Little Ice Age

The Little Ice Age was a cold period that lasted from about AD 1300 to about AD 1850 in Europe, North America, and Asia. This period was marked by rapid expansion of mountain glaciers, especially in the Alps, Norway, and Alaska. There were three maxima, beginning about 1650, about 1770, and 1850, each separated by slight warming intervals. Large temperature excursions during the Little Ice Age (~1400-1900 AD) and the Medieval Warm Period (~800-1300 AD) were possibly related to changes in the strength of North Atlantic thermohaline circulation (THC).

This scenario was the basis for the Hollywood disaster movie “The Day After Tomorrow.” In that movie the thermohaline circulation system shut down and an ice age instantly gripped the planet.

Aside from views of cattails and blackbirds, the marshes in the lower Hudson Valley near New York City offer an amazingly detailed history of the area's climate. Sediment layers from a tidal marsh in the Hudson River Estuary have preserved pollen from plants, seeds, and other materials.

From the pollen record found in sediments in Piermont Marsh of the lower Hudson Valley, a Medieval Warm period was evident from 800 to 1300 AD. Researchers know this from the striking increases in both charcoal, a sign of dry vegetation and fires, and pollen from pine and hickory trees. Prior to this warming spell, there were more oaks, which prefer a wetter climate.

During the Little Ice Age from the early 1400s to late 1800s, the vegetation changed again to plants that favored cooler and wetter climates. The core records revealed increases in spruce and hemlock that prefer cooler and wetter climates.

Scientific estimates regarding the onset of the Little Ice Age range from the 13th century to the 16th century, but there is little consensus. Although the cooling temperatures may have affected places as far away as South America and China, they were particularly evident in northern Europe. Advancing glaciers in mountain valleys destroyed towns, and paintings from the period depict people ice-skating on the Thames River in London and canals in the Netherlands, places that were ice-free before and after the Little Ice Age.

The dominant way scientists had defined the Little Ice Age was 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.

A 2012 study, led by the University of Colorado Boulder with co-authors at the National Center for Atmospheric Research (NCAR) and other organizations, suggests that an unusual, 50-year-long episode of four massive tropical volcanic eruptions triggered the Little Ice Age between 1275 and 1300 AD. The persistence of cold summers following the eruptions is best explained by a subsequent expansion of sea ice and a related weakening of Atlantic currents, according to computer simulations conducted for the study.

The study, which used analyses of patterns of dead vegetation, ice and sediment core data, and powerful computer climate models, provides new evidence in a longstanding scientific debate over the onset of the Little Ice Age. Scientists have theorized that the Little Ice Age was caused by decreased summer solar radiation, erupting volcanoes that cooled the planet by ejecting sulfates and other aerosol particles that reflected sunlight back into space, or a combination of the two.

“This is the first time anyone has clearly identified the specific onset of the cold times marking the start of the Little Ice Age,” says lead author Gifford Miller of the University of Colorado Boulder. “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.”

Miller and his colleagues radiocarbon-dated roughly 150 samples of dead plant material with roots intact, collected from beneath receding margins of ice caps on Baffin Island in the Canadian Arctic. They found a large cluster of “kill dates” between 1275 and 1300 AD, indicating the plants had been frozen and engulfed by ice during a relatively sudden event. The team saw a second spike in plant kill dates at about 1450 AD, indicating the quick onset of a second major cooling event.

Existing theories as to the cause of the Little Ice Age include 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. But Miller's study suggests that an unusual, 50-year-long episode of four massive tropical volcanic eruptions brought the onset of the Little Ice Age. Climate models produced by Miller and his team showed the persistence of cold summers following the eruptions is best explained by a sea ice-ocean feedback system that originated in the North Atlantic.

Powerful eruptions inject sulfur gases into the stratosphere, where the sulfur combines with water vapor to form sulfuric acid aerosols. These tiny particles scatter sunlight back to space and alter atmospheric circulation patterns. The result is that the Earth's surface cools and precipitation increases. Crops then fail to ripen properly, and famine and pestilence follow in scarcity's wake. This sequence of events is known to have happened after the seven greatest volcanic eruptions of the past two millennia. Mass hysteria has also ensued. People became terrified that the dimmed sun would never recover its brightness in 44 BC (Mount Etna), in AD 536, and in AD 626.

Somewhere in the tropics, a volcano exploded violently during the year 1258, producing a massive stratospheric aerosol veil that eventually blanketed the globe. Arctic and Antarctic ice cores suggest that this was the world largest volcanic eruption of the past millenium. According to contemporary chronicles, the stratospheric dry fog possibly manifested itself in Europe as a persistently cloudy aspect of the sky and also through an apparently total darkening of the eclipsed Moon. Based on a sudden temperature drop for several months in England, the eruption's initiation date can be inferred to have been probably January 1258. The frequent cold and rain that year led to severe crop damage and famine throughout much of Europe. Pestilence repeatedly broke out in 1258 and 1259; it occurred also in the Middle East, reportedly as plague. Another very cold winter followed in 1260-1261. The troubled period's wars, famines, pestilences, and earthquakes appear to have contributed in part to the rise of the European flagellant movement of 1260, one of the most bizarre social phenomena of the Middle Ages.

Sustained cooling from volcanoes would have sent some of the expanding Arctic sea ice down along the eastern coast of Greenland until it eventually melted in the North Atlantic. Since sea ice contains almost no salt, when it melted the surface water became less dense, preventing it from mixing with deeper North Atlantic water. This weakened heat transport back to the Arctic and created a self-sustaining feedback on the sea ice long after the effects of the volcanic aerosols subsided, according to simulations.

The existence of numerous temperature maxima between 2200 and 250 years BP (average ~70 years) suggests that multi-decadal processes typical of the North Atlantic Oscillation (NAO) are an inherent feature of late Holocene climate. However, late 19th and 20th century temperature extremes in Chesapeake Bay associated with NAO climate variability exceeded those of the prior 2000 years, including the interval 450-1000 AD, by 2-3°C, suggesting anomalous recent behavior of the climate system.

The number of sunspots has been shown to increase and decrease over time in a regular, approximately 11-year cycle, called the sunspot cycle. The exact length of the cycle can vary. It has been as short as eight years and as long as fourteen, but the number of sunspots always increases over time, and then returns to low again.

More sunspots mean increased solar activity, when great blooms of radiation known as solar flares or bursts of solar material known as coronal mass ejections (CMEs) shoot off the sun's surface. The highest number of sun spots in any given cycle is designated "solar maximum," while the lowest number is designated "solar minimum." Each cycle varies dramatically in intensity with some solar maxima being so low as to be almost indistinguishable from the preceding minimum.

Early records of sunspots indicate that the Sun went through a period of inactivity in the late 17th century. Very few sunspots were seen on the Sun from about 1645 to 1715.

One such set of cycles famously occurred from 1645 to 1715 and is known as the Maunder Minimum. Those who watched the sun could count enough change in sunspot number that they could still track cycles, but the overall sunspot number dropped drastically. One thirty-year period showed only 30 sunspots, one thousandth of what is typically seen.

The timing of the Maunder Minimum corresponded to what's called the Little Ice Age in Europe—a time of colder weather, heavier snowfall and the freezing of unusually large bodies of water such as the Thames and even the Baltic Sea. The Little Ice Age lasted longer than the Maunder Minimum, and there are other potential causes. Nevertheless it is believed by many scientists that the prolonged solar minima and its corresponding decrease in solar energy cooled Earth.