If Yellowstone were to erupt as a supervolcano, the consequences would be catastrophic on multiple levels:

• The eruption would likely start with an initial massive explosion where viscous magma packed with gas breaks the surface, creating a deafening blast. A plume of ash and pumice could shoot to the edge of space within minutes, causing pitch darkness below as ash spreads laterally.
• Pyroclastic flows—fast-moving surges of hot ash, gas, and pumice traveling at speeds over 180 mph—would devastate an area about 60 miles (100 km) around the vent, destroying everything living and built. The flows can cover 20,000 square kilometers, leaving deposits 10 to 100 meters thick, incinerating all life in their path.
• Ash fallout would heavily blanket much of the United States and parts of Canada within 24 hours, collapsing buildings under its weight, contaminating water supplies, smothering crops, and causing severe respiratory dangers. This ash would also carry toxic metals causing long-term environmental contamination.
• The eruption could last weeks or even a month with multiple phases and expanding vents.
• Most critically, sulfur dioxide gas blasted into the stratosphere would form aerosols that create a global “veil,” drastically reducing sunlight reaching Earth's surface. This would cool global temperatures significantly, with drops up to 10°C (18°F) worldwide, and up to 12°C (22°F) in the Northern Hemisphere. These colder temperatures could last 6 to 10 years.
• This global cooling would disrupt weather patterns, potentially causing monsoon failures and widespread famine, particularly in Asia.
• The economic impact would be enormous, shutting down agriculture across large swathes of North America, disrupting modern infrastructure, power, and communications, potentially bringing the US economy to its knees.
• Visibility would be like a perpetual cloudy winter day, with ash clouds potentially reaching Europe within days, albeit in much lighter amounts compared to North America.

Overall, a Yellowstone supereruption would be one of the most devastating natural disasters in human history with immediate destruction locally and severe global environmental, climatic, and societal consequences lasting for a decade or more. This scenario is based on scientific models and comparisons to past supervolcano eruptions such as Toba around 74,000 years ago and the scale of sulfur aerosol effects seen in smaller eruptions like Mount Pinatubo in 1991.