Ocean temperatures today are about the same as they were more than 100,000 years ago — at a time when sea levels were 20 to 30 feet higher.
The findings, published in the journal Science, highlight the key role that human activity has played in global warming and underscore concerns about the future impact of rising sea levels.
Over millions and billions of years, the Earth has gone through periods of cooling (when water freezes out of the oceans, causing glaciers to grow and sea levels to fall) and warming (when the ice melts and sea levels rise). Scientists often look for clues hidden in layers of ancient rock and ice to determine what conditions were like in that long-gone climate.
The last interglacial period, which took place some 129,000 to 116,000 years ago, is a particularly intriguing chapter in Earth's relatively recent history because of what it could tell us about today's climate, said lead author Jeremy Hoffman, a paleoclimatologist at the Science Museum of Virginia.
"The last interglacial is extremely interesting because it's the last time period in recent Earth history when global temperatures were a little bit higher and global sea level was about 6 to 9 meters higher — but carbon dioxide in the atmosphere was roughly at what it was during the pre-industrial era," said Hoffman, who conducted the work as a doctoral student at Oregon State University. "So it's a really interesting scientific question: What is it about the last interglacial that's so unique, that gave rise to higher sea levels?"
The problem is, researchers often assume climate change happened synchronously across the globe — that is, if it grew warm in one part, it also heated up in the others, and if it cooled in one area, it was cooling everywhere else at the same time.
It's already clear from climate patterns today that this simply isn't the case, Hoffman said. Even if Earth overall is warming at a given point in time, for example, some spots might be getting cooler while others heat up.
"What we know about how climate and temperature change on this planet is, it's not all at the same time or at the same rate," he said. "You can see these even today in human-caused climate change, how that's playing out on a global scale."
That means that if researchers studying an ancient climate were to take a reading from one area and extrapolate that to temperatures worldwide, their estimates could be wildly off, since that particular spot might be much colder or hotter than the global average.
This would skew our understanding of the past and how it relates to our present climate — and could interfere with our ability to predict future global changes.
Luckily, researchers have increasingly been finding ways to link different sets of data in different parts of the world — from ice cores in Antarctica to marine sediments at the bottom of ocean basins — to start developing a more coherent portrait of Earth's temperature at certain points in time.
For this paper, Hoffman and his colleagues analyzed published data on 104 sea surface temperature records during the last interglacial period, taken from 83 marine sediment core sites. They compared the data with temperature measurements during two periods: 1870 to 1889 and 1995 to 2014. They found that at the start of the last interglacial, the ocean's surface temperature was similar to the 1870-1889 average — but 4,000 years later, the temperature had jumped by about 0.9 degrees Fahrenheit — putting it on par with the 1995-2014 average.
During the last interglacial, that temperature jump took place over several thousand years. This time, it happened in barely over a century.
Of course, there's a key difference between then and now: Human activity has pumped carbon dioxide and other greenhouse gases into the atmosphere at an unprecedented rate.
"The fact of the matter is that we are putting heat-trapping gases in the atmosphere faster than the Earth was able to do by itself for 66 million years," he said. "So it just continues to really highlight the unique role that humans are playing in the climate system now."
The findings raise worrisome questions about future sea levels. Since 1901, levels have already climbed 8 inches, according to the Environmental Protection Agency, and are predicted to rise by 1 to 3 feet by the end of the century. Some 25 million people in the United States alone now live in areas threatened by coastal flooding.
"We are rapidly approaching ocean conditions that haven't been really seen on this planet for over 120,000 years," Hoffman said. "That's a very relevant thing as we head into the next couple decades, and we start making policy decisions to turn this train around."
The findings come on the heels of independent analyses by NASA and NOAA showing that Earth broke heat records in 2016, for the third year in a row — boosted in part by El Niño but driven in large part by global warming, which has been fueled by human activity.
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