The hands of the Doomsday Clock just moved 30 seconds closer to midnight Thursday morning, a signal from the group of scientists who control the metaphorical timepiece that they believe the threat of nuclear war and other potential catastrophes has increased.

The clock now reads two-and-a-half minutes to midnight, the Bulletin of Atomic Scientists announced at a press conference in Washington, D.C. It's the closest to global apocalypse the clock has been set since the early 1950s, when the U.S. and Soviet Union began testing thermonuclear devices.

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But why exactly did the scientists reset the clock? For the answer to that critical question and others related to the risks the world now faces, NBC MACH spoke with Dr. Lawrence Krauss, the Arizona State University scientist who chairs the Board of Sponsors of the Bulletin. Here, lightly edited, are his answers:

Why the change in the clock setting?

The Bulletin is extremely concerned about the willingness of governments — including the current U.S. administration — to ignore or discount sound science, empirical evidence, and considered expertise during their decision-making processes. Facts are stubborn things, and they must be taken into account if the future of humanity is to be preserved, long term.

In 2016, world leaders not only failed to deal adequately with the threats we face. They actually increased the risk of nuclear war and unchecked climate change through a variety of provocative statements and actions, including careless rhetoric about the use of nuclear weapons and the wanton defiance of scientific truths.

Theoretical physicist Lawrence Krauss, left, and former U.S. Ambassador to the United Nations Thomas Pickering announce The Bulletin of Atomic Scientist's decision to move the so-called "Doomsday Clock to two-and-a-half minutes to midnight at the National Press Club in Washington on Jan. 26. Jim Lo Scalzo / EPA

Two-and-a-half minutes to midnight sounds really scary. What exactly does that mean?

The clock is now closer to midnight than it has been for over half a century. International tensions are at an all-time recent high regarding nuclear weapons. On both sides of the Atlantic there is now saber-rattling over nuclear weapons, and there’s also talk of modernizing our own nuclear arsenal. Also, there’s been little concrete action to address climate change in the face of dramatic new evidence of its significance. Of course, countries like North Korea are also adding to global instability.

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Why would modernizing our nuclear arsenal make the world a more dangerous place?

It raises the possibility for the need to renew nuclear testing, something that the major nuclear nations have abstained from over the past 20 years. In addition, it sends a message that nuclear weapons might be useful offensive weapons. There is no rational strategic use of nuclear weapons, except to deter the use of nuclear weapons. It sends a message to non-nuclear nations that proliferation only applies to them, and instead encourages them to build their own weapons. And it maintains a huge nuclear arsenal, which is already far larger than necessary for deterrence, increasing the likelihood of accidental use. Finally, at a cost of $1 trillion, it destabilizes our economy.

Which factors in particular were taken into account to decide to change the clock setting?

All of these factors were taken into account. But we were particularly concerned about nuclear weapons. Then there’s the fact that international efforts to curb climate change seem to have slowed. There are no additional cuts in targeted goals for carbon emission beyond what was agreed to at the Paris COP21 summit in 2015. Finally, we considered the risk of terrorism, including cyberterrorism and even bioterrorism.

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We need to monitor the rise of autonomous artificial intelligence, especially as it applies to military systems, and of CRISPR, the powerful new gene-editing tool that offers hope for cures for some diseases, but makes the tools of potentially malicious genetic engineering more accessible. We should also encourage other governments to monitor these emerging technologies as well — and help industry establish guidelines to keep everyone safe.

How did the election of President Trump affect the decision to change the clock?

President Trump has made ill-considered comments about expanding and even deploying the American nuclear arsenal. He has expressed disbelief in the scientific consensus on global warming. He has shown a troubling propensity to discount or reject expert advice related to international security. And his nominees to head the Energy Department and the Environmental Protection Agency have disputed or questioned climate change. That’s true even though there is dramatic new evidence of climate change, with 2016 being the third year in a row as the hottest year in human history.

Trump’s strident and aggressive language, combined with his apparent lack of understanding of nuclear deterrence, is of great concern to all of us, and we’ll see if his actions follow his words. He’s only been in power a brief time, but his intemperate statements, lack of openness to expert advice, and questionable cabinet nominations have already made a bad international security situation that much worse.

What will it take to end the threat of nuclear war?

That won’t come unless the public gets mobilized and governments act. The U.S. and Russia should return to the negotiating table to seek further reductions in nuclear arms and to limit nuclear weapon modernization programs. In addition, both countries should reduce the alert levels of their nuclear weapons. While a world free of nuclear weapons may be the ultimate goal, it may also be a utopian goal. Right now there are more than 15,000 nuclear weapons worldwide. There is no rational reason — by a factor of 10 at least — for this number, and no reason to keep a significant fraction of weapons on high alert, which increases the chances of a nuclear accident.

What can private citizens do to help defuse the threats that face us?

Speak out. Write to your congressional representatives to oppose modernization of our nuclear arsenal. Talk to your neighbors. Only if politicians hear from the public will they respond.

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What can scientists do?

What we’re doing. Raise awareness of the problem, fight distortion with facts, speak to the community, and provide resources for people to learn.

How worried should we be?

The future is not written in stone. We have chosen the Doomsday Clock because we feel it allows us a rare opportunity to raise the profile of urgent issues that the public needs to be aware of. Its future, and our future, are in our hands. Only by understanding the potential threats, dealing with empirical reality with open eyes, and public pressure on world leaders to react accordingly can we step back from the brink. This is a time of both danger and opportunity. We need to send a loud message to our leaders that they cannot needlessly threaten our future.