The world's oceans have turned into a veritable sponge for our emissions, and new climate models suggest we've soaked them right through.
Since the 1950s, our planet's vast bodies of water have absorbed roughly 93 percent of the energy entering the climate system, and while most of that heating has been observed near the ocean surface, rising temperatures are now permeating even the deepest parts.
Real-world data on the deep ocean is hard to come by, but a new estimate, based on recent measurements and nearly a dozen climate models, suggests climate change has already impacted up to about half (20 to 55 percent) of the Atlantic, Pacific, and Indian Ocean basins.
What's more, in just six decades, these human-induced changes in temperature and salinity could very well spread to 80 percent of the world's oceans.
"We were interested in whether the levels of temperatures and salt were great enough to overcome natural variability in these deeper areas," explains climate scientist Yona Silvy from Sorbonne University in France. "That is, if they had risen or fallen higher than they ever would during the normal peaks and troughs."
Using temperature and salinity measurements from the deep ocean and plugging these into 11 current climate models, the team simulated ocean and atmospheric circulation over the years, with and without the contribution of human emissions.
During the second half of the 20th century, Silvy and her colleagues found human-induced warming was responsible for most observed ocean changes - "statistically" and "unambiguously" different from what would occur naturally. Because heat and salt impact ocean density and circulation, this could have widespread implications.
"This affects global ocean circulation, sea level rise, and poses a threat to human societies and ecosystems," says Silvy.
Most of the time, heat and salt from the surface of the ocean are transported relatively slowly to the ocean's interior, which means that many of the deepest parts experience a lag in human-induced changes.
Some deeper areas, however, circulate quicker, and thus respond faster to our emissions.
In the new model, for instance, the Southern Ocean, which is relatively well-ventilated, experienced human-induced changes quite quickly, showing up as early as the 1980s.
Meanwhile, in the Northern Hemisphere, oceans took a little longer to respond, with most changes calculated to appear sometime between 2010 and 2040.
Together, by 2020, the model shows somewhere between 20 percent and 55 percent of the world's oceans had been altered by anthropogenic climate change.
By mid-century, these changes could make up 50 to 60 percent of the world's oceans, and by 2080, 55 to 80 percent.
"This work suggests that a large portion of the observed change patterns in the ocean interior is human-induced and will continue to intensify with continuing CO2 emissions," the authors write.
Plus, even if emissions are slowed, the lag in ocean circulation means we are locked in to a certain amount of change going forwards.
We still don't fully understand the relationship between deeper changes to salt and heat and surface warming, or how these changes impact ocean circulation. It requires far more investigation, especially in the Southern Hemisphere where deep ocean data is few and far between, but investigate it we must.
The study was published in Nature Climate Change.