Prior to the advent of human-caused global warming in the 19th century, the surface layer of Earth's oceans had undergone 1,800 years of a steady cooling trend, according to a new study. During the latter half of this cooling period, the trend was most likely driven by large and frequent volcanic eruptions.An international team of researchers reported these findings in the August 17, 2015 issue of the journal Nature Geoscience. The study also indicates that the coolest temperatures occurred during the Little Ice Age–a period that spanned the 16th through 18th centuries and was known for cooler average temperatures over land.