Tambora volcano in Indonesia has been raised to level 2 alert (out of maximum 4) after an increase in volcanic earthquakes. In April 2011 there were 37 shallow volcanic earthquakes recorded, 167 in May, 277 in June, 363 in July, and 141 from 1-29 August. Continuous tremor was recorded on 29 and 30 August. Tourists and locals are advised to avoid the summit area of Tambora volcano. In 1815 Tambora had one of the largest eruptions on earth in the past 1000 years and ejected 100-150 cubic km of lava.
Tambora erupted in 1815 killing 92 000 people making 1816 the year without a summer as the global climate effects were felt. Aerosols from the Tambora eruption blocked out sunlight and reduced global temperatures by 3 deg C. Europe missed a summer, and India had crop failures following the Tambora eruption. 100 cubic km of magma was erupted. Ten thousand people were killed immediately from the pyroclastic flows and the eventual toll due to starvation and disease may have been as high as 117,000. The eruption caused a tsunami with a wave height of 10 m. (VolcanoLive)
What most people don't know is that after the 1815 eruption, Tambora has erupted at least 3 known times, all in the scale of VEI 0-2 (rather than the VEI 7 of 1815).
The PVBMG reports says that since April 2011, the volcano has seen intermittent swarms of earthquakes and now at the end of August, there seems to be an increase in these shallow, volcanic earthquakes along with the presence of tremor. Combine that in observations of steam plumes rising tens of meters over the edge of the caldera rim, then you might have the signs of impending activity - thus the elevation of the alert status from Level I to II. Now, what kind of activity might we expect? Most likely we would expect something small, akin to the VEI 0-2 eruptions in 1819, 1880 and 1967, which were mainly lava flows and domes confined to the caldera floor, and most definitely not another VEI 7 event like the 1815 eruption. (BigThink)