From the dawn of the Roman Empire until the late 19th century, a recent study suggested that the average surface temperatures on Earth gradually cooled. However, that cooling trend reversed rapidly and the 20th century became one of the warmest in human history.

Natural phenomenon, such as volcanic activity and fluctuations in solar activity, drove the global cooling, but the end to that chilly trend coincided with human’s increasing use of fossil fuels and widespread deforestation during the late Industrial Revolution.

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“The natural forces driving the cooling are still present today, but since the nineteenth century an additional, stronger, warming driver has been added: human activity,” said Paul E. Filmer of the National Science Foundation’s Geoscience Directorate in a press release. “We cannot match the temperature records since then without factoring in this new driver.”

Filmer joined an international team of 78 authors to scour biological and geological clues to Earth’s temperature history. Their study is being published in Nature Geoscience.

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Data from multiple sources helped to parse out Earth’s temporal temperature changes, including:

  • Trees – Analysis of tree-rings provided a record of the plants’ growth, which relates to air temperatures;

  • Pollen – Changes in preserved tree pollen abundance registers fluctuations in dominant species, which gives a clue to what the local climate;

  • Coral – Dome corals record sea surface temperatures in their annually banded skeletons

  • Ice – Temperature changes left a chemical signature on the water molecules trapped in ice cores from the Arctic, Antarctic and glaciers in the temperate regions;

  • Lakes – The physical and biological properties of sediments from lake bottoms recorded changes in surrounding ecosystems and the environment.

“The primary results: The long-term cooling trend, the century-scale differences between regions, and the warmth of the 20th century, are apparent no matter how you look at the data,” coauthor Nicholas McKay, a postdoctoral researcher at Northern Arizona University, said in a press release.

IMAGE: A river of melt-water running down the slope of a toe of the Athabasca Glacier. (Wing-Chi Poon, Wikimedia Commons)