Universal health care

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Universal_health_care

Universal health care (also called universal health coverage, universal coverage, or universal care) is a health care system in which all citizens of a particular country are assured some access to health care.

Some universal healthcare systems are tax- or compulsory fee-funded, while others are based on compulsory private health insurance. Universal health cares can differ on what services are covered, and how much of the cost is covered.

The World Health Organization describes universal healthcare as a situation where citizens can access health services without incurring financial hardship. Sustainable Development Goal 3 includes universal health coverage.

History

See also: List of countries with universal health care

Note: Links in table are "Healthcare in COUNTRY".

Universal health care start date

Country	Year
Algeria	1975
Armenia	2023
Australia	1975
Austria	1967
Bahrain	1957
Belgium	1945
Bhutan	1970
Brazil	1988
Brunei	1958
Canada	1966
China	2009
Cyprus	1980
Denmark	1973
Finland	1972
France	1974
Germany	1941
Greece	1983
Hong Kong	1993
Iceland	1990
Indonesia	2014
Ireland	1977
Israel	1995
Italy	1978
Japan	1961
Kuwait	1950
Luxembourg	1973
Malaysia	1980s
Netherlands	1966
New Zealand	1938
Norway	1956
Portugal	1979
Russia	1918
Saudi Arabia	2019
Singapore	1993
Slovenia	1972
South Korea	1988
Spain	1986
Sweden	1955
Switzerland	1994
Taiwan	1995
Turkey	2003
United Arab Emirates	1971
United Kingdom	1948

The first move towards a national health insurance system was launched in Germany in 1883, with the Sickness Insurance Law. Industrial employers were mandated to provide injury and illness insurance for their low-wage workers, and the system was funded and administered by employees and employers through "sick funds", which were drawn from deductions in workers' wages and from employers' contributions. This social health insurance model, named the Bismarck Model after Prussian Chancellor Otto von Bismarck, was the first form of universal care in modern times.

Other countries soon began to follow suit. In the United Kingdom, the National Insurance Act 1911 provided coverage for primary care (but not specialist or hospital care) for wage earners, covering about one-third of the population. The Russian Empire established a similar system in 1912, and other industrialized countries began following suit. By the 1930s, similar systems existed in virtually all of Western and Central Europe. Japan introduced an employee health insurance law in 1927, expanding further upon it in 1935 and 1940.

Following the Russian Revolution of 1917, the Bolsheviks established the world's first fully free and universal health care system in Soviet Russia in July 1918. The system was highly centralized, and while nominally any person regardless of his status was covered, the actual coverage, especially in the more remote and impoverished areas was virtually non-existent.

In New Zealand, a universal health care system was created in a series of steps, from 1938 to 1941. In Australia, the state of Queensland introduced a free public hospital system in 1946.

Following World War II, universal health care systems began to be set up around the world. On July 5, 1948, the United Kingdom launched its universal National Health Service. Universal health care was next introduced in the Nordic countries of Sweden (1955), Iceland (1956), Norway (1956), Denmark (1961) and Finland (1964). Universal health insurance was introduced in Japan in 1961, and in Canada through stages, starting with the province of Saskatchewan in 1962, followed by the rest of Canada from 1968 to 1972. A public healthcare system was introduced in Egypt following the Egyptian revolution of 1952. Centralized public healthcare systems were set up in the Eastern bloc countries. The Soviet Union extended universal health care to its rural residents in 1969. Kuwait and Bahrain introduced their universal healthcare systems in 1950 and 1957 respectively (prior to independence). Italy introduced its Servizio Sanitario Nazionale (National Health Service) in 1978. Universal health insurance was implemented in Australia in 1975 with the Medibank, which led to universal coverage under the current Medicare system from 1984.

From the 1970s to the 2000s, Western European countries began introducing universal coverage, most of them building upon previous health insurance programs to cover the whole population. For example, France built upon its 1928 national health insurance system, with subsequent legislation covering a larger and larger percentage of the population, until the remaining 1% of the population that was uninsured received coverage in 2000. Single payer healthcare systems were introduced in Finland (1972), Portugal (1979), Cyprus (1980), Spain (1986) and Iceland (1990). Switzerland introduced a universal healthcare system based on an insurance mandate in 1994. In addition, universal health coverage was introduced in some Asian countries, including Malaysia (1980s), South Korea (1989), Taiwan (1995), Singapore (1993), Israel (1995) and Thailand (2001).

Following the collapse of the Soviet Union, Russia retained and reformed its universal health care system, as did other now-independent former Soviet republics and Eastern bloc countries.

Beyond the 1990s, many countries in Latin America, the Caribbean, Africa and the Asia-Pacific region, including developing countries, took steps to bring their populations under universal health coverage, including China and Brazil's SUS which improved coverage up to 80% of the population. Taiwan implemented its system in 1995. India introduced a tax-payer funded decentralised universal healthcare system as well as comprehensive public and private health insurances that helped reduce mortality rates drastically and improved healthcare infrastructure across the country dramatically. A 2012 study examined progress being made by these countries, focusing on nine in particular: Ghana, Rwanda, Nigeria, Mali, Kenya, Indonesia, the Philippines and Vietnam.

Currently, most industrialized countries and many developing countries operate some form of publicly funded health care with universal coverage as the goal. According to the National Academy of Medicine and others, the United States is the only wealthy, industrialized nation that does not provide universal health care. The only forms of government-provided healthcare available are Medicare (for elderly patients above age 65 as well as people with disabilities), Medicaid (for low-income people), the Children's Health Insurance Program (for children in families of modest income, but too high to qualify for Medicaid), the Military Health System (active, reserve, and retired military personnel and dependants), and the Indian Health Service (members of federally recognized Native American tribes).

Funding models

See also: Health care economics

Health spending by country. Percent of GDP (Gross domestic product). For example: 11.2% for Canada in 2022. 16.6% for the United States in 2022.

Total healthcare cost per person. Public and private spending. US dollars PPP. For example: $6,319 for Canada in 2022. $12,555 for the US in 2022.

Universal health care in most countries has been achieved by a mixed model of funding. General taxation revenue is the primary source of funding, but in many countries it is supplemented by specific charge (which may be charged to the individual or an employer) or with the option of private payments (by direct or optional insurance) for services beyond those covered by the public system. Almost all European systems are financed through a mix of public and private contributions. Most universal health care systems are funded primarily by tax revenue (as in Portugal, India, Spain, Denmark and Sweden). Some nations, such as Germany, France, and Japan, employ a multi-payer system in which health care is funded by private and public contributions. However, much of the non-government funding comes from contributions from employers and employees to regulated non-profit sickness funds. Contributions are compulsory and defined according to law. A distinction is also made between municipal and national healthcare funding. For example, one model is that the bulk of the healthcare is funded by the municipality, specialty healthcare is provided and possibly funded by a larger entity, such as a municipal co-operation board or the state, and medications are paid for by a state agency. Universal health care financing can range from premiums to taxes and fees which increase with income. Universal health care systems can have redistributive effects.

Compulsory insurance

See also: List of countries by health insurance coverage

This is usually enforced via legislation requiring residents to purchase insurance, but sometimes the government provides the insurance. Sometimes there may be a choice of multiple public and private funds providing a standard service (as in Germany) or sometimes just a single public fund. Healthcare in Switzerland is based on compulsory insurance.

In some European countries where private insurance and universal health care coexist, such as Germany, Belgium and the Netherlands, the problem of adverse selection is overcome by using a risk compensation pool to equalize, as far as possible, the risks between funds. Thus, a fund with a predominantly healthy, younger population has to pay into a compensation pool and a fund with an older and predominantly less healthy population would receive funds from the pool. In this way, sickness funds compete on price and there is no advantage in eliminating people with higher risks because they are compensated for by means of risk-adjusted capitation payments. Funds are not allowed to pick and choose their policyholders or deny coverage, but they compete mainly on price and service. In some countries, the basic coverage level is set by the government and cannot be modified.

The Republic of Ireland at one time had a "community rating" system by VHI, effectively a single-payer or common risk pool. The government later opened VHI to competition, but without a compensation pool. That resulted in foreign insurance companies entering the Irish market and offering much less expensive health insurance to relatively healthy segments of the market, which then made higher profits at VHI's expense. The government later reintroduced community rating by a pooling arrangement and at least one main major insurance company, Bupa, withdrew from the Irish market.

In Poland, people are obliged to pay a percentage of the average monthly wage to the state, even if they are covered by private insurance. People working under a employment contract pay a percentage of their wage, while entrepreneurs pay a fixed rate, based on the average national wage. Unemployed people are insured by the labor office.

Among the potential solutions posited by economists are single-payer systems as well as other methods of ensuring that health insurance is universal, such as by requiring all citizens to purchase insurance or by limiting the ability of insurance companies to deny insurance to individuals or vary price between individuals.

Private insurance

See also: Health insurance marketplace

In private health insurance, premiums are paid directly from employers, associations, individuals and families to insurance companies, which pool risks across their membership base. Private insurance includes policies sold by commercial for-profit firms, non-profit companies and community health insurers. Generally, private insurance is voluntary in contrast to social insurance programs, which tend to be compulsory.

In some countries with universal coverage, private insurance often excludes certain health conditions that are expensive and the state health care system can provide coverage. For example, in the United Kingdom, one of the largest private health care providers is Bupa, which has a long list of general exclusions even in its highest coverage policy, most of which are routinely provided by the National Health Service. In the Netherlands, which has regulated competition for its main insurance system (but is subject to a budget cap), insurers must cover a basic package for all enrollees, but may choose which additional services they offer in supplementary plans; which most people possess.

The Planning Commission of India has also suggested that the country should embrace insurance to achieve universal health coverage. General tax revenue is currently used to meet the essential health requirements of all people.

Community-based health insurance

A particular form of private health insurance that has often emerged, if financial risk protection mechanisms have only a limited impact, is community-based health insurance. Individual members of a specific community pay to a collective health fund which they can draw from when they need medical care. Contributions are not risk-related and there is generally a high level of community involvement in the running of these plans. Community-based health insurance generally only play a limited role in helping countries move towards universal health coverage. Challenges includes inequitable access by the poorest that health service utilization of members generally increase after enrollment.

Tax-based financing

Main article: Publicly funded health care

In tax-based financing, individuals contribute to the provision of health services through various taxes. These are typically pooled across the whole population unless local governments raise and retain tax revenues. Some countries (notably Spain, the United Kingdom, Ireland, New Zealand, Italy, Brazil, Portugal, India and the Nordic countries) choose to fund public health care directly from taxation alone. Other countries with insurance-based systems effectively meet the cost of insuring those unable to insure themselves via social security arrangements funded from taxation, either by directly paying their medical bills or by paying for insurance premiums for those affected. Public subsidies can increase the health care coverage for low income persons.

Single-payer

Main articles: Single-payer healthcare and National health insurance

Single-payer health care is a system in which the government, rather than private insurers, pays for all health care costs. Single-payer systems may contract for healthcare services from private organizations, or own and employ healthcare resources and personnel (as was the case in England before the introduction of the Health and Social Care Act). In some instances, such as Italy and Spain, both these realities may exist at the same time. "Single-payer" thus describes only the funding mechanism and refers to health care financed by a single public body from a single fund and does not specify the type of delivery or for whom doctors work. Although the fund holder is usually the state, some forms of single-payer use a mixed public-private system.

Bismark model

In a social health insurance system, contributions from workers, the self-employed, enterprises and governments are pooled into single or multiple funds on a compulsory basis. This is based on risk pooling. The social health insurance model is also referred to as the Bismarck Model, after German Chancellor Otto von Bismarck, who introduced the first universal health care system in Germany in the 19th century. The funds typically contract with a mix of public and private providers for the provision of a specified benefit package. Preventive and public health care may be provided by these funds or responsibility kept solely by the Ministry of Health. Within social health insurance, a number of functions may be executed by parastatal or non-governmental sickness funds, or in a few cases, by private health insurance companies. Social health insurance is used in a number of Western European countries and increasingly in Eastern Europe as well as in Israel and Japan.

Implementation and comparisons

For a more comprehensive list, see List of countries with universal health care.

See also: Health system and Health care systems by country

Health spending per capita, in US$ purchasing power parity-adjusted, among various OECD countries. For later data see List of countries by total health expenditure per capita.

Universal health care systems vary according to the degree of government involvement in providing care or health insurance. In some countries, such as Canada, the UK, Italy, Australia, and the Nordic countries, the government has a high degree of involvement in the commissioning or delivery of health care services and access is based on residence rights, not on the purchase of insurance. Others have a much more pluralistic delivery system, based on obligatory health with contributory insurance rates related to salaries or income and usually funded by employers and beneficiaries jointly. Subnational disparities remain a barrier to achieving total coverage in emerging economies.

Sometimes, the health funds are derived from a mixture of insurance premiums, salary-related mandatory contributions by employees or employers to regulated sickness funds, and by government taxes. These insurance based systems tend to reimburse private or public medical providers, often at heavily regulated rates, through mutual or publicly owned medical insurers. A few countries, such as the Netherlands and Switzerland, operate via privately owned but heavily regulated private insurers, which are not allowed to make a profit from the mandatory element of insurance but can profit by selling supplemental insurance.

Universal health care is a broad concept that has been implemented in several ways. The common denominator for all such programs is some form of government action aimed at extending access to health care as widely as possible and setting minimum standards. Most implement universal health care through legislation, regulation, and taxation. Legislation and regulation direct what care must be provided, to whom, and on what basis. Usually, some costs are borne by the patient at the time of consumption, but the bulk of costs come from a combination of compulsory insurance and tax revenues. Some programs are paid for entirely out of tax revenues. In others, tax revenues are used either to fund insurance for the very poor or for those needing long-term chronic care.

A critical concept in the delivery of universal healthcare is that of population healthcare. This is a way of organizing the delivery, and allocating resources, of healthcare (and potentially social care) based on populations in a given geography with a common need (such as asthma, end of life, urgent care). Rather than focus on institutions such as hospitals, primary care, community care etc. the system focuses on the population with a common as a whole. This includes people currently being treated, and those that are not being treated but should be (i.e. where there is health inequity). This approach encourages integrated care and a more effective use of resources.

The United Kingdom National Audit Office in 2003 published an international comparison of ten different health care systems in ten developed countries, nine universal systems against one non-universal system (the United States), and their relative costs and key health outcomes. A wider international comparison of 16 countries, each with universal health care, was published by the World Health Organization in 2004. In some cases, government involvement also includes directly managing the health care system, but many countries use mixed public-private systems to deliver universal health care.

Health Coverage Reports

The 2023 report from the WHO and the World Bank indicates that the advancement towards Universal Health Coverage (UHC) by the year 2030 has not progressed since 2015. The UHC Service Coverage Index (SCI) has remained constant at a score of 68 from 2019 to 2021. It is reported that catastrophic out-of-pocket (OOP) health expenditures have impacted over 1 billion individuals globally. Additionally, in the year 2019, it was found that 2 billion people experienced financial difficulties due to health expenses, with ongoing, significant disparities in coverage. The report suggests several strategies to mitigate these challenges: it calls for the acceleration of essential health services, sustained attention to infectious disease management, improvement in health workforce and infrastructure, the elimination of financial barriers to care, an increase in pre-paid and pooled health financing, policy initiatives to curtail OOP expenses, a focus on primary healthcare to reinforce overall health systems, and the fortification of collaborative efforts to achieve UHC. These measures aim to increase health service coverage by an additional 477 million individuals by the year 2023 and to continue progress towards covering an extra billion people by the 2030 deadline.

Politics and criticism

See also: Health politics and Health policy

Universal health care is politically contested, with debates focusing on the proper scope of government, efficiency, equity, individual liberty, and long-term economic and health outcomes.

Critics of single-payer models argue that they lead to longer wait times and lower quality of care due to budget constraints, overburdened staff, and non-price rationing. In Canada, for example, many patients have historically sought care in the United States to avoid delays. Government-run or heavily price-controlled systems are often described by critics as less efficient than competitive private alternatives, with risks of waste, mismanagement, overuse, and eventual insolvency or fiscal strain. Maintaining such systems can require high taxes, as seen in the Nordic countries, Australia, and New Zealand. In single-payer systems, governments sometimes impose price controls and global budgets to contain spending. This produces health care rationing through queues. In the United Kingdom's National Health Service there are documented cases of corridor care and delays. Critics contend single-payer implementation would substantially raise government expenditures and tax burdens.

From libertarian perspective, these outcomes stem from alleged incentive problems they consider inherent to the system. When most payments are made by third parties (government or insurers) rather than patients directly, individuals may have less incentive to economize, which in turn could drive up utilization and costs.

A 2025 Pew Research Center poll found 66% support that all Americans have some health care coverage. Given the high prevalence of uninsured people in the United States universal health care coverage would increase health care access for more than 25 million Americans. In the United States the support for private or public implementation of universal health care is mixed. Some argue single-payer healthcare represents "unnecessary government overreach" into the lives of American citizens and employers as it denies them individual choice and personal responsibility. It may limit the choices available to patients, as they argue government may control which treatments and medications are covered.

According to a 2020 study published in The Lancet, the proposed universal healthcare could save 68,000 lives and $450 billion in national healthcare expenditure annually through redistribution and reduced administrative costs. A 2022 study published in the PNAS found that a universal healthcare system could have saved 212,000 lives and averted over $100 billion in medical costs during the COVID-19 pandemic in the United States in 2020 alone.

Impact winter

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Impact_winter

Artistic impression of an asteroid slamming into tropical, shallow seas of the sulfur-rich Yucatán Peninsula in what is today Southeast Mexico. The aftermath of this immense asteroid collision, which occurred approximately 66 million years ago, is believed to have caused the mass extinction of non-avian dinosaurs and many other species on Earth. The impact spewed hundreds of billions of tons of sulfur into the atmosphere, producing a worldwide blackout and freezing temperatures which persisted for at least a decade.

An impact winter is a hypothesized period of prolonged cold climate due to an impact event upon the Earth's planetary surface by a large asteroid or comet. If such meteors were to survive atmospheric entry and successfully impact land or a shallow body of water, it would eject an enormous amount of debris, dust, ash and other particulate materials into Earth's atmosphere, blocking off irradiance from the Sun. This would lead to global dimming and cause the climatic temperature to decrease drastically. If an asteroid or comet with the diameter of about 5 km (3.1 mi) or more were to hit in a deep body of water or explode mid-air before hitting the surface, there would still be an enormous amount of debris released into the atmosphere.

It has been proposed that the rapid climate shifts associated with impact winters could cause severe floral disruptions and subsequently cascading collapse of food chains, leading to mass extinctions of the Earth's biosphere that wipe out many of the world's extant species. An example of such an event would be the Cretaceous–Paleogene extinction event, which probably involved a worldwide impact winter caused by the 10 km (6.2 mi) diameter Chicxulub impactor, and led to extinction of nearly all tetrapods weighing more than 25 kilograms (55 lb), which included all non-avian dinosaurs, pterosaurs and Mesozoic marine reptiles.

Possibility of impact

Each year, the Earth is hit by 5 m (16 ft) diameter meteoroids that deliver an explosion 50 km (31 mi) above the surface with the power equivalent of one kiloton of TNT. The Earth is hit every day by a meteor less than 5 m (16 ft) in diameter that disintegrates before reaching the surface. The meteors that do make it to the surface tend to strike unpopulated areas and cause no harm. A human is more likely to die in a fire, flood, or other natural disaster than to die because of an asteroid or comet impact. Another study in 1994 found a 1-in-10,000 chance that the Earth will be hit by a large asteroid or comet with a diameter of about 2 km (1.2 mi) during the next century. This object would be capable of disrupting the ecosphere and would kill a large fraction of the world's population. One such object, Asteroid 1950 DA, currently has a 0.038% chance of colliding with Earth in the year 2880, though when first discovered the probability was 0.3%. The probability goes down as orbits are refined with additional measurements.

Over 300 short-period comets pass near larger planets, such as Saturn and Jupiter, which can change the comets' trajectories and could potentially put them into an Earth-crossing orbit. This could happen for long-period comets also but the chance is highest for short-period comets. The chance of these directly impacting Earth is far lower than a near-Earth object (NEO) impact. Victor Clube and Bill Napier support a controversial theory that a short-period comet in an Earth-crossing orbit does not need to impact to be hazardous, as it could disintegrate and cause a dust veil with possibilities of a "nuclear winter" scenario with long-term global cooling lasting for thousands of years (which they consider to be similar in probability to a 1 km impact).

Necessary impact factors

The Earth experiences a never-ending barrage of cosmic debris. Small particles burn up as they enter the atmosphere and are visible as meteors. Many of them go unnoticed by the average person even though not all of them burn up before they hit the Earth's surface. Those that strike the surface are known as meteorites. Thus, not every object that hits the Earth will cause an extinction-level event or even cause any real harm. Objects release most of their kinetic energy in the atmosphere and will explode if they experience a column of atmosphere greater than or equal to their mass. Extinction-level impacts on the Earth occur about every 100 million years. Although extinction events happen very rarely, large projectiles can do severe damage. This section will discuss the nature of the hazards posed by projectiles as a function of their size and composition.

Size

A large asteroid or comet could collide with the Earth's surface with the force of hundreds to thousands of times the force of all the nuclear bombs on the Earth. For example, the Cretaceous–Paleogene extinction event has been proposed to have caused extinction of all non-avian dinosaurs 66 million years ago. Early estimates of this asteroid's size put it at about 10 km (6.2 mi) in diameter. This means it hit with nearly a force of 100,000,000 MT (418 ZJ). That is over six billion times larger than the atomic bomb yield (16 kilotons, 67 TJ) that was dropped on Hiroshima during WW2. This impactor excavated the Chicxulub crater that is 180 km (110 mi) in diameter. With an object this size, dust and debris would still be ejected into the atmosphere even if it hit the ocean, which is only 4 km (2.5 mi) deep. An asteroid, meteor, or comet would remain intact through the atmosphere by virtue of its sheer mass. However, an object smaller than 3 km (1.9 mi) would have to have a strong iron composition to breach the lower atmosphere — the troposphere or the lower levels of the stratosphere.

Composition

There are three different composition types for an asteroid or comet: metallic, stony and icy. The composition of the object determines whether or not it will make it to the Earth's surface in one piece, disintegrate before breaching the atmosphere, or break up and explode just before reaching the surface. A metallic object tends to be made up of iron and nickel alloys. These metallic objects are the most likely to impact the surface because they stand up better to the stresses of ram pressure induced flattening and fragmentation during deceleration in the atmosphere. The stony objects, like chondritic meteorites, tend to burn, break up, or explode before leaving the upper atmosphere. Those that do make it to the surface need a minimum energy of about 10 Mt (4×10¹⁶ J) or about 50 m (160 ft) diameter to breach the lower atmosphere (this is for a stony object hitting at 20 kilometres per second (40,000 mph)). The porous comet-like objects are made up of low-density silicates, organics, ice, volatile and often burn up in the upper atmosphere because of their low bulk density (≤1 g/cm³ (60 lb/cu ft)).

Possible mechanisms

See also: Younger Dryas impact hypothesis

Although the asteroids and comets that impact the Earth hit with many times the explosive force of a volcano, the mechanisms of an impact winter are similar to those that occur after a mega-volcanic eruption-induced volcanic winter. In this scenario massive amounts of debris injected into the atmosphere would block some of the Sun's radiation for an extended period of time and lower the mean global temperature by as much as 20°C after a year. The two main mechanisms that could lead to an impact winter are mass ejection of regolith and multiple firestorms.

Mass ejection of regolith

See also: Supervolcano, Toba catastrophe theory, and Yellowstone caldera § Hazards

This diagram shows the size distribution in micrometres of various types of atmospheric particulate matter.

In a study conducted by Curt Covey et al., it was found that an asteroid about 10 km (6.2 mi) in diameter with the explosive force of about 10⁸ MT could send upward of about 2.5 × 10¹⁵ kg of 1 μm sized aerosol particles into the atmosphere. Anything larger would fall quickly back to the surface. These particles would then be spread throughout the atmosphere and absorb or refract the sunlight before it is able to reach the surface, cooling the planet in a similar fashion as the sulfurous aerosol rising from a megavolcano, producing deep global dimming. This is controversially purported to have occurred following the Toba eruption.

These pulverized rock particles would remain in the atmosphere until dry deposition and due to their size, they would also act as cloud condensation nuclei and would be washed out by wet deposition/precipitation, but even then, about 15% of the sun's radiation might not reach the surface. After the first 20 days, the land temperature might drop quickly, by about 13°C. After about a year, the temperature could rebound by about 6°C, but by this time about one-third of the Northern Hemisphere might be covered in ice.

However, this effect could be largely mitigated, even reversed, by a release of enormous quantities of water vapor and carbon dioxide caused by the initial global heat pulse after the impact. If the asteroid hit an ocean (which would be the case with the majority of impact events), water vapor would form the majority of any ejected matter, and would likely result in a major greenhouse effect and a net increase in temperature.

If the impact event is sufficiently energetic it might cause mantle plume (volcanism) at the antipodal point (the opposite side of the world). This volcanism could alone therefore create a volcanic winter, irrespective of the other impact effects.

Multiple firestorms

See also: Nuclear winter

In combination with the initial debris ejected into the atmosphere, if the impactor is extremely large (3 km (1.9 mi) or more), like at the Cretaceous–Paleogene extinction event (estimated 10 km (6.2 mi)), there might be the ignition of multiple fire storms, possibly with a global reach into every dense and therefore firestorm-prone forest. These wood fires might release enough amounts of water vapor, ash, soot, tar and carbon dioxide into the atmosphere to perturb the climate on their own and cause the pulverized rock dust cloud blocking the sun to last longer. Alternatively it could cause it to last for a much shorter time, as there would be more water vapor for the rocky aerosol particles to form cloud condensation nuclei. If it causes the dust cloud to last longer, it would prolong the Earth's cooling time, possibly causing thicker ice sheets to form.

Past events

Main articles: Chicxulub crater and Cretaceous–Paleogene extinction event

In 2016, a scientific drilling project drilled deep into the peak ring of the Chicxulub impact crater to obtain rock core samples from the impact itself. This crater is one of the best known impact craters and was the impact responsible for the extinction of the non-avian dinosaurs.

The discoveries were widely seen as confirming current theories related to both the crater impact and its effects. They confirmed that the rock comprising the peak ring had been subjected to immense pressures and forces, and had been melted by immense heat and shocked by immense pressure from its usual state into its present form in just minutes. The fact that the peak ring was made of granite was also significant, since granite is not a rock found in sea-floor deposits — it originates much deeper in the earth and had been ejected to the surface by the immense pressures of impact. Gypsum, a sulfate-containing rock that is usually present in the shallow seabed of the region, had been almost entirely removed and must therefore have been almost entirely vaporized and entered the atmosphere, and that the event was immediately followed by a huge megatsunami (a massive movement of sea waters) sufficient to lay down the largest known layer of sand separated by grain size directly above the peak ring.

These strongly support the hypothesis that the impactor was large enough to create a 120-mile peak ring, eject molten granite from deep within the earth, create colossal water movements, and eject an immense quantity of vaporized rock and sulfates into the atmosphere, where they would have persisted for a long time. This global dispersion of dust and sulfates would have led to a sudden and catastrophic effect on the climate worldwide by causing large temperature drops, devastating the food chain.

Impact on humans

Artist's impression of the Toba eruption, circa 74,000 years ago. Some scientists believe this eruption led to a population collapse and subsequent genetic bottleneck in humans.

An impact winter would have a devastating effect on humans, as well as the other species on Earth. With the sun's radiation being severely diminished, the first species to die would be plants and animals who survive through the process of photosynthesis. This lack of food would ultimately lead to other mass extinctions of other animals that are higher up on the food chain and possibly kill up to 25% of the human population. Depending on location and size of the initial impact, the cost of clean-up efforts could be so high as to cause an economic crisis for the survivors. These factors would make life on Earth, for humans, extremely difficult.

Agriculture

See also: Svalbard seedbank

With the Earth's atmosphere full of dust and other material, radiation from the sun would be refracted and scattered back into space and absorbed by this debris. The first effect on the Earth, after the blast wave and potential multiple fire storms, would be the death of most, if not all, of the photosynthetic life forms on Earth. Those in the ocean that survive would possibly become dormant until the sun came out again. Those on land could possibly be kept alive in underground microclimates, with one such example being the Zbrašov aragonite caves. Greenhouses in underground complexes with fossil or nuclear energy power stations could conceivably keep artificial sunlight growing lamps on until the atmosphere began to clear. Meanwhile, those outside that were not killed by the lack of sunlight would most likely be killed or kept dormant by the extreme cold of the impact winter. This death of plants might lead to a long period of famine if enough people survived the initial blast wave and would result in increased food costs in undeveloped countries only a few months after the first crop failures. Developed countries wouldn't encounter famine unless the cooling event was to last longer than a year, due to larger canned food and grain stockpiles in these countries. However, if the impactor was similar in size to the K/T boundary impactor, agricultural losses might not be compensated with imports to the northern hemisphere from the southern hemisphere or vice versa. The only way to keep from starving would be for each country to amass at least a year's worth of food for their people. Not many countries have this; the world's average cereal stock levels are only about 30% of the yearly production.

Economics

The cost to clean up after an asteroid or comet impact would cost billions to trillions of dollars, depending on the location impacted. An impact in New York City (the 16th most populated city in the world) could cost billions of dollars in financial losses and it could take years for the financial sector (i.e. stock market) to recover. However, the probability of such a naturally specifically aimed impact would be exceedingly low.

Survivability

See also: Asteroid impact avoidance, Duck and cover, and Civil defense

As of February 20, 2018, there are 17,841 near-Earth objects known. 8,059 potentially hazardous objects are known; they are larger than 140 m (460 ft) and may approach the Earth closer than 20 times the distance to the Moon. 888 NEAs larger than 1 km have been discovered, or 96.5% of an estimated total of about 920.