There are several competing theories to explain the possible cause of the climate changes experienced during the MWP and the LIA. These include sunspot variations, volcanic eruptions, changes in the large-scale ocean current conveyor belt, and to a lesser extent, changes in the earth's albedo. None of these theories on their own offers conclusive evidence; it is more likely that each has played a role. One should also note that these causes CANNOT explain the warming experienced from 1900 to present.
Note to general public:
My position on the current global warming is the same as the overwhelming majority of international climate scientists: the current rate of global warming is unprecedented and is being caused by humans. In no way should my summary of the research regarding climate change and the Viking civilization/Little Ice Age be used to "prove" the current global warming is due to a natural cycle. Human forcing (output of greenhouse gases) was just not as large a factor before the 1900s as natural forcing mechanisms. That would be like comparing the number of traffic fatalities today vs. a time when there were no cars!
I highly recommend that you read the information being presented by the Intergovernmental Panel on Climate Change (IPCC) at http://www.ipcc.ch/. Please view my two .PDF files Global Warming: Man or Myth? and IPCC WGI FAQ which address many of the questions asked about the human impact on the current climate change in a very simple format.
It is true that there are natural climate change cycles but most of these are on the order of thousands of years to millions of years. The current global warming is not a natural cycle and is of utmost importance because modern-day humans live on time scales much smaller than the natural cycles. Therefore, mankind cannot just simply wait thousands of years for a natural cooling to occur.
It is my opinion that those who still proclaim that "the jury is still out" or that modern-day climate change is natural, are either ignorant about the scientific evidence or are politically motivated to ignore it.
Because the sun is Earth's greatest source of energy and is the driving force behind its atmospheric circulation, any variation in solar output will influence the weather. Scientists have observed that the number of sunspots on the surface of the sun has been determined to correspond to solar output variability. More sunspots correspond to a higher solar energy output while fewer sunspots correspond to a lower solar output. A record of sunspot numbers has been recorded through time by various indicators including naked eye observations, auroral reports, and C14 isotope concentrations in tree rings (Schaefer, 1977.) Fig. 8 shows that during the MWP there was a high number of sunspots referred to as the Medieval Maximum, while during the LIA there were two periods of very low sunspot numbers called the Spörer Minimum and Maunder Minimum. Although a direct link has not yet been established between sunspot variability and climate change, the data is highly suggestive.
Ash and other small particulate matter injected into the stratosphere can effectively reduce incoming solar radiation received at the earth's surface. Sulfur compounds from eruptions condense into very tiny sulfuric acid droplets that form clouds which may stay suspended in the stratosphere for years, further reducing incoming sunlight (Pollock et al., 1976.) Fig. 9 illustrates the process.
Large eruptions at low latitudes can cause the greatest global climate change. Weaker eruptions only send their eruptive materials into the troposphere where weather processes quickly remove them and high latitude eruptions only send their materials into one hemisphere. The explosion of Mt. Tambora in 1815 led to the year 1816 being called "the year without a summer" across much of Europe. The eruption of Mt. Pinatubo in 1991 provided a good example of how a large low-latitude eruption can quickly influence global climate. Fig. 10 shows how in nine days the sulfur dioxide plume had spread into both hemispheres and around half the planet.
The result was an estimated 1oC global cooling that lasted two years. It is unlikely that a single eruption can cause long-term cooling over hundreds of years such as during the LIA. Robock (1979) has shown that there was an increase in the frequency of large eruptions during the LIA that corresponds quite well with the coolest years during this time period.
Fig. 11 shows the large-scale ocean current conveyor belt.
Warm waters in the upper 1500 meters flow northward to the vicinity of Iceland. Winter cooling increases the density of the water permitting it to sink to great depths. Once at depth, the water flows the length of the Atlantic and becomes mixed into the deep southern hemisphere current. Because the ocean and atmosphere are a coupled system any changes in this large-scale ocean circulation could cause large-scale atmospheric changes on the order of hundreds of years (Miller, 2000.) The ocean is both a heat source for the atmosphere by releasing carbon dioxide, a greenhouse gas, and a heat sink by conducting heat away from the air that rests upon it. Surface water that comes into contact with air is referred to as ventilated water. Broecker et al. (1999) have demonstrated that very high rates of deep water ventilation occurred during the LIA, which means the oceans were removing heat from the atmosphere at a greater rate than normal during that period. That could explain the dramatic cooling observed during the LIA.
Albedo is a measure of the reflectivity of a surface. Snow and ice have a high albedo because their properties allow them to reflect up to 90% of incoming sunlight. After a global cooling event has begun, it can become self-perpetuating. With increased snow cover and glaciation, the planet's surface will have a higher albedo, which in turn will cause more incoming sunlight to be reflected. With less sunlight being absorbed at the earth's surface there will be a subsequent cooling effect. This cooling effect may cause even more snow cover and glaciation that would increase the planet's albedo even more. As the climate cooled during the LIA, earth's albedo increased due to more snow and greater glaciation. The process can last for many years however, it eventually does subside because cooler oceans experience less evaporation which leads to a decrease in cloud cover. Reduced cloud cover allows more sunlight to reach the surface which results in higher global air temperatures.