An impact winter is a suggested period of long-lasting cold weather caused by a large asteroid or comet hitting Earth's surface. If an asteroid hits land or a shallow body of water, it would throw a large amount of dust, ash, and other material into the atmosphere, blocking sunlight. This would cause global temperatures to drop suddenly. If an asteroid or comet with a size of about 5 km (3.1 mi) or larger hits a large deep body of water or explodes before reaching Earth's surface, there would still be a large amount of debris thrown into the atmosphere. It has been suggested that an impact winter could lead to mass extinction, causing many species to disappear. The Cretaceous–Paleogene extinction event probably involved an impact winter and led to the mass extinction of most tetrapods weighing more than 25 kilograms (55 pounds).
Possibility of impact
Each year, Earth is struck by meteoroids that are 5 meters (16 feet) in diameter. These meteoroids explode 50 kilometers (31 miles) above Earth’s surface with energy equal to one kiloton of TNT. Every day, Earth is hit by smaller meteors, less than 5 meters (16 feet) in diameter, which break apart before reaching Earth’s surface. Most meteors that reach Earth’s surface land in areas with no people and cause no harm. A person is more likely to die in a fire, flood, or other natural disaster than to die from an asteroid or comet impact. A 1994 study found a 1-in-10,000 chance that Earth will be hit by a large asteroid or comet about 2 kilometers (1.2 miles) in diameter during the next century. Such an object could disrupt Earth’s environment and kill a large part of the world’s population. One example, Asteroid 1950 DA, has a 0.038% chance of hitting Earth in 2880. When first discovered, its chance of collision was 0.3%. This probability decreases as scientists make more measurements of its orbit.
Over 300 short-period comets pass near large planets like Saturn and Jupiter. These planets can change the comets’ paths and possibly send them into orbits that cross Earth’s path. This could also happen with long-period comets, but it is more likely for short-period comets. The chance of these comets directly hitting Earth is much lower than the chance of a near-Earth object (NEO) impact. Scientists Victor Clube and Bill Napier proposed a theory that a short-period comet in an Earth-crossing orbit could be dangerous even without hitting Earth. If the comet breaks apart, it might create a dust cloud that could lead to a "nuclear winter" scenario, causing long-term global cooling lasting thousands of years. They believe this scenario is as likely as a 1-kilometer asteroid impact.
Necessary impact factors
The Earth is constantly hit by space debris. Small pieces of this debris burn up when they enter the atmosphere and are seen as meteors. Many of these pieces are not noticed by people because not all of them burn up completely before reaching Earth. Those that land on Earth are called meteorites. Not every object that hits Earth causes major harm or leads to an extinction event. Most of the energy from these objects is released in the atmosphere, and they often explode if the atmosphere they pass through is equal to or heavier than the object itself. Major impacts that cause extinction-level events happen about every 100 million years. Although these events are rare, large objects can cause serious damage. This section will explain how the dangers from space objects depend on their size and material.
A large asteroid or comet could hit Earth with a force hundreds to thousands of times greater than all the nuclear bombs on Earth combined. For example, the Cretaceous–Paleogene extinction event, which happened 66 million years ago, is believed to have caused the extinction of all non-avian dinosaurs. Early estimates suggest the asteroid involved was about 10 km (6.2 mi) in diameter. This object hit Earth with a force of about 100,000,000 megatons (418 zettajoules). This is over six billion times more powerful than the atomic bomb dropped on Hiroshima during World War II, which had a force of 16 kilotons (67 terajoules). This impact created the Chicxulub crater, which is 180 km (110 mi) in diameter. Even if such a large object hit the ocean, which is only 4 km (2.5 mi) deep, it would still send dust and debris into the atmosphere. Objects as large as asteroids, meteors, or comets can remain whole as they pass through the atmosphere because of their great mass. However, objects smaller than 3 km (1.9 mi) would need to be made mostly of iron to survive the lower atmosphere, such as the troposphere or lower parts of the stratosphere.
Asteroids and comets can be divided into three main types based on their composition: metallic, stony, and icy. The material of an object determines whether it will reach Earth’s surface intact, break apart before entering the atmosphere, or explode just before hitting Earth. Metallic objects are mostly made of iron and nickel. These objects are most likely to hit Earth’s surface because they are better at withstanding the forces of air pressure and breaking apart as they slow down in the atmosphere. Stony objects, like chondritic meteorites, often burn up, break apart, or explode before leaving the upper atmosphere. Those that reach Earth’s surface need at least 10 megatons (4 × 10^16 joules) of energy or a size of about 50 m (160 ft) to pass through the lower atmosphere (this applies to a stony object moving at 20 kilometers per second (40,000 mph)). Porous, comet-like objects are made of low-density materials such as silicates, organic compounds, and ice. These objects often burn up in the upper atmosphere because of their low density (≤1 g/cm³ (60 lb/cu ft)).
Possible mechanisms
Although asteroids and comets that hit Earth release much more energy than volcanoes, the way they cause an "impact winter" is similar to how very large volcanic eruptions can lead to a "volcanic winter." In both cases, large amounts of debris enter the atmosphere, blocking sunlight for a long time and lowering Earth's average temperature by as much as 20°C within a year. Two main factors can cause an impact winter: the large-scale throwing of soil and rock into the air, and the creation of many large fires.
A study by Curt Covey and others found that an asteroid about 10 km (6.2 mi) wide, with the power of 10 megatons, could send about 2.5×10^15 kg of tiny aerosol particles (1 micrometer in size) into the atmosphere. Larger particles would fall back to Earth quickly. These tiny particles would spread globally, absorbing or bending sunlight before it reaches Earth’s surface, causing cooling similar to the sulfur particles from a very large volcanic eruption. This process is thought to have happened after the Toba eruption, though this remains debated.
These crushed rock particles would stay in the atmosphere until they fall back to Earth through dry air or are washed out by rain. Even then, about 15% of sunlight might not reach Earth’s surface. Within 20 days, land temperatures could drop by about 13°C. After one year, temperatures might rise slightly by 6°C, but about one-third of the Northern Hemisphere could be covered in ice.
However, this cooling could be reduced or even reversed by large amounts of water vapor and carbon dioxide released after the initial heat from the impact. If the asteroid struck an ocean (which is common), water vapor would dominate the material thrown into the air, creating a strong greenhouse effect and raising Earth’s temperature.
If the impact is powerful enough, it might trigger volcanic activity at the antipodal point (the opposite side of Earth). This volcanic activity could cause a volcanic winter on its own, without needing other effects from the impact.
If the impactor is extremely large (3 km or more), like the one that caused the Cretaceous–Paleogene extinction event (estimated at 10 km wide), it might start many large fires across dense forests worldwide. These fires could release water vapor, ash, soot, tar, and carbon dioxide into the atmosphere, affecting the climate. This could either extend the time the dust cloud blocks sunlight, leading to longer cooling and thicker ice sheets, or shorten it, depending on how much water vapor is available to help the dust form clouds.
Past events
In 2016, scientists drilled deep into the peak ring of the Chicxulub impact crater to collect rock samples from the impact itself. This crater is one of the most well-known impact craters and was the event that caused the extinction of non-avian dinosaurs.
The findings supported existing theories about the crater and its effects. They showed that the rock in the peak ring had been exposed to very strong pressure and force, melted by intense heat, and changed rapidly due to extreme pressure. The presence of granite in the peak ring was important because granite is not found in sea-floor deposits—it forms much deeper underground and was brought to the surface by the impact. Gypsum, a rock that contains sulfate and is usually found in shallow seabed areas, was nearly completely removed. This suggests it was mostly turned into vapor and entered the atmosphere. The impact also caused a massive megatsunami, which moved large amounts of seawater and created the largest known layer of sand, sorted by grain size, above the peak ring.
These findings support the idea that the impactor was large enough to form a 120-mile-wide peak ring, eject molten granite from deep within Earth, cause enormous water movements, and send large amounts of vaporized rock and sulfates into the atmosphere. This widespread distribution of dust and sulfates likely caused sudden and severe climate changes worldwide, leading to sharp drops in temperature and disrupting the food chain.
Impact on humans
An impact winter would cause serious harm to humans and many other living things on Earth. If the sun's light is blocked, plants and animals that need sunlight to live would die first. This loss of food would lead to the deaths of many animals higher up in the food chain and could kill about 25% of the human population. The cost of cleaning up after an impact might be so high that it could cause economic problems for people who survive. These challenges would make life on Earth very difficult for humans.
After an impact, dust and other materials in the atmosphere would block sunlight. This would prevent sunlight from reaching Earth, and the first major effect would be the death of most plants and algae that use sunlight to grow. Some ocean plants might survive and go into a sleeping state until sunlight returns. On land, some plants might survive in underground areas, such as the Zbrašov aragonite caves. Greenhouses in underground areas powered by fossil or nuclear energy could use artificial lights to grow plants until the atmosphere clears. People outside who survive the initial impact might freeze to death or go into a sleeping state due to the cold. The loss of plants could lead to long-term food shortages if enough people survive the initial disaster. In countries without large food supplies, food prices might rise quickly after crops fail. Wealthier countries might avoid famine unless the cold lasts more than a year, because they have more stored food. However, if the impact was as large as the one that caused the K/T boundary event, food shortages could be worse because countries might not be able to trade food between the northern and southern hemispheres. To avoid starvation, each country would need to store at least a year’s worth of food for its people. Most countries do not have this; the world’s average cereal stock is only about 30% of what is produced each year.
Cleaning up after an asteroid or comet impact could cost billions or trillions of dollars, depending on where it hits. If an impact occurred in a city like New York City (the 16th most populated city in the world), it could cause billions of dollars in damage and take years for the financial system to recover. However, the chance of an asteroid hitting a specific place on Earth is very low.
As of February 20, 2018, scientists had identified 17,841 near-Earth objects. Of these, 8,059 are considered potentially dangerous because they are larger than 140 meters (460 feet) and could come within 20 times the distance to the Moon. Scientists have discovered 888 near-Earth asteroids larger than 1 kilometer, which is about 96.5% of the estimated total of 920 such objects.