The recent volcanic activity in Iceland and Chile has shown us that all it takes is a couple of good volcanic eruptions to completely screw up a large amount of air travel.
In 2010 it was absolute chaos when planes across Europe were grounded for weeks and travel was severely disrupted all thanks that Icelandic volcanic whose name nobody can pronounce.
After a few weeks the ash cleared enough for flights to resume but if the eruption had continued then the disruptions could have lasted for months!!

In the Southern Hemisphere right now, flights have been grounded in New Zealand, Australia and parts of South America due to ash from the Chilean volcano, Puyehue-Cordon Caulle.
So when you stop and think about it, all it would take is for two or more large volcanos in the Northern and Southern hemispheres to erupt at around the same time for WORLD-WIDE air travel to grind to a screeching halt!
Pretty scary when you think about!
But wait, there’s another risk lurking on the horizon.

Another threat of large volcanic eruptions is a volcanic winter.
I know what you’re thinking, “What the hell is a volcano winter???”
Well when volcano spew out truckloads of ash it rises up into the stratosphere and sits up there for a long time.
If there is enough ash then it has the potential to obscure the sun’s rays and raise the Earth’s albedo (increasing the Earth’s reflectivity of solar radiation).
This results in a lowering of temperature right across the planet which can have massive effects on agriculture, horticulture and life in general.

The scariest part of this whole thing is that is isn’t just a theory, it’s actually happened on several occasions throughout history!
Here’s a few examples (thank you Wikipedia)

. The extreme weather events of 535–536 are most likely linked to a volcanic eruption.

. The Great Famine of 1315–1317 in Europe may have been precipitated by a volcanic event,[2] perhaps that of Kaharoa, New Zealand, which lasted about five years.[3]

. In 1452 or 1453, a cataclysmic eruption of the submarine volcano Kuwae caused worldwide disruptions.
. In 1600, the Huaynaputina in Peru erupted.
Tree ring studies show that 1601 was cold. Russia had its worst famine in 1601 to 1603. From 1600 to 1602, Switzerland, Latvia and Estonia had exceptionally cold winters. The wine harvest was late in 1601 in France, and in Peru and Germany wine production collapsed. Peach trees bloomed late in China, and Lake Suwa in Japan froze early.[4]

. A paper written by Benjamin Franklin in 1783 blamed the unusually cool summer of 1783 on volcanic dust coming from Iceland, where the eruption of Laki volcano had released enormous amounts of sulfur dioxide, resulting in the death of much of the island’s livestock and a catastrophic famine which killed a quarter of the population. Temperatures in the northern hemisphere dropped by about 1 °C in the year following the Laki eruption.

. The 1815 eruption of Mount Tambora, a stratovolcano in Indonesia, occasioned mid-summer frosts in New York State and June snowfalls in New England and Newfoundland and Labrador in what came to be known as the “Year Without a Summer” of 1816.

. In 1883, the explosion of Krakatoa (Krakatau) also created volcanic winter-like conditions. The next four years after the explosion were unusually cold, and the winter of 1887 to 1888 included powerful blizzards.[5] Record snowfalls were recorded worldwide.

.The 1991 explosion of Mount Pinatubo, another stratovolcano in the Philippines, cooled global temperatures for about 2–3 years.[6]

As you can see the ramifications of a large amount of volcanic ash are rather severe!
If a volcanic winter were to happen today it would have a massive effects of the world’s food production and could easily spark food shortages and famine which could lead to social unrest, war, etc
It’s a very scary but very real threat!