© Smithsonian, NASA Landsat7 image
Mt. Tambora
The eruption of the Mt. Tambora volcano in April 1815 was the largest and most deadly volcanic eruption in recorded history. Its sunlight dimming particles caused a major cooling in the global climate that led to 1816's "Year Without a Summer".

Experts are now saying that Mount Tambora is ready to erupt again as a sequence of earthquakes has been shaking the island at increasing frequency since April. Columns of ash are already venting as high as 4,600 feet. (Note: Tambora was about four times more powerful than Indonesia's much better-known Krakatoa blast of 1883 - history's second-deadliest).

While it appears that Tambora is on the verge of erupting, no one knows with confidence how big it will be. Given the potential consequences, Indonesian authorities have raised the volcano alert to its second-highest level. Active disaster preparedness is underway with evacuation routes mapped and armed forces pre- deployed if the worst occurs (alert status reaching the highest level).

The 1815 explosion of Tambora was heard 1,600 miles away, ash fell at least 810 miles away, and pitch darkness was observed up to 370 miles from the mountain summit for up to two days. More than 71,000 people were killed directly on the spot within the first few days following the eruption

Of great significance in the longer term was that the Tambora eruption ejected immense amounts of volcanic dust more than 140,000 ft into the stratosphere (25 times more than the 1980 Mount St. Helens and five times more than the 1991 Mount Pinatubo eruptions). While ash particles settled out of the atmosphere within a couple weeks to a few months after the eruptions, stratospheric sulfate aerosols remained in the upper atmosphere up to a few years while spreading around the globe by the prevailing winds. The aerosol veil dimmed sunlight at lower atmospheric levels. The consequent global cooling started a chain of events which collectively altered the course of human history.

During the 1816 "year without a summer", northern areas of the United States experienced extreme frosts and heavy snow well into July and August. For example, during the period June 4-10 a major snow and ice storm buried the region from Canada to Pennsylvania along and to the east of the Appalachian mountain chain. In July and August, lake and river ice were observed as far south as Pennsylvania and northern Maryland. Temperatures occasionally rose in the Northeast to as high as 95, but it's reported that relief came at the expense of cold fronts plunging the thermometer to near freezing within hours.

In the mid-Atlantic, CWG's Kevin Ambrose found:
Thomas Jefferson's records for 1816 show that the ice in his ice house at Monticello did not melt until October 11, 1816, which was a month longer than normal. Also, the Maryland Weather Service in 1906 studied the Summer of 1816 and concluded it was 8 degrees F below normal in Baltimore. The next coldest summer was 1836, which was 4.3 degrees below normal.
If you think that's harsh, the following "volcanic winter" brought exceptional cold and heavy snowfalls. New York City dropped to -26 which resulted in freezing of New York's Upper Bay. (Anyone know if this has happened since?).

The impacts of bitter cold period extended into Atlantic Canada and parts of Western Europe and led to the worst famine of the 19th century. It brought widespread food riots, untold personal suffering, and migration of large populations in search of food.

Reports indicate it touched off the wave of emigration (from the hardest hit Northeast) that led to the settlement of what is now the American Midwest. Worldwide, the aftermath of the Tambora eruption led to the additional death of tens of thousands people.

Will 2012 be another year without a summer?

Chief of Indonesia's Geological Disaster Mitigation and Volcanology Center told Viva News the tremendous Tambora eruption is unlikely to repeat.

Tambora in 1815 had tall peak with sizeable magma chamber. There is a very slight chance that the volcano will have as huge an explosion as it did in 1815.

Similarly, Our Amazing Planet reported any Tambora eruption should be smaller than 1815's blast:
In fact, having had such a large eruption as recently as 1815 (geologically speaking) makes it less likely that a similar eruption will happen anytime soon, [Erik] Klemetti said [a volcanologist at Denison University].

Any new eruption would likely be similar to these smaller eruptions, because as with most volcanoes, there are long periods between huge eruptions, which take time to build up. For very large eruptions, the period in between can be upwards of hundreds to thousands of years.
However, the track record of predicting the strength of volcanic eruptions even with the latest monitoring capabilities is not 100 percent reliable.

So the question for now remains unanswerable. The odds seem low but only time will tell.