The discovery provides support for climate models and provides answers for a debate known as the "Holocene temperature conundrum" concerning how temperatures have changed over the Holocene, an epoch that describes the past 11,700 years of Earth's history.
Scientists have debated how temperature evolved over the Last Interglacial and Holocene eras, with conflicting data concerning when the highest temperatures occurred. While some reconstructions suggest that the average temperature during the Holocene peaked about 6,000 to 10,000 years ago, climate models suggest that temperatures have risen over the past 12,000 years.
Samantha Bova, lead author of the study, stated in a Rutgers article that it "eliminates any doubts" concerning the role of carbon emissions in global warming.
“Our reconstruction shows that the first half of the Holocene was colder than in industrial times due to the cooling effects of remnant ice sheets from the previous glacial period – contrary to previous reconstructions of global temperatures,” said Bova, a postdoctoral researcher associate in the lab of co-author Yair Rosenthal, a distinguished professor in the Department of Marine and Coastal Sciences and Department of Earth and Planetary Sciences at Rutgers University–New Brunswick.
Rosenthal was born and raised in Israel and graduated from the Hebrew University in 1985.
“The late Holocene warming was indeed caused by the increase in greenhouse gases, as predicted by climate models, and that eliminates any doubts about the key role of carbon dioxide in global warming,” she said.
The Rutgers-led study used marine calcareous (calcium carbonate-containing) fossils from foraminifers - single-celled organisms that live at the ocean surface - to reconstruct the history of the temperatures of the two most recent warm intervals on the planet: the Last Interglacial period from 128,000 to 115,000 years ago and the Holocene.
The fossils were collected within a core of bottom sediments near the mouth of the Sepik River off northern Papua New Guinea during Expedition 363 of the International Ocean Discovery Program. The core features rapidly accumulating sediments that allowed scientists to recreate the temperature history of the western Pacific warm pool, according to a Rutgers article.
The study found that previous reconstructions of temperature history in the Holocene era reflected seasonal, rather than annual, temperatures. The scientists worked to develop a method to transform the data into average annual temperatures and demonstrated that global mean annual sea surface temperatures have steadily increased since the beginning of the Holocene, at first due to retreating ice sheets 12,000 to 6,500 years ago and then due to rising greenhouse gas concentrations.
Bova believes that the study also gives confidence in the ability of climate models to predict the future, not just simulate the past, according to Space.com.
The scientists also found that the Holocene and last interglacial Period differed due to the amount of remnant glacial ice sheets and the greenhouse gas levels. The team's reconstructions demonstrate that modern global temperature has exceeded annual levels over the Holocene era and probably approaches the levels of the last interglacial period.
“The apparent discrepancy between climate models and data has cast doubts among skeptics about the role of greenhouse gases in climate change during the Holocene and possibly in the future,” said Rosenthal in a Rutgers article. “We found that post-industrial warming has indeed accelerated the long and steady trend of warming throughout the past 10,000 years."Our study also underscores the importance of seasonal changes, specifically Northern Hemisphere summers, in driving many climate systems. Our method can, for the first time, use seasonal temperatures to come up with annual averages.”
Shital P. Godad, a former Rutgers researcher now at National Taiwan University, as well as scientists at Ohio State University and Nanjing Normal University contributed to the study.