Scientists looking at the earth's climate history of the past 10,000 years say their research has thrown up a puzzle; some studies show consistent warming, others suggest it was cooling until human industrial activity reversed that trend.

When asked by the Intergovernmental Panel on Climate Change to provide figures for the panel's latest summary, the researchers knew what they had to say would cause problems.

"We have been building models and there are now robust contradictions," says Zhengyu Liu, a researcher at the Center for Climatic Research of the University of Wisconsin-Madison. "Data from observation says global cooling. The physical model says it has to be warming."

Liu, along with colleagues from the U.S., Europe and Asia, report in the Proceedings of the National Academy of Sciences a consistent warming throughout the period of the current geological period, known as the Holocene, but they acknowledge their findings run counter to a previous study describing a period before modern human influences when the world was, in fact, cooling.

That study presented evidence of a worldwide cooling trend beginning about 7,000 years ago that continued until humans began affecting the climate, most notably since the beginning of the industrial revolution and through the 20th century.

While none of the scientists disagree with the idea that humans have had a global impact on climate, the mystery of whether it was warming or cooling beforehand still remains.

"The question is, 'Who is right?'" says Liu. "Or, maybe none of us is completely right. It could be partly a data problem, since some of the data in last year's study contradicts itself. It could partly be a model problem because of some missing physical mechanisms."

Liu and his research colleagues created computer models incorporating climate influences such as changes in greenhouse gases, the intensity of sunlight reaching the Earth through our atmosphere, and the advance and retreat of ice sheet coverage.

They ran three models with different combinations of those influences and found all three models suggested consistent warming during the 10,000 years studied.

And yet, a study last year that analyzed data in ice cores, sediments of phytoplankton and other evidence from 73 global sites suggests a period of cooling climate everywhere on Earth that lasted until humans began to leave their mark.

Lui said such samples might not accurately address the larger global picture. For example, he said, biological samples from cores in strata laid down in summer could yield different results from samples in the same location in wintertime sediments.

"In the Northern Atlantic, there is cooling and warming data the (climate change) community hasn't been able to figure out," says Liu.

The previous study's authors had been aware of that limitation and acknowledged it in their published report.

Further research is needed by all scientists to secure more data about what they've dubbed the Holocene temperature conundrum, he says.

"Both communities have to look back critically and see what is missing," Liu says. "I think it is a puzzle."

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