During the late Middle Ages, many of the glaciers in Arctic Canada and Iceland experienced abrupt increases in their size, due to substantially cooler summer months. This advance of glaciers has been termed the Little Ice Age (LIA), and the expansion of the glaciers has only been undone in the last decade or two. There are several hypotheses as to the cause of the glacial expansion during the LIA, but until now, no definitive answers have emerged. However, the retreat of these glaciers over the last few decades has allowed scientists to study the plants that were killed by the advance, determine the dates of the expansion of the glaciers, and connect the timing with global events to try to assign a cause to the LIA.
First, though, it helps to explain a little astronomy. As the Earth orbits the sun, there are 3 separate rotations occurring. The fastest is the cycle of day and night, caused by the Earth moving about its own axis of rotation. The next longest is the Earth’s rotation around the sun, which causes the seasons. Imagine a plane that cuts through the center of the Earth and the Sun, and has on it the ellipse that the Earth follows around the sun. This is called the plane of the ecliptic. The axis of rotation of the Earth (i.e., the one it rotates around to cause day and night) is tilted with respect to this plane - that is to say, it is not perpendicular to the plane. This is what causes the seasons, as described in the video below.
In addition to these rotations that occur on human time scales, there is a much longer process called the precession of the equinoxes, which occurs on nearly a 26,000 year cycle. As the Earth precesses, the insolation (the amount of sunlight received on the surface of the Earth) decreases, and therefore, the summer temperatures in the Northern Hemisphere have been slowly declining over the last 8000 years. However, this decrease in insolation is insufficient to explain several cold snaps that have occurred on the time scale of centuries instead of millennia.
The present work was done by a group of researchers with representatives from five different institutions from Colorado, California, Iceland and Scotland. They traveled to Arctic Canada and Iceland to collect moss samples for use in radiocarbon dating. These moss samples had all been recently exposed by retreating glaciers, and by comparing the carbon content of dead samples with living samples, the authors were able to accurately determine the date of the advance of the glaciers. This is because organisms that are alive have a constant amount of one particular isotope of carbon (14C; carbon usually has a mass of 12 atomic mass units (amu), the superscript 14 indicates that the mass of this carbon is 14 amu). Once an organism dies, no new 14C is taken in, and the 14C begins to decay naturally with a known rate. By comparing the amount of 14C in living organisms and organisms that died due to glaciers, the authors could place a date on when their samples died. The results of the radiocarbon dating show that there were two abrupt periods of glacial expansion, between 1275 and 1300 AD, and again around 1450 AD. Very little vegetation had dates of death after 1450 AD, indicating that the wide expanse of these glaciers stuck around from that time until the early 20th century, according to the authors.
In addition to examining the dates of expansion of the glaciers, the authors conducted computer simulations to determine the cause of the Little Ice Age. It is relatively well known (but don’t read the comments on the linked article, ugh) that volcanic eruptions can cause short term effects on global temperatures. Indeed, there were four explosive volcanic events around the time of the first glacial expansion (1275-1300 AD) and an additional explosion in 1452 AD, corresponding to the second expansion. However, what was unknown was how the relatively short term effects of volcanic explosions can cause many centuries of depressed Arctic temperatures, allowing the glaciers to remain until the 20th century. The computer simulations showed that there was a positive feedback mechanism between the glacier expansion caused by the volcanic explosions and summertime air temperatures, due to the reflective effect of the ice. Glacial and sea ice have a very strong cooling effect on the air temperature because they tend to reflect sunlight back into space. Because the air is cooler above glaciers and sea ice, more sea ice is formed and more sunlight is reflected in a loop that the authors assert caused the maintenance of the glacial sheets until the modern era. The authors conclude that the combination of volcanic explosions and lower-than-modern solar insolation triggered the glacial expansion and a positive feedback loop between the glaciers and the air temperatures maintained the glacial extent until the 20th century.