Why are we so worried about terrorism? After all, terrorism has been part of human experience for centuries. Sometimes – when assessing endless airport lines, billions of dollars in homeland security expenditures, or constant media analysis – you cannot help but wonder what all the fuss is about.

A short answer is technology. Yes, people have been killing innocent civilians to spread terror as long as people have wielded weapons. But never before have such horrific weapons been so widely available to people who can travel and communicate so freely – a degree of lethality previously reserved for nation-states. That is why every American should be familiar with the threat of chemical, biological and nuclear terrorism.

Of the three, chemical weapons are the easiest to obtain and use, though they are also the least deadly. An individual with the right know-how could purchase components for a chemical like the nerve gas sarin on the open market, or a terrorist group could buy or steal a weapon from one of the dozen or so nations that has stockpiles. Released into an enclosed space like a subway or arena, a properly weaponized chemical agent could kill hundreds, even thousands. A terrorist could achieve more deadly results by blowing up part of a chemical plant, releasing a cloud of toxic agents.

Biological weapons have the potential to cause greater harm. We have seen the capacity of a terrorist to spread death and disease– while evading capture – with anthrax. Future attacks could be far more deadly if they involve more infectious biological agents like smallpox and plague. Most experts fear that a biological attack of some sort is likely, in part because information on creating bio-weapons, and the equipment to produce them, are widely available.

Biological attacks are extremely hard to defend against. Even with a recent government subsidy, pharmaceutical companies lack incentive to develop vaccines for which there is little profit. Public health experts worry about our ability to detect an attack before it has spread beyond containment, and fear that our hospitals and local health agencies would be overrun by people seeking vaccines or antidotes. Meanwhile, terrorism analysts are left guessing at a seemingly infinite possibility of methods – from contamination of the food supply to delivery via a crop-duster – while revolutionary advances in biotechnology make it hard to anticipate what the threat will be in a decade.

The most catastrophic threat is an attack with a nuclear weapon. Though nuclear terrorism is also the most unlikely of the three, the sheer magnitude of the danger causes deep concern. It is extremely difficult to build, transport and use a nuclear device, but the number of countries with a nuclear weapon has been growing, and last year's revelation of a global black market of nuclear secrets shows how the most deadly of technologies does not belong solely to nation-states.

It should be stressed that nuclear, biological and chemical materials are all extremely difficult to acquire, build, and turn into workable weapons. Movies dwell on scenarios where whole cities are wiped out, but the more likely scenarios involve less horrific destruction: a "dirty bomb" emitting radiation rather than a nuclear explosion; an anthrax-style biological attack rather than a smallpox epidemic; the release of a modest amount of dangerous chemicals rather than a mass-casualty scenario. Still, an attack with one of these weapons that kills only a handful of people – less than a fire or major car accident – could cause tens of billions of dollars in damage, cleanup and economic fallout, not to mention widespread panic.

Nor should we take comfort in the unlikelihood of worst-case scenarios. If an attack does cause mass destruction, society might change in ways that are hard to imagine – with a serious erosion, if not an end to civil liberties and open society as we know it. There is no doubting the terrorists' intent and no reason to underestimate their capability – learning to master a chemical agent is no more unlikely than learning to fly, penetrating our borders and airline security, simultaneously gaining control of passenger airliners, and crashing them into buildings. As twenty or fifty years go by, the advances and diffusion of knowledge in the nuclear, chemical, and biological sciences will only increase, further raising the risks.

Preventing catastrophic terrorism should be the number one security objective of the 21st century – for all governments. We are better prepared than we were on 9/11, but much work remains. Steady advances in technology should galvanize our efforts to increase outreach, understanding, and cooperation with the Islamic world, and among all peoples – so science serves as a vehicle for peace and enlightenment, not war and terror.

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