A bit off the main topic of this blog, but interesting and relevant nonetheless is this recent news item from Indonesia:
Lava and hot gas clouds from a volcano in eastern Indonesia are threatening more villages, officials said on Tuesday.
...The volcano, on the diving resort island of Siau off Sulawesi and 2,325 km (1,445 miles) northeast of the capital Jakarta, is one of Indonesia's most active volcanoes.
...The alert status on the 1,827 meter (5,994 feet) volcano was raised to maximum at the weekend after hot clouds started moving eastwards, posing a threat to hundreds of people.
The relevance to climate is the possibility that it might explode with sufficient force to inject aerosol into the stratosphere. A recent example of the climates impacts of volcanic eruptions would be Mt Pinatubo in 1991:
The cloud over the earth [from the eruption] reduced global temperatures. In 1992 and 1993, the average temperature in the Northern Hemisphere was reduced 0.5 to 0.6°C and the entire planet was cooled 0.4 to 0.5°C. The maximum reduction in global temperature occurred in August 1992 with a reduction of 0.73°C. The eruption is believed to have influenced such events as 1993 floods along the Mississippi river and the drought in the Sahel region of Africa. The United States experienced its third coldest and third wettest summer in 77 years during 1992.
The short- and longer-term climate impacts are potentially quite significant. Other 'recent' eruptions with a global climate impact were observed Mt. Agung in 1963 and El Chichon in 1982. How likely is an event of this magnitude from this volcano? (Keeping in mind that I have no specific knowledge in this field, and may not know what I am talking about...)
The eruption history of Karangetang suggests that this is unlikely. The volcano erupts quite often, with small to moderate eruptions. The largest Volcanic Explosivity Index (VEI) from this volcano is 3 (Moderate to Large)**. This table suggests that stratospheric injection is 'Possible'. These events are rare in the history of this volcano, more likely is a 1 or 2. These facts suggest that most likely, this volcano will not have a significant impact on the global climate.
**For comparison with other eruptions, Pinatubo has VEI=6 , and the other two eruptions noted above had VEI=4. Its apparently not all VEI, though. Location is also important in determining the overall climate impact. Tropical eruptions are more efficient at cooling the globe. I would also guess that the amount of ejecta, particularly sulfur dioxide, from the volcano would also play a role.