Using data obtained from 73 ice and sediment core monitoring sites around the world, scientists have reconstructed Earth's temperature history back to the end of the last Ice Age.
The analysis by researchers at Oregon State University (OSU) and Harvard University has revealed that the planet today is warmer than it's been during 70 to 80 percent of the last 11,300 years.
Previous research on past global temperature change has largely focused on the last 2,000 years, said lead paper author Shaun Marcott of OSU.
Extending the reconstruction of global temperatures back to the end of the last Ice Age puts today's climate into a larger context.
"We already knew that on a global scale, Earth is warmer today than it was over much of the past 2,000 years. Now we know that it is warmer than most of the past 11,300 years," Marcott said.
The research was funded by the Paleoclimate Program in NSF’s Division of Atmospheric and Geospace Sciences.
"The last century stands out as the anomaly in this record of global temperature since the end of the last ice age," said Candace Major, program director in the National Science Foundation's (NSF) Division of Ocean Sciences.
"This research shows that we've experienced almost the same range of temperature change since the beginning of the industrial revolution," added Major, "as over the previous 11,000 years of Earth history--but this change happened a lot more quickly."
What that history shows, the researchers said, is that during the last 5,000 years, the Earth on average cooled about 1.3 degrees Fahrenheit--until the last 100 years, when it warmed about 1.3 degrees F.
The largest changes were in the Northern Hemisphere, where there are more land masses and larger human populations than in the Southern Hemisphere.
Climate models project that global temperature will rise another 2.0 to 11.5 degrees F by the end of this century, largely dependent on the magnitude of carbon emissions.
"What is most troubling," Clark said, "is that this warming will be significantly greater than at any time during the past 11,300 years."
Marcott noted that one of the natural factors affecting global temperatures during the last 11,300 years is a gradual change in the distribution of solar insolation linked with Earth's position relative to the sun.
"During the warmest period of the Holocene, the Earth was positioned such that Northern Hemisphere summers warmed more," Marcott explained.
"As the Earth's orientation changed, Northern Hemisphere summers became cooler, and we should now be near the bottom of this long-term cooling trend--but obviously, we're not," he said.
The research team, which included Jeremy Shakun of Harvard and Alan Mix of OSU, primarily used fossils from ocean sediment cores and terrestrial archives to reconstruct the temperature history.
The chemical and physical characteristics of the fossils--including the species as well as their chemical composition and isotopic ratios--provide reliable proxy records for past temperatures by calibrating them to modern temperature records.
Analyses of data from the 73 sites allow a global picture of the Earth's history and provide a new context for climate change analysis.
"The Earth's climate is complex and responds to multiple forcings, including carbon dioxide and solar insolation," Marcott said.
"Both changed very slowly over the past 11,000 years. But in the last 100 years, the increase in carbon dioxide through increased emissions from human activities has been significant. It's the only variable that can best explain the rapid increase in global temperatures," he added.
Results of the study are published this week in a paper in the journal Science.