A new study published Thursday strengthens the consensus that the warming of the world's oceans is accelerating.
It's a trend that climate models have long predicted, but it had been difficult to confirm until recently.
The findings are vindication of the scientific community's work so far and lend greater weight to the projections for warming through the end of this century, said Gavin Schmidt, a leading climate scientist at Columbia University who was not involved in the study.
The new paper, published in the journal Science, reviews four studies conducted over the past decade and was partly a response to a controversy over one of them, an article published in the journal Nature on Nov. 1. The authors of the November article were forced to issue a correction after discovering they had made errors in their assumptions and that the uncertainty in their findings was much greater than they had thought.
While the November paper made some "disquieting" assumptions, the corrected version is closely in line with three other studies that used different techniques, said Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research and coauthor of the new review.
The overall point—that warming is accelerating—holds true, and it helps explain why we're starting to see the effects of warming through stronger storms and severe weather, he said.
"Global warming is here, it has major consequences, and it's going to be very, very difficult to get this under control," Trenberth said. "That doesn't mean we shouldn't try, because anytime we can slow this down if not stop it, it allows us to adapt to it, to plan for it, to deal with some of the expected consequences in a much better fashion."
'The Best Measure' of Global Warming
Understanding how ocean temperatures have changed is particularly valuable for understanding climate change.
More than 90 percent of the excess heat from greenhouse gas emissions has been absorbed into the oceans, where that energy can fuel larger and more intense hurricanes and contributes to sea level rise as warming seawater expands.
Ocean temperatures are also much less variable than surface temperatures, which can swing greatly from year to year, and therefore give a clearer signal of global warming.
"The change in ocean heat content is the best measure we have of the global energy imbalance of the whole planet," Schmidt said.
The new paper cites projections showing ocean heat continuing to rise for several decades, though the rise would be less in a low-emissions scenario where global greenhouse gas emissions peak soon and fall quickly. In a high-emissions scenario, the temperature of the oceans' top 2000 meters would rise about 0.8 degrees Celsius (about 1.4 degrees Fahrenheit) from current conditions by the end of the century. The resulting thermal expansion of the oceans' water would add about 1 foot of sea level rise on top of the rise from melting ice sheets and glaciers, the paper says.
Data Show IPCC Underestimated Warming
The four studies discussed in the analysis show that the most recent Intergovernmental Panel on Climate Change assessment, published in 2013, underestimated the amount of heat the oceans have absorbed. With the IPCC now working on a new oceans review, the authors of the new analysis wanted to draw attention to the recent data and the consensus.
The studies in the new review were able to draw on a much stronger record of ocean data collected since 2007, made possible in part by thousands of ocean-going sensors—called Argo floats—that have been deployed around the world to measure and report conditions in the upper oceans. The data has helped scientists better understand how heat is distributed across the top two kilometers of the oceans, and it has allowed them to make better estimates dating back decades.
"The biggest takeaway is that these are things that we predicted as a community 30 years ago," Schmidt said. "And as we've understood the system more and as our data has become more refined and our methodologies more complete, what we're finding is that, yes, we did know what we were talking about 30 years ago, and we still know what we're talking about now."
2018: Another Record-Warm Year
The trend in ocean warming also continued in 2018: New data set to be published next week will show that last year was the warmest on record in the oceans, Trenberth said.
One 2018 hotspot occurred where Hurricane Florence grew into one of the wettest storms on record before striking the Carolinas. The previous year, one of the warmest areas was in the Gulf of Mexico, where Hurricane Harvey erupted into the monster cyclone that flooded Houston and devastated parts of the Texas coast, he said.
With 2018 the warmest year yet in the oceans, there's no doubt about what is happening, Trenberth said. "It emphasizes the unequivocal fact that the ocean is warming, the planet is warming, and it has consequences."