An important source of uncertainty in anticipating the effects of climate change on agriculture is limited understanding of crop responses to extremely high temperatures. This uncertainty partly reflects the relative lack of observations of crop behaviour in farmers’ fields under extreme heat. We used nine years of satellite measurements of wheat growth in northern India to monitor rates of wheat senescence following exposure to temperatures greater than 34 °C. We detect a statistically significant acceleration of senescence from extreme heat, above and beyond the effects of increased average temperatures. Simulations with two commonly used process-based crop models indicate that existing models underestimate the effects of heat on senescence. As the onset of senescence is an important limit to grain filling, and therefore grain yields, crop models probably underestimate yield losses for +2 °C by as much as 50% for some sowing dates. These results imply that warming presents an even greater challenge to wheat than implied by previous modelling studies, and that the effectiveness of adaptations will depend on how well they reduce crop sensitivity to very hot days.

Matthew Kroenig's argument for preventive military action to combat Tehran's nuclear program -- "Time to Attack Iran" (January/February 2012) -- suffers from three problems. First, its view of Iranian leaders' risk calculations is self-contradictory. Second, it misreads nuclear history. And third, it underestimates the United States' ability to contain a nuclear Iran. When these problems are addressed, it is clear that, contrary to what Kroenig contends, attacking Iran is not "the least bad option." 

Kroenig's view of the way Iranian leaders are willing to take on risks is deeply incongruous. In his view, a nuclear bomb will push Tehran to block U.S. initiatives in the Middle East, unleash conventional and terrorist aggression on U.S. forces and allies, and possibly engage in a nuclear exchange with Israel. This would mean Iranian leaders are reckless: given the United States' conventional and nuclear superiority, any of these actions would provoke considerable retaliation from Washington. And, of course, a nuclear exchange with Israel would invite annihilation. At the same time, Kroenig suggests that Tehran would remain remarkably timid after a preventive strike from the United States. Presented with clear redlines, Iran would not retaliate against U.S. troops and allies or attempt to close the Strait of Hormuz. Kroenig's inconsistency is clear: If Iranian leaders are as reckless as he seems to believe, a preventive strike would likely escalate to a full-blown war. If they are not, then there is no reason to think that a nuclear Iran would be uncontainable. In short, a preventive attack on Iran can hardly be both limited and necessary.

Kroenig's argument misreads nuclear history at least three times. First, he writes that a targeted preventive strike would likely wipe out the nuclear program in Iran, as strikes against Iraq in 1981 and Syria in 2007 did in those countries. These comparisons are misleading. Recent research based on captured Iraqi documents demonstrates that the 1981 Israeli attack on the Osirak reactor, near Baghdad, actually spurred a covert nuclear weapons program at other sites. Indeed, Iraqi President Saddam Hussein remained determined to revive his nuclear program until he was removed from power in 2003. What prevented him from achieving that goal was the decade-long U.S.-led containment regime put in place after the 1991 Gulf War. The Iraqi case suggests that any attacks that do not depose the Iranian regime, too, would cause it to accelerate its efforts to acquire nuclear weapons. Kroenig's prescription might therefore precipitate the very outcome he is trying to avoid. 

As for Syria, Damascus' nuclear program was just budding. The country boasted only one exploratory facility, which was shattered easily by a single aerial bombing carried out by Israel in September 2007 under the cloak of night. But Iran's nuclear program is much more advanced and is already of industrial proportions. Any attack on Tehran would involve destroying numerous nuclear-program and air-defense targets, making it far more costly and less likely to succeed than the Israeli raid against Syria's Deir ez-Zor reactor. More, Iran's advanced program reflects Tehran's greater resolve to develop nuclear capabilities, so, post-attack, Tehran would be ever more likely to double down on developing a weapon. Furthermore, although Kroenig hopes that a targeted strike would destabilize the Iranian regime, there is no basis for such optimism. Being a civilian, parliamentary, oil-rich theocracy, Iran is relatively stable. Put simply, a preventive strike against Iran can hardly be both limited and effective.

Kroenig misreads history again when he considers a nuclear exchange between Iran and Israel. In his view, they "lack nearly all the safeguards that helped the United States and the Soviet Union avoid a nuclear exchange during the Cold War." Yet the United States and the Soviet Union avoided a nuclear exchange even during the hottest crisis of the Cold War, the Cuban Missile Crisis, at a moment in which Soviet retaliatory capability was still uncertain, there were no clear direct communication channels between the two leaderships, and Soviet experience managing their nuclear arsenal was no longer than five years. Moreover, the historical record shows that even young and unstable nuclear powers have avoided nuclear escalation despite acute crises. Pakistan and India avoided nuclear war in Kargil in 1999, as well as after the terrorist attacks targeting the Indian parliament in 2001 and Mumbai in 2008. When national survival is at stake, even opaque and supposedly "irrational" regimes with nuclear weapons have historically behaved in prudent ways.

Kroenig's final abuse of history comes when he posits a cascade of nuclear proliferation across the Middle East in response to an Iranian bomb. He mentions Saudi Arabia, and implies that Egypt, Iraq, and Turkey might all follow suit. Yet none of these states, which can count on U.S. support against Iran, nuclearized in response to Israel's nuclearization (against which they cannot count on U.S. backing, mind you). And more generally, the United States has a successful record of preventing clients from acquiring nuclear weapons in response to a regional enemy, such as South Korea and Japan in response to North Korean nuclear acquisition. (Washington agreed with Pakistani nuclearization in response to India.) 

Taking the long view, Kroenig's argument reveals an unwarranted skepticism about Washington's ability to contain a nuclear Iran. This skepticism is all the more surprising considering Kroenig's work on the benefits of U.S. nuclear superiority. Existing U.S. security guarantees, based on current capabilities, give allies little incentive to nuclearize. Egypt and Saudi Arabia are among the largest recipients of U.S. military support, and Turkey is a member of NATO. Reinforcing U.S. ties with friends in the region would be easier, cheaper, and less risky than attacking the Iranian nuclear program. 

Instead, the United States should heed the lessons of the North Korean nuclearization. Not so long ago, Washington had to face an aggressive regime in Pyongyang intent on developing nuclear weapons. The United States rejected a preventive strike in 1994 for fear that the outcome would be worse than its target's nuclear acquisition. This was the right decision. After North Korea acquired nuclear weapons, none of the consequences that Kroenig's argument would predict materialized. U.S. security guarantees contained Pyongyang and persuaded South Korea and Japan not to acquire nuclear weapons. Nobody believes that the world is better off with a bomb in North Korea -- but the record shows that it hasn't brought the end of the world, either.

Military action against Iran would be a profound strategic miscalculation. For all the talk of retrenchment, the U.S. military might remains the most powerful in the world, and it can successfully minimize consequences of an Iranian bomb, should one come to pass, by containing Tehran's ambitions, dissuading regional proliferation, and providing security assurances to its allies.

The nuclear weapons news of late has been alarming. David Sanger reported in "The New York Times" on January 9 that Iran's top nuclear official had announced his country was near initiating uranium enrichment at a new plant. And the recent leadership change in North Korea means added uncertainty about one of the world's most unpredictable nuclear weapons states. Both developments mean the danger is rising that nuclear weapons or the means to make them will spread in this year.

The ominous news brings to mind a comment that Robert M. Gates made a few years ago while working as President Obama's Secretary of Defense. "If you were to ask most of the leaders of the last administration or the current administration what might keep them awake at night," he told me, "it's the prospect of a [nuclear] weapon or nuclear material falling into the hands of Al Qaeda or some other extremists."

I was interviewing Gates for a book about nuclear threats. The book, "The Partnership: Five Cold Warriors and Their Quest to Ban the Bomb," [Harper, $29.99] examines the acute state of nuclear dangers today, including the spread of nuclear materials and technology to unstable nations like Pakistan, North Korea and Iran. If a terror group like Al Qaeda is ever going to get its hands on a nuclear weapon, or more likely the fissile material needed to make one, the source is likely to be one of those three nations. North Korea and Pakistan have a frightening history of exporting nuclear weapons technology. Iran may be next.

Despite the denials of Iranian leaders, Tehran seems well on the way to building its first nuclear weapon. Iran already has enough enriched uranium to make several warheads once the uranium is raised to a higher level of enrichment. The enrichment process can move very quickly from a low level to high, bomb-grade levels. Some upgrading of known Iranian enrichment facilities are required to get there, and these changes would be visible to the outside world. Still, Iran may well have hidden enrichment programs already cranking out highly enriched uranium. If it does move openly to higher enrichment, Israel and the United States will be tempted to attack Iran's nuclear installations.

A simple but powerful nuclear weapon can be fabricated with just a small amount of highly enriched uranium. The hardest part of making a uranium bomb is producing highly enriched uranium, something that requires advanced, industrial-scale technologies beyond the reach of a terror group. But with just 60 pounds of highly enriched uranium, a small, savvy group of engineers with some basic laboratory equipment could construct a fission bomb in a garage. The bomb mechanism is so straightforward that the United States did not bother to test a uranium weapon before dropping one over Hiroshima in 1945. And it is not wildly improbable to imagine Iran giving highly enriched uranium to a terror group.

The continuation of the Kim dynasty in North Korea - now in its third generation with the recent installation of Kim Jong-un as the new supreme leader - does not augur well for more responsible behavior by North Korea. With its active nuclear weapons program, hunger for hard currency and record of selling nuclear weapons goods to Libya and Syria, North Korea is one of the most dangerous nations on earth.

While North Korea is unlikely to sell a nuclear weapon to a terror group, it could provide the materials and knowhow to make a crude but powerful bomb. The United States, for all its intelligence-gathering hardware like spy satellites, does not know a great deal about the North Korean program. Washington was surprised to learn in 2010 that North Korea had constructed a uranium enrichment plant outfitted with the latest centrifuge technology. News about the existence of the plant came from a group of American scholars who were shown the facility during a visit to the North Korean nuclear complex at Yongbyon.

The plant is not a problem if it is producing low enriched uranium to fuel a small, light water reactor. But the plant could be used to produce highly enriched uranium. The rapid construction of the plant - it was built in just 18 months - suggested that the North Koreans might have honed their techniques at another enrichment facility, as yet undetected by the United States.

I recently asked my Stanford colleague Sig Hecker, one of the scholars who visited the enrichment plant in 2010, to outline what to watch for in the North Korean weapons program in coming weeks to determine if the new leadership is planning any change in nuclear policy and/or operations. Sig served as director of the Los Alamos National Laboratory 1986-1997. He has been a frequent visitor to North Korea, one of the few Americans to get a first-hand look at the North Korean nuclear program.

His response:

I believe that there will be a period of quiet on the diplomatic front, both for mourning and to rethink strategy. Just before Kim Jong-il died, American and North Korean diplomats came close to an agreement of American food aid in return for some concessions on the nuclear program (some reports indicated that Pyongyang would stop enrichment - but I have yet to hear official confirmation from the UnitedStates - and we never may). What to look for is to see when North Korean diplomats are ready to re-engage with Americans in quiet bilateral talks, mostlikely in China.

On the technical front: I would expect "normal operations" at Yongbyon. That means they will continue with the experimental light water reactor construction- although little will be seen from overheads because it is winter time. Much of the interior components will be fabricated in shops. I also expect them to continue with operations of the centrifuge enrichment facility - either to make more low enriched uranium for reactor fuel or to get the facility to operate fully (which it may not have been when we visited). Both of these operations will continue regardless of which way Pyongyang eventually decides to go with the nuclear program. I don't see any reason why they would cut back on these operations now.

As for potential provocative actions - they could prepare for another nuclear test -- but that is highly unlikely, if for no other reason than it is winter. Their tests occurred in October 2006 and May 2009. Nevertheless, the third test tunnel appears to have been dug some time ago (South Korean news reports and overhead imagery) and one should watch closely for activity at the test site (particularly come spring). We should also look for potential missile tests - the new launch site on the west coast should be watched for another potential long-range missile launch. (They have had three attempts from the old launch site in the east: 1998 over Japan, 2006 a complete failure, and 2009 two out of three stagesworked.) They also have not flight-tested the Musudan road-mobile missile."

It would not surprise me if North Korea conducted another nuclear test in 2012. If Kim Jong-un is looking for a way to flex North Korean military power and remind his impoverished people that their nation matters to the rest of the world, detonating a nuclear weapon will do the trick.

Iran's nuclear program will also likely generate news and international anxiety this year. Iranian threats to attack US naval vessels in the Persian Gulf may seem self-defeating, but a military confrontation between Iran and the United States is not out of the question.

There is no greater danger to American and global security than the spread of nuclear weapons and the means to make them.

OVER the last three years, as I delved into the world of American nuclear weapons, I felt increasingly as though I had stepped into a time warp. Despite the nearly total rearrangement of the international security landscape since the demise of the Soviet Union in 1991, the rise of Islamic terrorism and the spread of nuclear materials and technology to volatile nations like Pakistan, North Korea and Iran, the Defense Department remains enthralled by cold war nuclear strategies and practices.

Barack Obama took office determined to change that. He has made progress on many fronts. Last week, he outlined a new, no-frills defense strategy, downsizing conventional forces. He now needs to double down on his commitment to refashion nuclear forces. He should trim the American nuclear arsenal by two-thirds to bring it down to a sensible size, order the Pentagon to scale back nuclear war-fighting plans so they are relevant to contemporary threats, remove most American intercontinental, land-based missiles from high alert and drop the quaint notion that a fleet of aging B-52 bombers can effectively deliver nuclear weapons to distant targets.

This agenda is not only desirable, it is doable without undercutting American security. It would save tens of billions of dollars a year, a relatively small amount by Pentagon standards, but every billion counts as Leon E. Panetta, the defense secretary, trims his budget. And the steps can safely be taken without requiring reciprocal moves by Russia that must be codified in a treaty.

For the last few months, the Obama administration has been conducting a classified review of the doctrines and operations that determine the shape and potential uses of America’s nuclear armaments. If the president pushes back against the defenders of the old order at the Pentagon and other redoubts of the nuclear priesthood, he can preserve American security while making the United States a more credible leader on one of today’s most critical issues — containing the spread of nuclear weapons. Like a chain smoker asking others to give up cigarettes, the United States, with its bloated arsenal, sounds hypocritical when it puts pressure on other nations to cut weapons and stop producing bomb-grade highly enriched uranium, the key ingredient of a crude nuclear weapon.

American actions alone won’t end the proliferation danger, but American leadership is essential to any hope of containing the threat.

Sam Nunn, the former chairman of the Senate Armed Services Committee, and anything but a dove over the years, rightly warns that the spread of weapons and the means to make them may soon reach a combustible stage where New York, Washington, Moscow, Tokyo or London is at risk of a nuclear terrorist attack.

Mr. Nunn and other keepers of America’s cold-war armory, George P. Shultz and Henry A. Kissinger, former Republican secretaries of state, and William J. Perry, a former Democratic defense secretary, have banded together in recent years to press, among other things, for cutting nuclear forces, de-alerting missiles and, ultimately, eliminating nuclear arms. Mr. Obama has embraced their aims and welcomed them to the Oval Office. Their high-powered, bipartisan alliance, if adroitly employed by the White House, ought to provide some political cover as Mr. Obama reshapes nuclear policy while running for a second term.

There is no national security rationale for maintaining an arsenal of some 5,000 warheads, with nearly 2,000 arms ready to use on short notice and the rest in reserve. We don’t need thousands of warheads, or even hundreds, to counter threats from countries like Iran or North Korea.

The only conceivable use of so many weapons would be a full-scale nuclear war with Russia, which has more warheads than the United States. But two decades after the collapse of the Soviet Union, even Vladimir V. Putin, with his authoritarian bent, is not about to put Russia on a collision course with the United States that leads to nuclear war. China, equally unlikely to escalate tensions to the nuclear brink, probably has fewer than 400 warheads and a policy to use them only in self-defense. Pakistan has roughly 100, North Korea fewer than 10 and Iran, so far, zero.

The United States could live quite securely with fewer than 1,500 warheads, half in reserve. Defenders of the nuclear faith claim we need 5,000 weapons as a hedge against warheads that may become defective over time. But an elaborate Energy Department program to maintain and refurbish warheads, the Stockpile Stewardship Program, has proved highly effective.

Another oft-cited reason for increasing our arsenal is that the Pentagon’s nuclear war-fighting plans still call for striking hundreds of targets in Russia and China, as well as dozens of sites in a number of other publicly unidentified nations — presumably Iran, North Korea and Syria — considered potentially hostile to the United States and eager to possess unconventional weapons.

Washington’s current nuclear war plans remain far too outsize to deal with any plausible attack on America. Mr. Obama could remove some nations from the hit list, starting with China, and tell his generals to limit the number of targets in the countries that remain.

The oversize American nuclear arsenal features an equally outdated reliance on long-distance bombers. The days when lumbering B-52 bombers could play a central role in delivering nuclear weapons — memorably spoofed in Stanley Kubrick’s “Dr. Strangelove” — ended decades ago. Mr. Obama should ground the bombers and depend on land- and sea-based missiles.

The high-alert status of America’s intercontinental ballistic missiles is another anachronism. There are few circumstances that might require the United States to quickly launch nuclear-tipped missiles, and missiles on high alert are an invitation to an accident, or impulsive action. In the first year of his presidency, Mr. Obama outlined an ambitious nuclear weapons agenda. Absent new action, Washington will remain frozen in a costly cold war posture.

Philip Taubman is a former New York Times bureau chief in Moscow and Washington and the author of “The Partnership: Five Cold Warriors and Their Quest to Ban the Bomb.”