The study, using results from analyse of dead vegetation, ice sediment core data and computer climate models provided evidence that the major cause of the Little Ice Age was volcanic eruptions that ejected sulfates and other aerosol particles that reflected sunlight back into space.
The Little Ice Age is often considered to have persisted as late as the 19th century. During the period, glaciers advanced on major towns in northern Europe and the Thames River in London froze. Other parts of Europe that are now free of Ice also froze, such as the canals in the Netherlands. Science Daily
reports that paintings from the period of the Little Ice Age show people ice-skating on the Thames River in London and the canals in the Netherlands.
According to Gifford Miller, geologist and professor at the University of Colorado, Boulder, and lead researcher in the study, the Little Ice Age began long before its effect was felt in Europe. He explains: "The dominant way scientists have defined the Little Ice Age is by the expansion of big valley glaciers in the Alps and in Norway. But the time in which European glaciers advanced far enough to demolish villages would have been long after the onset of the cold period."
Little Ice Age caused by aerosols spewed from massive volcanic eruptions
During the Little Ice Age, global temperatures dipped by one degrees Celsius or 1.8 degrees Fahrenheit, leading to the disappearance of many Alpine villages under expanding glaciers. The New York Times
reports that in the winter of 1780, New Yorkers could walk the five miles from Manhattan to Staten Island on ice as thick as eight feet.
reports Miller, commenting on the significance of the study, said: "This is the first time anyone has clearly identified the specific onset of the cold times marking the start of the Little Ice Age. We also have provided an understandable climate feedback system that explains how this cold period could be sustained for a long period of time."
According to the study, the Little Ice Age appears to have been caused by massive tropical volcanoes whose eruptions spewed tiny particles of aerosols into the Earth's atmosphere. When the aerosols are suspended in the air, they reflect solar radiation back into space and cause the planet below to cool. Four huge eruptions of tropical volcanoes occurred between 1275 and 1300.
The sea ice feedback mechanism
The researchers believe that the cooling effect on the atmosphere continued after the aerosols had dissipated by sea ice feedback in the North Atlantic Ocean. This refers to the mechanism by which expanding sea ice melting into the North Atlantic interferes with the mixing between surface and deep waters causing water flowing back to the Arctic to be cooler and thus preventing large areas of sea ice from melting.
Simulations showed cooling from volcanic eruptions would have sent some of the expanding Arctic sea ice down along the eastern coast of Greenland into the North Atlantic. Sea ice contains very little salt, so when it melted into the sea, the surface water became less dense, preventing mixing with deeper and warmer North Atlantic water. This affected heat transport back to the Arctic and initiated a self-sustaining sea ice feedback long after the cooling effects of volcanic aerosols had worn off.
The sea ice, in turn, reflects sunlight back into the atmosphere and initiates a feedback loop that prolongs the cooling effect in the atmosphere.
Evidence from clusters of plant 'kill dates'
Miller and his co-workers, through radiocarbon dating of dead plants on Baffin Island in the Canadian Arctic, found that many plants at both low and high altitudes died between A.D. 1275 and A.D. 1300. This was evidence that Baffin Island froze suddenly. Many plants died around A.D. 1450, indicating there was also a major cooling around this period. The plant "kill date" ranges coincided with a half-century period of highest volcanic activity in the past millennium.
The New York Times
reports that to further confirm their conclusions from evidence obtained on Baffin Island, the scientists analyzed sediment cores from a glacial lake in Iceland. The thickest layer of sediments matched the dates of the plant deaths on Baffin. Thickness of the sediment core indicates increased erosion due to expansion of the ice cap between the late 13th century and the 15th century.
Miller, according to Space.com
, explained the significance of the finding of thickened layers of sediment core in Iceland :"This gave us a great deal more confidence that there was a major perturbation to the Northern Hemisphere climate near the end of the 13th century."
Climate simulation models
reports the scientists also did a climate simulation that showed that major volcanic eruptions occurring close to each other could have cooled the Northern Hemisphere sufficiently to induce sea ice expansion and then a feedback loop as already described. According to Otto-Bliesner, climatologist at the National Center for Atmospheric Research, and co-author of the study: "Our simulations showed that the volcanic eruptions may have had a profound cooling effect. The eruptions could have triggered a chain reaction, affecting sea ice and ocean currents in a way that lowered temperatures for centuries."
According to Science Daily
, the researchers set solar radiation at a constant level in the climate simulation models they built and the results they got indicated that the Little Ice Age would have occurred without decreased summer solar radiation. The scientists thus concluded that decreased solar radiation may not have played a major role in causing the Little Ice Age, as some scientists have speculated in the past.
Volcanic eruptions, Little Ice Age and Global Warming
New York Times
reports that Otto-Bliesner said the study has importance for current debate on global warming. She said: “I think people might look at the Little Ice Age and think that all we need to save us from rising temperatures are some volcanic eruptions or the geo-engineering equivalent. But when you see what happened when global temperatures dropped by just one degree and you look at current predictions of six or seven degree increases for the future, you realize how precarious things are for life as we know it...I don’t see a lot of hope that we can somehow compensate for the climate trajectory we’re on."
The study was published in the journal Geophysical Research Letters.