A super-eruption may not give warning
A large natural lake, called Lake Toba, in North Sumatra, Indonesia peacefully fills what is actually an ancient caldera of a supervolcano.
Volcanic eruptions can affect a wide swath of the world, depending on the size of the eruption. Ash launched high into the atmosphere affects air travel with aircraft avoiding the ash clouds, or areas where ash could potentially be present. The ash particulates can not only affect visibility, they can also cause a lot of damage to an aircraft engine.
I was living in Federal Way, Washington in 1980 when Mt. Saint Helens erupted. We were too far away to see it. I remember about a week later there was a grayish dust over some of the things in our backyard. Turns out it was ash dust that had travelled around the world before it settled down.
Supervolcanoes are one of the disasters that would seriously affect much of the world. Thankfully scientists estimate there are only 5-10 volcanoes in the world that are capable of super eruptions. The bad news is a single eruption from one of these would cause more climate change faster than humans can.
Disaster movies like to show raise the tension by showing the events that some believe would occur before a super-eruption. Some of these are believed to be a significant increase in earthquakes (the frequent uptick in earthquake swarms at Yellowstone often cause concern of a volcanic eruption) or rapid ground uplift, most likely caused by a sudden inflow of magma into the volcano's reservoir.
This study Growth and thermal maturation of the Toba magma reservoir, which analyzed the chemistry of zircons (minerals produced by volcanic eruptions) around Toba, suggests that the theorized rapid magma increase into the volcanic reservoir is not necessary prior to an eruption. What they discovered is a long-term increase, magma collecting silently and slowly, led to the super eruptions.
The concern is a super-eruption is very capable of global climate effects. Imagine a volcanic-caused winter that lasts a decade. This study, Did the Toba volcanic eruption of ∼74 ka B.P. produce widespread glaciation? suggests a Toba eruption may have resulted in widespread glaciation that dropped temperatures around the world.
Neither article speculates on the effects a super-eruption would have on our world short of the eruption having "serious effects on plant and animal life."
Here's my take.
Without the expected "warning" signs, a super-eruption would certainly take most by surprise. Not only would such an eruption affect travel, it would seriously affect the transportation of items around the world. Do you think COVID disrupted "normal", this size of eruption would grind just about everything to a halt. Very little (if any) international shipping would occur for the first year or two.
Crops will fail around the world and little, or no, international shipping of food would happen with governments keeping what little food they have for their own people. Any food production would mostly occur at a local level, with little at the regional level.
Famine resulting from the severely decreased food production would be the biggest hazard and cause of death around the world. The question of survival will likely rest in how much of the world is affected in its crop production and how quickly food production can pivot to produce in alternative ways. Oh, and how much seed is available for crops will be a determining factor.
If Yellowstone is has a super-eruption, I'm not getting much warning. But if Toba or one of the other supervolcanoes erupts those of us not in the immediate area should get a day or more of warning. Remember my experience with the ash from Mt. Saint Helens? We might get more or less time before the effects of the super-eruptions start affecting us.
But don't expect to rush out and get prepared then. The stores will be packed quickly and emptied just as fast. People will be stockpiling as much as they can, as quick as they can. Stores will do their best to limit quantities. Fear and uncertainty will grip the hearts of many.
Many people (the unprepared and those who are entitled and think they should be provided with everything they want) will demand the government to do something, but there isn't much the government can do to stop the impending disaster. But, those in power may issue emergency regulations allowing government agencies to take supplies for the "good of the people."
Probably best to keep a low profile as much as possible.
Just another way humanity can be humbled, and another reason to become better prepared.
And don't be surprised if a super-eruption ends up triggering some big earthquakes in other parts of the world.