1,800-Year-Long Cooling Trend Results from a Series of Volcanic Eruptions

Frequent volcanic eruptions are now thought to have contributed to an 1,800-year-long cooling period--coolest during the 16th and 18th centuries--brought to an end in the 19th century by global warming. 

"Today, the Earth is warming about 20 times faster than it cooled during the past 1,800 years," Michael Evans, second author of the study and an associate professor at the University of Maryland, said in a statement. "This study truly highlights the profound effects we are having on our climate today."

Researchers also found that the concurrence of cooling events on both land and sea indicated that the period of global cooling was altered by human-caused global warming. This is evident because oceans absorb and trap much more heat, for a longer period of time, than the atmosphere does. So, they can often defend themselves against short-term fluctuating conditions. However, this is not the case when a series of volcanic events occur within a relatively short period of time. Their research was recently published in the journal Nature Geoscience. 

"Volcanic eruptions have a short-term cooling effect on the atmosphere, but our results showed that when volcanic eruptions occurred more frequently, there was long-term ocean cooling," said lead author Helen McGregor, at the University of Wollongong in Australia. "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."

When investigating the cause behind this cooling trend, researchers examined climate models to determine 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. According to the release, they found that only volcanic events resulted in a cooling trend that matched the team's real-world observations. Understanding how such factors changed ocean temperatures in the past can help explain future climate changes.

"Model simulations by others have shown us that the oceans can impart a substantial delay in the warming of the surface climate," said Evans in a statement. "With much of the heat from global warming entering our oceans, recent ocean surface warming may foreshadow additional future warming, in the same way ocean cooling appeared as a long-term response to large and frequent volcanic events in recent centuries."

Evans added that there is still much to be learned about the ocean's role in climate change, in the statement. 

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