A nuclear weapon is a weapon which derives its destructive force from nuclear reactions of fission or fusion. As a result, even a nuclear weapon with a relatively small yield is significantly more powerful than the largest conventional explosives, and a single weapon is capable of destroying an entire city. The first nuclear weapons were created in the United States, by an international team including many displaced emigrant scientists from central Europe with assistance from the United Kingdom and Canada, during World War II as part of the top-secret Manhattan Project While the first weapons were developed primarily out of fear that Nazi Germany would develop them first, they were eventually used against the Japanese cities of Hiroshima and Nagasaki in August 1945. The Soviet Union developed and tested their first nuclear weapon in 1949, based partially on information obtained from Soviet espionage in the United States. Both the U.S. and USSR would go on to develop weapons powered by nuclear fusion (hydrogen bombs) by the mid-1950s. With the invention of reliable rocketry during the 1960s, it became possible for nuclear weapons to be delivered anywhere in the world on a very short notice, and the two Cold War superpowers adopted a strategy of deterrence to maintain a shaky peace. Nuclear weapons were symbols of military and national power, and nuclear testing was often used both to test new designs as well as to send political messages. Other nations also developed nuclear weapons during this time, including the United Kingdom, France, and China. These five members of the 'nuclear club' agreed to attempt to limit the spread of nuclear proliferation to other nations, though at least three other countries (India, South Africa, Pakistan, and most likely Israel) developed nuclear arms during this time. At the end of the Cold War in the early 1990s, the Russian Federation inherited the weapons of the former USSR, and along with the U.S., pledged to reduce their stockpile for increased international safety. Nuclear proliferation has continued, though, with Pakistan testing their first weapons in 1998, and North Korea claiming to have developed nuclear weapons in 2004 and on recently May 2009. In January 2005, Pakistani metallurgist Abdul Qadeer Khan confessed to selling nuclear technology and information of nuclear weapons to Iran, Libya, and North Korea in a massive, international proliferation ring. Nuclear weapons have been at the heart of many national and international political disputes and have played a major part in popular culture since their dramatic public debut in the 1940s and have usually symbolized the ultimate ability of mankind to utilize the strength of nature for destruction. There have been (at least) four major false alarms, the most recent in 1995, that almost resulted in the U.S. or USSR/Russia launching its weapons in retaliation for a supposed attack. Additionally, during the Cold War the U.S. and USSR came close to nuclear warfare several times, most notably during the Cuban Missile Crisis. As of 2005, there are estimated to be at least 29,000 nuclear weapons held by at least seven countries, 96 percent of them in the possession of the United States and Russia. The environmental destruction that certain scientists contend would probably result from the hundreds of nuclear explosions in a nuclear war. The damaging effects of the light, heat, blast, and radiation caused by nuclear explosions had long been known to scientists, but such explosions' indirect effects on the environment remained largely ignored for decades. In the 1970s, however, several studies posited that the layer of ozone in the stratosphere that shields living things from much of the Sun's harmful ultraviolet radiation might be depleted by the large amounts of nitrogen oxides produced by nuclear explosions. Further studies speculated that large amounts of dust kicked up into the atmosphere by nuclear explosions might block sunlight from reaching the Earth's surface, leading to a temporary cooling of the air. Scientists then began to take into account the smoke produced by vast forests set ablaze by nuclear fireballs, and in 1983 an ambitious study, known as the TTAPS study (from the initials of the last names of its authors, R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, and Carl Sagan), took into consideration the crucial factor of smoke and soot arising from the burning petroleum fuels and plastics in nuclear-devastated cities. (Smoke from such materials absorbs sunlight much more effectively than smoke from burning wood.) The TTAPS study coined the term "nuclear winter," and its ominous hypotheses about the environmental effects of a nuclear war came under intensive study by both the American and Soviet scientific communities. The basic cause of nuclear winter, as hypothesized by researchers, would be the numerous and immense fireballs caused by exploding nuclear warheads. These fireballs would ignite huge uncontrolled fires (firestorms) over any and all cities and forests that were within range of them. Great plumes of smoke, soot, and dust would be sent aloft from these fires, lifted by their own heating to high altitudes where they could drift for weeks before dropping back or being washed out of the atmosphere onto the ground. Several hundred million tons of this smoke and soot would be shepherded by strong west-to-east winds until they would form a uniform belt of particles encircling the Northern Hemisphere from 30° to 60° latitude. These thick black clouds could block out all but a fraction of the Sun's light for a period as long as several weeks. Surface temperatures would plunge for a few weeks as a consequence, perhaps by as much as 11° to 22° C (20° to 40° F). The conditions of semidarkness, killing frosts, and subfreezing temperatures, combined with high doses of radiation from nuclear fallout, would interrupt plant photosynthesis and could thus destroy much of the Earth's vegetation and animal life. The extreme cold, high radiation levels, and the widespread destruction of industrial, medical, and transportation infrastructures along with food supplies and crops would trigger a massive death toll from starvation, exposure, and disease. A nuclear war could thus reduce the Earth's human population to a fraction of its previous numbers. A number of scientists have disputed the results of the original calculations, and, though such a nuclear war would undoubtedly be devastating, the degree of damage to life on Earth remains controversial.