Global warming halts 1,800 years of steady ocean cooling

Global warming caused by human activities has halted 1,800 years of steady cooling trend in the oceans of the world, shows new research.

"Today, the Earth is warming about 20 times faster than it cooled during the past 1,800 years," said second author of the study Michael Evans, associate professor at University of Maryland in the US.

"This study truly highlights the profound effects we are having on our climate today," Evans noted.

During the latter half of this cooling period, the trend was most likely driven by large and frequent volcanic eruptions.

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, the study found.

The concurrence of cooling events on both land and sea suggests that a global cooling phenomenon was erased by subsequent human-caused global warming.

"With this research, we now have new insight into the century-scale global sea-surface temperature variations that came before man-made greenhouse gas forcing," lead author Helen McGregor from University of Wollongong in New South Wales, Australia noted.

The scientists combined 57 previously published marine surface temperature reconstructions that cover all of the world's oceans, from near-polar to tropical regions.

The team compiled the data within 200-year brackets to observe long-term trends, and then compared the findings to land-based reconstructions, which revealed similar cooling trends.

To investigate the cause of the cooling trend, the researchers turned to climate models.

They examined how sea-surface temperatures reacted to various "forcing" factors, such as changes in solar output, Earth's orbit, land use, volcanic activity and greenhouse gases.

The study was published in the journal Nature Geoscience.