Scientists have disagreed for many years over the precise cause for a period of cooling global temperatures that began after the Middle Ages and lasted into the late 19th century, commonly known as the Little Ice Age.
Now, a new study led by CU-Boulder Professor and Institute for Alpine and Arctic Research (INSTAAR) Fellow Gifford Miller indicates that the Little Ice Age began abruptly between A.D. 1275 and 1300, triggered by repeated, explosive volcanism and sustained by a self-perpetuating sea ice-ocean feedback system in the North Atlantic Ocean.
''This is the first time anyone has clearly identified the specific onset of the cold times marking the start of the Little Ice Age,'' said Miller. ''We also have provided an understandable climate feedback system that explains how this cold period could be sustained for a long period of time. If the climate system is hit again and again by cold conditions over a relatively short period-in this case, from volcanic eruptions-there appears to be a cumulative cooling effect.''
Most scientists believed the Little Ice Age was caused either by decreased summer solar radiation, erupting volcanoes that cooled the planet by ejecting shiny aerosol particles that reflected sunlight back into space, or a combination of both, said Miller.
The new study, funded by the National Science Foundation and the Icelandic Science Foundation, suggests that the onset of the Little Ice Age was caused by an unusual, 50-year-long episode of four massive tropical volcanic eruptions. Climate models used in the new study showed that the persistence of cold summers following the eruptions is best explained by a sea ice-ocean feedback system originating in the North Atlantic Ocean.
"Our simulations showed that the volcanic eruptions may have had a profound cooling effect,'' says NCAR scientist Bette Otto-Bliesner, a co-author of the study. ''The eruptions could have triggered a chain reaction, affecting sea ice and ocean currents in a way that lowered temperatures for centuries."
The researchers set the solar radiation at a constant level in the climate models, and Miller said the Little Ice Age likely would have occurred without decreased summer solar radiation at the time. ''Estimates of the sun's variability over time are getting smaller, it's now thought by some scientists to have varied little more in the last millennia than during a standard 11-year solar cycle,'' he said.
One of the primary questions pertaining to the Little Ice Age is how unusual the warming of Earth is today, he said. A previous study led by Miller in 2008 on Baffin Island indicated temperatures today are the warmest in at least 2,000 years.