The world will end. It's just a question of time and opportunity.

Although millenia of religious doomsaying have yielded no results, the post-religious techno-scientific infosphere of the 21st century actually articulates many more possible apocalypses than at any other time in human history. Some are new and human-made, such as the threat of massive nuclear holocaust, while others we simply never had the tools to see before - the threat of a rogue asteroid, or the troubling evidence in the fossil record of periodic mass extinction.

In order to function, most of us don't spend much time in our daily lives thinking about the myriad looming world-deaths on the horizon; yet, in most cases, to deny their existence is to deny the body of known scientific knowledge itself.

It doesn't pay to bet against science.

In celebration of Earth Day 2005 we have attempted to catalogue the known apocalypses, and break them down for analysis. We have also scored them from 0-10 in the areas of imminence, certainty and style (a 0 in either imminence or certainty being grounds for immediate disqualification).

For our purposes, we have defined "The End of the World" as global catastrophic change. The planet does not actually have to be destroyed, nor all the cockroaches killed, for an apocalypse to be considered for this list. Humanity doesn't even need to be eradicated, although that certainly helps.

It just has to be bad.

The top-rated apocalypses are listed below, in order of imminence.

global warming

cause: human

scenario: We had initially ranked nuclear holocaust as the most imminent end, since it could happen within the hour, but then we realized that global warming trumped even that since global warming is already happening.

If you harbor lingering doubts about global warming, then you've been misled-the scientific community of Earth is quite united: the global average temperature is creeping upward as a result of the proliferation of human-made greenhouse gases in the atmosphere.

The Earth's average temperature has increased by .6 degrees Celsius in the past century. That might not seem like much, but it is. The United Nations' Intergovernmental Panel on Climate Change has set the threshold for "dangerous warming" at only 2.5 degrees Celsius above current temperatures.

It's not possible to outline here all the possible repercussions of global warming, nor is it possible as yet to determine which will rise to the fore. Even though a couple of degrees may not seem like much, the oceans will rise, shorelines will recede, and consequences and reactions will cascade outward.

Agricultural zones will change, and as climate zones shift north, vegetation will not be able to move quickly enough to match it. Storms may become more severe, as more energy is held in the system. Melted fresh water from the polar caps could change the dynamics of thermohaline circulation, a.k.a. "the global conveyor belt," a system of massive ocean currents-the Gulf Stream among them-that helps regulate global climate, conceivably causing temperatures to plummet in some parts of the world, devasting regional agriculture.

Some projections estimate the death of nearly one quarter of all plant and animal species by 2050 due to global warming-related causes.

We just don't know. That's the point-we've tampered enormously with an enormously complex system, altering the planet on a scale never even approached before, without any thought to how it might turn out. We have yet to even fully own up to what we've done, never mind come up with a compelling solution.

The United States is the world's largest producer of greenhouse gases, yet we've decided not to participate in the Kyoto Protocol, an international accord meant to decrease greenhouse gas emissions. Our emissions are so extensive that we can, in fact, cancel out any progress made by the other 141 countries.

imminence: 10

certainty: 10

style: 2

nuclear holocaust

cause: human

scenario: An oldie but goody-just because there's no cold war doesn't mean the danger is gone.

Sure, 1985's high water mark of 65,000 nuclear weapons worldwide has fallen considerably to around 20,000, but 20,000 is plenty-ans the pieces from the dismantled bombs are within easy reach, anyway.

The tension between the superpowers has been replaced by the complexity and anxiety of proliferation as more and more countries develop or aquire their own nuclear weapons. There may be as many as 11 nations now with nuclear weapons capabilities.

Iraq wasn't one of them.

With more fingers on more buttons, the likelihood of mishap spirals upward. It's the global equivalent of having a loaded handgun in a house with children: of course you only have it for protection, of course you've explained to the children that the gun is bad, of course you keep it on a high shelf where it's out of sight... but every year kids get shot, they shoot their friends, spouses get shot, people shoot themselves, their dogs, and their friends who climb through the window to play a joke on them. That's the way humans are-prone to error, and not marked by good judgement.

The Doomsday Clock, a rating system for global nuclear threat maintained by the Bulletin of the Atomic Scientists, currently stands at seven minutes until midnight.

imminence: 9

certainty: 3

style: 3

yet another mass extinction

cause: human, natural-terrestrial or natural-cosmic

scenario: Look at the fossil record and it's plain as day. Every so often, lots and lots of things die at the same time.

It happens again and again, with varying degrees of severity, sometimes eliminating as much as 70 percent of all Earth species.

Generally, we don't know why. Some specific mass extinctions have theories that go with them, such as the asteroid that wiped out 50 percent of life on Earth and killed off the dinosaurs, but there is no overarching explanation. Maybe the reason is different each time, but if the past is a predictor of the future then it will happen again.

The Twilight Zone twist is that we are actually already in a mass extinction event, and we are the cause. E.O. Wilson estimates that 50 percent of all life on Earth will be extinct in 100 years if the current rate of environmental destruction continues.

imminence: 8

certainty: 7

style: 2

plague

cause: human or natural-terrestrial

scenario: A classic with a modern twist. Plagues periodically come out of the jungle, or jump species and wreak havok, but in the past there were at least natural geographical barriers to their spread.

Now, a fast-moving disease can hop on a plane and go global in a few hours.

It doesn't even have to be a new disease. Two weeks ago, samples of the 1957 pandemic flu virus were accidentally sent out to thousands of labs around the globe when they were mistakenly included in test kits. It's a strain against which most people no longer have an immunity, nor is there any amount of vaccine for it currently available. While an outbreak has not occurred, any virus would be green with envy to have such distribution.

Such as the Marburg virus currently staging an outbreak in Angola, currently killing 9 out of 10 infected. It has killed 235 so far, including 16 doctors and nurses, and there is no vaccine.

International health organizations are working to contain the outbreak.

imminence: 8

certainty: 5

style: 2

ozone layer depletion

cause: human

scenario: A thin layer of ozone (O3) in the upper atmosphere protects us from the sun's ultraviolet radiation. UV radiation is destructive to DNA, and without this protection the havoc that it could wreak on the life forms of Earth would go far beyond the simple skin cancers that it causes currently.

The ozone layer crisis may be out of fashion, but it's not over. Due to the distribution of human-made chloroflourocarbons into the upper atmosphere-chemicals that destroy ozone-the ozone layer over the northern hemisphere has been depleting by about 4 percent per decade for the last several decades.

The ozone holes that appear seasonally over the North and South poles are a result of seasonal concentrations of these human-made chemicals.

A study released by the American Geophysical Union in 2003 shows that the depletion may be slowing due to the effect of widespread international bans on CFCs, but it has not stopped.

imminence: 8

certainty: 5

style: 1

asteroid or comet

cause: natural-cosmic

scenario: Asteroids happen. Scientists are in pretty close agreement that the mass extinction of the dinosaurs 65 million years ago was caused by an asteroid that impacted in Mexico, and evidence of other impacts abound throughout the solar system, from the Barringer Meteor Crater in Arizona to the scarred face of the moon to the collision of comet Shoemaker-Levy 9 with Jupiter in 1994, during which time astronomers watched 20 separate comet pieces cascade into the Jovian atmosphere.

It's for the purpose of assessing the threat posed by these objects that NASA established the Near Earth Object Program (http://neo.jpl.nasa.gov/). Each detected object is given a number from 0-10 on the Torino Scale, with zero being no chance for impact and 8-10 being certain collisions, ranked by severity. Currently, all known objects but three have ratings of zero. Of the three objects with a measurable chance of impact, all three rate only at one on the Torino scale. However, one of them, asteroid 2004MN4, topped the chart with a rating of 4 back in December 2004 before being downgraded as additional observations of its trajectory came in. Asteroid 2004MN4 is a 400-meter asteroid that will fly by the Earth in 2029-but it has been proposed that the Earth's gravity may alter the asteroid's course enough to cause an impact a few years later.

Be warned: asteroids and comets are dark objects, and our ability to detect them is relatively limited and recent. Between 1997 and 2004, the average number of near-Earth asteroids detected has skyrocketed from about 50 per year to around 500. In other words, we've just begun to look.

imminence: 7

certainty: 8

style: 7

magnetic field reversal

cause: natural-terrestrial

scenario: There is a second, inner Earth, made of iron, and over it flow liquid iron seas. We call it the core, and it is the relative motion of this hard iron sphere and its fluid covering that generates the outer Earth's magnetic field.

Periodically, the Earth's magnetic field collapses and reverses-North becomes South, and South becomes North. This occurs, on average, every 250,000 years, although the interval is by no means regular. The last reversal was about 780,000 years ago, and measurements from the last 150 years show that the magnetic field is currently weakening at a rate that will lead to its collapse in 1,000-2,000 years if field decay continues at the current rate.

The consequences of a field collapse are uncertain. The Earth's magnetic field protects us from solar radiation, which would destroy satellites and damage the atmosphere. The effects on the migratory patterns of animals are also a concern.

Although it's highly likely that humanity would be able to adapt to a world altered by magnetic field changes, it's worth noting that the breakdown of Mars' magnetic field is believed to have caused its atmosphere to boil away into space.

imminence: 6

certainty: 9

style: 5

ice age

cause: natural-terrestrial

scenario: Ice ages come and go and the glaciers advance and retreat, but the mechanism for their fluctuation is not clearly understood. Technically, we are currently in an ice age now, but in an interglacial period. As recently as 10,000-15,000 years ago, the Wisconsinan glaciation carved out the Great Lakes, while 600-800 million years ago the world is believed to have achieved a "Snowball Earth" state in which the oceans froze over all the way to the equator.

Ice ages are so commonplace and routine that they are clearly a natural part of the Earth's climatic processes. There is little question that the ice will return, it's just a matter of how soon and how fast.

While a new ice age would most likely come on very, very, very slowly, a world in which Ohio becomes a blue state by virtue of being buried under 100 feet of ice would nonetheless be a very different world than what we know today.

imminence: 5

certainty: 8

style: 5

superspecies

cause: human or natural-terrestrial

scenario: Remember Jurassic Park? Scientists have found trace proteins in Iguanadon bones and dinosaur eggs, and now, according to a recent New Scientist article, they've found what is believed to be traces of Tyrannosaurus Rex bone marrow preserved inside a fossil. Others are exploring the idea of "resurrection ecologies," hatching miraculously preserved hundred-year-old shrimp eggs to study a hundred-years-gone ecosystem. Plants are being genetically engineered to absorb heavy metals from waste sites.

Chimeras abound: according to a recent Washington Post article, lamb fetuses injected with human stem cells have grown into sheep with livers 80 percent human, while human stem cells injected into pig fetuses have yielded pigs with human/pig blood. Chicken and quail brains have been mingled, and there are proposals to introduce human brain cells into mice.

What could possibly go wrong?

In 1999 it was discovered that pollen from genetically modified corn-which is the norm, not the exception-had the unintended consequence of killing Monarch butterflies it came into contact with.

That was just a warning shot across the bow. As we continue to experiment with biological combinations and modifications that have no precedent or counteragent in the natural world, we will continue to run an ever-escalating risk of starting something that we can't finish. Whether it's a lethal pig virus that is able to jump to humans by gestating in a human/pig chimera, or a species of moss that we accidentally imbue with the ability to dissolve iron, or simply a very prolific and unstoppable snail that we recreate from some ancient genetic stew, it will only be a matter of time before someone makes a mistake or a miscalculation.

imminence: 5

certainty: 6

style: 6

supervolcano

cause: natural-terrestrial

scenario: Some volcanic eruptions are bigger than others. A zero on the Volcanic Explosivity Index means the eruption was non-explosive, and only a little matter oozes out. An 8, however, is considered mega-colossal, and the ash ejected into the atmosphere will plunge the world into a volcanic winter.

Toba in Indonesia was one such supervolcano, a mega-colossal eruption 73,000 years ago that may have decreased the average temperature of the Earth by 15 degrees Celsius. There is some speculation that the human race barely survived this event, possibly whittled down to a few thousand individuals by the catastrophe.

Yellowstone National Park is actually the remains of another planet-changing mega-colossal eruption from about two million years ago.

More recently, the Mount Tambora eruption of 1815 rated a 7 on the VEI and left behind a hole four miles in diameter. The global climate effects caused crops to fail in Europe and America the following year.

We have no reason to think that there will not be more VEI 7 and VEI 8 volcanoes in Earth's future, we just don't know when.

imminence: 5

certainty: 9

style: 7

miscellaneous

cause: to be determined

scenario: The more we looked, the more a catch-all category seemed necessary. There is so much that we don't know about the universe, the planet and the ultimate effects of our actions. There may be cosmic threats that we haven't even dreamed of, just as there might be butterfly-effect-type consequences to any of the countless irresponsible things we do every day to our environment. A critical keystone species could be eradicated, causing whole sections of the food chain to collapse. Global biodiversity could reach a critical low, making the entire system less robust and more susceptible to mishap. Some studies have shown declining sperm counts worldwide over the past 50 years; other studies have refuted them. Pollution in the oceans could result in vast algal blooms that upset the ecosystem of the entire ocean, or not. If we create artificial intelligence, will it grow to hate us, as we've long suspected? What sorts of weapons will be invented in the next 50 years? The next 100? The next 1,000?

We had a category for "miscellaneous genocidal madman," but then it seemed like we should have a category for "miscellaneous genocidal regime" to be fair. But then what about a cult like Aum Shinrikyo, or a fundamentalist religious group like Al-Qaeda? Or what if a corporation with access to a biological weapon collapsed in scandal and a mentally unstable CEO decided to take the whole world down with him/her?

So, here we are. Miscellaneous. Score: even fives, right down the middle, because we just don't know.

imminence: 5

certainty: 5

style: 5

burst of gamma radiation

cause: natural-cosmic

scenario: Gamma radiation bursts are a known cosmic phenomenon and are detected about once a month. Lasting only about 10 seconds each, all the bursts detected so far have originated in other galaxies and therefore have been too weak to harm Earth. Thought to be caused by neutron star collisions or by black hole-producing events, a gamma radiation burst that originated in our galaxy would be a different thing altogether. In a recent article in The Guardian, Dr Adrian Melott, of the University of Kansas and co-author of a recent paper on the subject, observed that "a gamma ray burst originating within 6,000 light years from Earth would have a devastating effect on life."

The article goes on to explain that such a burst would wipe out the plankton in the upper levels of the ocean and destroy the ozone layer. Scientists are currently investigating whether the Ordovican mass extinction 450 million years ago, during which roughly 60 percent of marine invertebrate species were eradicated, may have been caused by such a burst.

We currently have no warning or defense against a major gamma ray event.

imminence: 4

certainty: 2

style: 4

alien invasion

cause: other

scenario: We felt we had no choice but to include the alien invasion scenario, because the logic is so hard to refute: In a universe of millions of galaxies and billions of star systems, there is bound to be other life. And since life on Earth achieved sentience, there's no reason to think that life elsewhere couldn't as well. And if we can get to the moon in a decade, then it's not unreasonable to think that another species with a few thousand-or million years' head start could unravel the riddle of insterstellar travel. And if they find us, there's a good chance they'll decide to kill us, since that's what we'd do.

imminence: 2

certainty: 1

style: 9

death of the sun

cause: natural-cosmic

scenario: While going supernova would be more dramatic, current astronomical science holds that the sun does not have enough mass to do so and will instead, in about 4.5 billion years, expand into a red giant before dissolving into a combination nebula/white dwarf. When it expands into a red giant, it may become large enough to absorb the Earth, but at the very least the heat will cook and destroy all life on the planet, rendering it uninhabitable.

imminence: 2

certainty: 10

style: 8

the inevitable heat death of the universe

cause: natural-cosmic

scenario: All good things come to an end. While it's still possible that the universe might someday collapse back onto itself in a Big Crunch-the mirror opposite of the Big Bang-current theory points to an eternally-expanding universe. In that case, the universe will continually cool as it expands, until there is not enough heat to sustain life. Eventually, the Second Law of Thermodynamics dictates that all matter in the universe will reach a state of uniform energy, somewhere just barely above absolute zero.

Without variations in energy, nothing can happen.

It will be very quiet then.

imminence: 1

certainty: 10

style: 10