What Led the Vikings to Leave Greenland?

During the Medieval Warming Period around 1000 CE the vikings in Greenland prospered. Because they were more farmers than fisherman, their agriculture flourished. With the melting of the ice on and around the island, the Northwest Passage opened for travel and trade with European countries. As the temperatures dropped falling into what is known as the Little Ice Age, a drop of about 1.1 degree Fahrenheit by around 1700 CE, many things happened. This climate change meant a change in the amount and placement of ice, changing their boating routes. Around the same time, the demands for their exported goods was lowered and the more difficult boating routes along with less income made it more difficult to attain the necessary imported supplies. Colder temperatures also brought freezing ground, and difficulty farming; although evidence suggests they were able to adapt to fishing fairly easily. Therefore because the drop in temperature brought changes to the climate and altered their land and boating routes, the vikings also had to adapt and leave Greenland.

Figure 1: Temperature from 1AD to 2000AD

Image Source: http://wattsupwiththat.files.wordpress.com/2010/09/loehle_v_fig1.png

This figure shows the temperature from the year 1AD to 2000AD. As you can see there is a gradual drop from around the year 1000 to around 1700. This is the drop from the Medieval Warming Period into the Little Ice Age, about .6 degrees Celsius or about 1.1 degrees Fahrenheit. This is the period in which the Vikings in Greenland experienced changes and decided to leave.

What Caused the Ice Ages?

There are many factors that together can be used to try and explain what caused the ice ages the Earth has experienced. In order for an event to occur many things need to happen. There need to be certain boundary conditions, a trigger (which without those specific conditions would be irrelevant), feedbacks and a tipping point. 
To explain the ice ages we have to set our boundary conditions: the Earth rotating on it’s axis. This is how the Earth is and how it rotates around the Sun, plain and simple. The trigger would then be the Sun’s output of radiation as well as the Earth’s position on the axis as well as in relation to the Sun. These things alone do not cause an ice age, but when factored all together the Earth experiences a feedback. The Earth may be farther away from the Sun at a point and temperature drops because of the lessened amount of radiation reaching the surface. The temperature drops, which causes ice to form. The more ice that forms, the more radiation is reflected from the surface, which causes the temperature to drop even more. The tipping point is the resulting glaciers and ice sheets that cover large masses of land. 

Figure 1: Earth's Axis and Rotation
Image Source: www.eoearth.org
In order to help illustrate the effects that the Earth's position in relation to the Sun has, here is a figure to illustrate how the Earth's axis changes. You can see how the position of the Earth on it's axis has changed as well as it's relative position to the Sun.