In late January, the Bulletin of the Atomic Scientists moved the “Doomsday Clock” 30 seconds closer to midnight. The clock is symbolic, with midnight representing the end of the world; the group moves the minute and second hands based on its analysis of various threats to humanity. Now, at two and a half minutes to midnight, the Doomsday Clock’s minute hand is closer to the catastrophic hour than it’s been since 1953. The decision to move it was in part because of Trump’s recent comments on nuclear arms, as well as nuclear tests by North Korea and new ballistics missiles being built by Russia.

Right now, nearly 15,000 nuclear weapons are estimated to exist in the world (that's down from the 70,000-plus that existed in the Cold War era), with the U.S. and Russia owning approximately 93% of those. The remaining 7% is owned by six other countries: France, China, the United Kingdom, Pakistan, India, and Israel.

Here’s what you need to know about nuclear weapons.

What are nuclear weapons and proliferation?

Nuclear weapons are explosive devices that derive their destructive force from a combination of chemical explosives and nuclear reactions. They can be fired using airplanes, submarines, or missiles launched from silos. They can destroy entire cities, wipe out millions of people, and cause long-term, devastating effects to the environment and to human health.

The first nuclear weapons were developed during World War II, and they’ve only been used in warfare twice, when the United States bombed the Japanese cities of Hiroshima and Nagasaki in 1945. Since then, other countries have acquired nuclear weapons, and more than 2,000 nuclear tests have been conducted.

Under the Treaty on the Nonproliferation of Nuclear Weapons (NPT), the term "nuclear proliferation" refers to the spread of nuclear weapons (including weapon material, information, and technology) to states that don't already have them, while nonproliferation refers to preventing such a spread.

What is the Treaty on the Nonproliferation of Nuclear Weapons and why is it important?

The Treaty on the Nonproliferation of Nuclear Weapons (NPT) is an international agreement covering three pillars: disarmament, nonproliferation, and peaceful uses of nuclear energy. The core of the NPT states that "countries with nuclear weapons will move towards disarmament; countries without nuclear weapons will not acquire them; and all countries can access peaceful nuclear technology."

The NPT was developed in the aftermath of the 1962 Cuban Missile Crisis (when the U.S. and the Soviet Union teetered on the brink of nuclear war following the discovery of Soviet missiles in Cuba) and has been in effect since 1970. There are 190 countries that are signatories to the NPT, including five nuclear states: the U.S., Russia, the U.K., France, and China. North Korea signed the NPT in 1985 but withdrew in 2003. India, Israel, and Pakistan have never signed the agreement.

“This treaty is just a piece of paper, but it has done a great deal in terms of limiting the creation of new nuclear capable states and fostering international cooperation,” Angelica Gheen, a radiation-health physicist at a large research university, tells Teen Vogue. Along with nonproliferation, "this has led to an environment of global cooperation on nuclear security...and it allowed for [South Africa] to successfully disarm with international resources," a process that took place from 1989 to 1991, culminating in South Africa joining the NPT in '91.

Some believe that nuclear proliferation can actually prevent war, with the dangerous weapons acting as deterrents to countries considering attacks. However, some studies state otherwise. Research has also shown that the closer a country is to acquiring nuclear weapons, the more likely it is to be attacked.