The ground suddenly shakes beneath your feet. A powerful earthquake throws you to the ground. Moments later, a blinding flash lights up the horizon. The air feels impossibly hot, and a towering wave of molten rock races across the landscape. Then you realize the impossible has happened.Earth’s atmosphere is on fire.How long would it take before everything burns? Could anyone survive? And could such a disaster actually happen?The idea sounds like something from a science fiction movie, but it is based on one of the most powerful forces in the Universe: nuclear fusion.Fusion is the process that powers the Sun. It occurs when two light atomic nuclei combine to form a heavier one, releasing enormous amounts of energy. Every second, billions of these reactions occur inside stars, producing the light and heat that make life on Earth possible.Scientists have recreated fusion reactions on Earth in laboratories, and several nations possess hydrogen bombs that rely on this same process. The most efficient fusion reaction combines two forms of hydrogen known as deuterium and tritium.There is one enormous problem, however. For fusion to begin, temperatures must reach roughly 100 million°C, about seven times hotter than the core of the Sun. Now imagine that somehow those conditions were achieved throughout Earth’s atmosphere.The result would be the greatest firestorm imaginable.Unlike an ordinary wildfire, there would be nowhere to escape because the atmosphere surrounds the entire planet. The heat would spread almost instantly, igniting forests, cities, and everything else exposed to the air. Anyone caught on the surface would have virtually no chance of survival.As temperatures continued climbing, the world itself would begin to melt.Steel softens and melts at around 1,510°C, causing bridges, skyscrapers, and vehicles to collapse into glowing rivers of metal. At roughly 1,700°C, sand would begin melting into glass, transforming deserts into enormous sheets of solidified silica.The oceans would fare no better.Water contains hydrogen, one of the fuels for fusion. If the reaction somehow spread through the atmosphere, Earth’s oceans would gradually disappear as the hydrogen became part of the ongoing nuclear reaction. Steam would fill the skies before the water itself vanished entirely.Could hiding underground save you? Unfortunately, probably not.Rock begins melting at temperatures around 1,300°C. If the inferno lasted long enough, Earth’s crust would soften and flow. The planet could eventually resemble a giant ocean of lava, much like the scorching surface of Mercury.Imagine returning years later after escaping into space.Instead of blue oceans and green continents, you would find a glowing, lifeless world covered in molten rock. Even the atmosphere would be different. Much of the hydrogen involved in the fusion reactions would have been converted into helium.Breathing would become impossible. Without a normal atmosphere, your lungs would fill with helium instead of oxygen, causing suffocation within minutes. Thankfully, this nightmare remains firmly in the realm of theory.Earth simply does not contain enough of the right fusion fuel to ignite its atmosphere. Deuterium, one of the necessary hydrogen isotopes, makes up only about 0.016% of the hydrogen in our oceans. Tritium is even rarer and decays naturally over time. Ordinary hydrogen, called protium, is far more common, but fusing it requires temperatures even higher than those found in hydrogen bombs.In other words, nature has stacked the odds heavily in our favor.Even the most powerful nuclear explosions ever created cannot set the atmosphere ablaze. Scientists studied this possibility before the first atomic bomb tests and concluded that the conditions required simply cannot occur on Earth.So while a burning sky would create one of the most spectacular apocalyptic scenes imaginable, it remains one disaster we can safely cross off humanity’s list of existential threats. The atmosphere may surround us every second of our lives, but thankfully, it is far more difficult to ignite than science fiction would have you believe.