The first period of global cooling occurred because of two volcanic eruptions occurring in the sixth century.
The first eruption, probably of an Icelandic volcano, occurred in 536.
The second eruption (in 540 or 541) was much more powerful and occurred in Mount Ilopango, El Salvador. This eruption was so massive that it ranks in the top ten of all volcanic incidents in the last 2,000 years. This event is sometimes referred to as the Tierra Blanca Joven (TBJ) eruption.

Both of these sixth century eruptions produced extreme meteorological consequences for Europe and Asia. The period from 536 though 546 has been determined to be the coldest decade of the last 2,000 years resulting in crop failures, prevalent from Ireland through China producing poor nutrition and famine throughout the Northern Hemisphere. Specifically, the historian Michael McCormick has said that the year 536 was probably the worst year to be alive.

The resulting cold weather, combined with a poorly-fed populace, produced a breeding ground for disease. In the 540's, the "Justinian Plague", named for Emperor Justinian (484 - 565), ravaged through the Eastern Roman Empire. All of these factors contributed to the decline of the Eastern Roman Empire. Some historians believe that this was the beginning of the "Dark Ages".

The volcano Ilopango, located in the midst of the Mayan civilization, also created serious problems for the Mayans. However, unlike their European counterparts, the Mayans were able to establish rescue crews and founded new cities for the people who had been displaced by the eruption. Decades later, they were able to return to their previous level of civilization. Basically, the Mayans recovered rather well from this volcanic eruption, whereas Europe and the Eastern Roman Empire slipped into the Dark Ages.

The second cooling period occurred in 1816, due to the 1815 eruption of Mount Tambora on the island of Sumbawa, Indonesia.
What were the consequences of this eruption?
This massive eruption in April 10, 1815 was the largest in human history and spewed so much volcanic dust and sulfur into the troposphere (even into the stratosphere), that it blocked out a significant portion of sunlight creating The Year Without A Summer in 1816. The Volcanic Explosivity Index ranks eruptions on a scale from zero to eight and the Tambora eruption was calssified as VEI 7.

As was the case with the sixth century volcanic eruptions, the global cooling was much more pronounced in the Northern Hemisphere. The decreased sunlight produced crop failures, which then caused famine. The cooler weather had one positive effect for America. Farmers, disappointed with their poor crop returns, decided to head west, in hopes of finding more hospitable weather. These westward migrations led to a population increase in the west, creating the states of Indiana in 1816 and Illinois in 1818.

An interesting anecdote of the inclement weather of 1816 is that during that summer, Lord Byron, Percy Shelley, Mary Wollstonecraft (Shelley) and John Polidori (future author of "The Vampyre" - 1819) were vacationing at Lake Geneva, Switzerland. The miserable weather pretty much confined them indoors, prompting Lord Byron to create a "ghost story" writing contest in which all his visitors would participate. This inspired Mary Shelley into writing Frankenstein or the Modern Prometheus (eventually published in 1818).

After mentioning these eruptions, you must be wondering why hasn't Krakatoa been discussed?
After all, you've probably never heard of Ilopango nor Tambora but you've definitely heard of Krakatoa, right?

Looking at the table below, let's compare the Tambora eruption to Krakatoa.