oceans, surface temperatures worldwide would have risen by 36° Celsius since the 1970s.

Instead, thanks to the oceans' capacity to absorb so much heat, global temperatures have only risen about 0.5° since the 1970s, according to the National Oceanic and Atmospheric Administration (NOAA).

"Without this oceanic buffer, global temperature rises would have gone much, much speedier," Andersen said.

However, the massive amount of heat absorbed by the oceans has sent the water temperatures soaring, the report finds, rapidly depleting the biodiversity of marine and coastal species—from microbes to whales—and changing weather patterns worldwide.

"Up to now, the ocean has shielded us from the worst impacts of climate change," the report's preface notes. "The costs is that its chemistry has been altered as it absorbed significant amounts of the extra carbon dioxide we put into the atmosphere, but it has also warmed at an alarming rate in recent decades."

And the oceans can't take much more: the alarming, rapid rise in sea temperatures indicate that they have reached capacity and will soon release that extra carbon dioxide back into the atmosphere in what the IUCN report characterizes as a "positive feedback loop."

Coastal regions whose economies rely heavily on fisheries are already feeling the effects of a rapidly warming ocean, while coastal cities are greatly threatened by sea level rise from melting polar ice caps.

The report also predicts that ocean temperatures will rise by an additional 1° to 4° Celsius by 2100, with the most warming occurring in the Southern Hemisphere. The polar regions are predicted to warm even more than the rest of the world's oceans.

"Ocean warming is one of this generation's greatest hidden challenges—and one for which we are completely unprepared," Andersen said, according to the Independent.

"The only way to preserve the rich diversity of marine life, and to safeguard the protection and resources the ocean provides us with, is to cut greenhouse gas emissions rapidly and substantially," Andersen added.

This article previously stated that the oceans have absorbed 93 percent of CO2 emissions emitted by human activity since the 1970s. In fact, oceans have absorbed 93 percent of the excess heat created by the greenhouse effect and human activity since the 1970s. We regret the error.