Among the many sea changes in policy President Barack Obama is bringing to Washington is his support for the Comprehensive Nuclear Test Ban Treaty (CTBT). Ever since the U.S. Senate rejected the CTBT in 1999, the treaty has languished in parliamentary purgatory. Yet during last fall's presidential campaign, Obama told the journal Arms Control Today, "As president, I will reach out to the Senate to secure the ratification of the CTBT at the earliest practical date." We urge the president to make good on his promise.

The CTBT would prohibit the U.S. and every other signatory from conducting test explosions, no matter how small, of nuclear weapons underground, in space or anywhere else. If ratified, the ban would affect existing stockpiles as well as any future weapons. We addressed the issues of managing an aging stockpile without testing in 2007 [see "A Need for New Warheads?"; Scientific American, November]. In brief, the weapons scientists interviewed by staff editor David Biello concurred with the conclusion of a 2002 report by a committee of the National Academy of Sciences (NAS): that the ongoing stockpile stewardship program can maintain and verify the reliability of U.S. nuclear weapons without explosive testing. Indeed, the National Nuclear Security Administration recently announced that an eight-year program to refurbish B61 nuclear bombs and ensure their reliability was completed without explosive testing a year ahead of schedule. As for future nuclear weapons, since their capability and reliability could not be tested, parties to the treaty would be unlikely to risk deploying them in their own military arsenals.

But wouldn't a ratified treaty that goes into force leave the U.S. and other countries that abided by it vulnerable to cheaters that clandestinely develop and test their nuclear capabilities? Isn't that reason to enough to reject the treaty? The answer may seem counterintuitive, but the CTBT would make the world a safer, more secure place for the U.S. than a world without the treaty.

An essential element of the CTBT is a monitoring system intended to support compliance and deter cheating by ensuring that cheaters are unmasked [see "Monitoring for Nuclear Explosions," by Paul G. Richards and Won-Young Kim, on page 70]. Of course, no policing system is perfect. But the NAS report concluded that once the planned International Monitoring System of the CTBT is fully operational (it is about two-thirds complete today), no underground test with an explosive yield of more than one kiloton could "be confidently hidden." Moreover, even if a test smaller than a kiloton were somehow concealed, it would do little to harm the strategic interests of the U.S. Because of their prior experience with nuclear testing, Russia and China could learn the most from a low-yield test but the NAS report concludes that such a test could only marginally increase the great threat they already pose. New, aspiring nuclear powers would be less likely to derive technical benefits from a test small enough to hide, and they would be far less skilled at keeping it hidden.

Still, successful cheating could be harmful to U.S. interests. But compared with what? The only realistic alternative to living with the risk that parties to the CTBT will cheat at a low, undetectable yield is living in a world without the treaty the world we inhabit today. In that world, India, Pakistan and North Korea can test at whatever explosive yield they like, as they have done. Iran's nuclear ambitions deeply threaten the stability of the Middle East, but with the CTBT in force an Iranian nuclear explosion would risk even greater international condemnation than it does today. And testing begets testing: the anxiety among the nonnuclear neighbors of a testing state makes the pressure for the neighbors to "go nuclear" almost irresistible. Such proliferation is far more dangerous to the U.S. not to mention the countries directly involved than the worst-case risk of putting the CTBT in force.

The NAS draws the same conclusion in its 2002 report. Yet there is some hope that the reasoning from enlightened self-interest that leads to its conclusion will finally get the high-level hearing it deserves in Washington. The chair of the committee and lead author of the NAS report was John P. Holdren, now Obama's choice to become the presidential science adviser.

Editor's Note: This story was originally published with the title "Acceptable Risks for Arms Control"