While humanity has survived the apocalypse on Dec. 21, which some people believe the Mayans predicted hundreds of years ago, experts say there are legitimate threats that could potentially wipe out the human race.

Here’s a look at some plausible scenarios.

Earth-killing asteroids

News that a five-kilometre asteroid zipped past Earth last week wowed stargazers, but its hefty size gave apocalypse watchers a bit of a scare.

If an asteroid measuring about one kilometre in diameter struck the Earth, the impact would be enough to leave a 10-kilometre-wide crater and kill 90 per cent of humanity, says Jay Melosh, professor of planetary sciences at Purdue University.  

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This NASA radar image from 2011 shows asteroid 2005 YU55. This 400-metre-wide asteroid made a close, but harmless, swing by the Earth in November last year. (NASA/JPL-Caltech/Reuters)

"Human civilizations might suffer very badly. It would raise the dust into the air, cause atmospheric changes far away from the impact site that could lead to global crop failures for a couple years in a row," he said.

But Melosh says the likelihood of a space rock that size actually striking the Earth is incredibly small.

"Pea size asteroids hit us — thousands a day," he told CBC News. "But rocks the size of the one that killed the dinosaurs — maybe once in a hundred million years."

Of the reasonable threats to humanity, a civilization-ending asteroid impact is one of the few that can actually be monitored with ample warning time. And right now, nothing is due to hit Earth within the next 100 years, Melosh said.

A survey of all the civilization-threatening asteroids measuring one-kilometre wide or more is close to complete, and space watchers believe that 90 per cent of them have been found, Melosh added.

The most threatening asteroid due to make a close pass is called Apophis, which measures less than a kilometre wide, in 2029, said Melosh. However, if its orbit changes slightly or has been miscalculated, it might hit the Earth in 2036, he said.

Still, the probability of something that size striking the Earth in our lifetime is small, says Brett Gladman, professor of planetary astronomy at the University of British Columbia.

Even if that scenario materializes, we will have years, or even more than a decade, of advance warning, he said.

Despite what Hollywood might say, blowing up the asteroid is not an optimal way to stop it, because the fragments still pose a risk, he said.  

Instead, scientists have technology that can divert the asteroid from its path, including strapping a rocket to the asteroid to nudge its orbit slightly, or possibly painting one half of the asteroid white, Gladman said. Doing so deflects sunlight (which exerts a tiny amount of pressure on the asteroid) and in turn shifts its path, he added.

"This extremely rare but extremely catastrophic asteroid impact event is one of the few [scenarios] where we actually could, by figuring out where all the asteroids are that could do this, actually eliminate the possibility that this is a danger," he said.

Nuclear warfare

North Korea's launch last week of a three-stage rocket — similar in design to a model capable of carrying a nuclear-tipped warhead as far as California — stoked global fears and drew a wave of condemnation.

As well, Iran's nuclear ambitions remain a concern for the international community.

Still, these countries are quite far from developing nuclear capability, says Kennette Benedict, the executive director of the Bulletin of the Atomic Scientists, which informs the public of threats to the survival and development of humanity.

Benedict says the largest threats, in terms of volume and immediacy, reside in the U.S. and Russia. As a holdover from their cold-war antagonism, these two countries are estimated to house a combined 18,000 nuclear weapons — at least a thousand of which could be deployed within minutes, she added.

Benedict isn’t concerned that some geopolitical conflict would compel these two world powers to deliberately fire on each other. Rather, her fear is that some misunderstanding or third-party hacking into military command could prompt either side to push the launch button, she said.

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Iranian President Mahmoud Ahmadinejad, centre, visits the Natanz Uranium Enrichment Facility, south of the capital Tehran, in 2008. Experts are less worried about Iran and other entitities developing their nuclear capabilities than an accidental launch of existing nuclear weapons in the U.S. and Russia. (Iranian President's Office/Associated Press)

And some of these nuclear weapons could be launched within an hour to any place in the world, with a typical rocket carrying 50 weapons capable of wiping out 100 million people in very little time, she said.

"We’re pretty concerned about what's called the ‘launch readiness’ of these weapons," Kennette told CBC News from Chicago.

Alan Robock, a professor of environmental sciences at Rutgers University who has studied the climactic consequences of nuclear conflict, says there have been some close calls.

He cites one such incident back in 1995, when the Russians briefly mistook a Norwegian weather rocket as a U.S. nuclear attack.

Nuclear war is "the biggest potential danger to the planet right now, and to humans, and it could happen tomorrow by intent or by mistake," he said.

Still, even if there were nuclear warfare at a global level, pockets of humanity would likely endure, said Robock.

Infectious disease

Widespread disease, one of the four horsemen of the apocalypse according to the Bible, is another potential threat to humanity.

The fast spread of diseases like bird flu (H5N1) and SARS show how transmittable these viruses can be in our increasingly interconnected world. What are the chances of a deadly, ultracontagious bug seriously harming, if not ending the human race?

"The threat is always there," said Dr. Bhagirath Singh, the former scientific director of the Institute of Infection and Immunity and a professor at the University of Western Ontario.

"I don't think we know everything there is to know about infections and infectious diseases. Because these things come in waves, and sometimes they just jump from species to species, from birds to humans or pigs and animals. The problem is the unknown," said Dr. Singh.  

'I don't think we know everything there is to know about infections and infectious diseases.'—Dr. Bhagirath Singh, professor at the University of Western Ontario.

Viruses can spread to different countries, and around the world, within days, he said.

"Within three months, the flu virus goes around the world. But with SARS, you know, you take a plane ride and it’s nine hours from the [international] spread. And then within three days, you'll have lots of people infected. So, we are talking about days, not months and years."  

Over the years, however, humans have learned to adapt, and our immune systems have learned to live with different infections, he added.  

With a population of seven billion, and rising, however, there will always be people who outlive any widespread wave of disease, he said.

Climate change

For Alan Weisman, journalist and author of The World Without Us, the most pressing threat to the human race is the shifting climate of the Earth.

The changes — from more intense weather to diminishing natural resources — have been happening within the last five years at a faster pace than imagined, he said.

"We are pushing the atmosphere and the chemistry of the sea… it's happening at breathtaking speed," he said.  

Professor Robock, whose research also covers climate change, said its effects put people "living on the margin" at the most risk.  

"If you're a rich American or Canadian, you can do pretty well for a while," he said. "We produce a surplus of agriculture here, and we can afford air-conditioning in the summer."  

'We are pushing the atmosphere and the chemistry of the sea.'—Alan Weisman, author of World Without Us

People living in parts of sub-Saharan Africa who survive on little means are unlikely to fare as well, he said.  

"In the short term, I see global warming causing stress and potential warfare in areas where resources are limited," he said. "But that doesn't threaten the existence of humanity in the short term."  

A time frame for when the effects of climate change will directly threaten the human race is hard to predict.  

Just this past September, the global average temperature matched the all-time high, according to the National Oceanic and Atmospheric Administration.  

"The way that we are going right now, we're looking at an easily five- to seven-degree [global average temperature]

change by the end of this century," Weisman said.

"It's utterly crazy. I’m not sure humanity can survive that."  

The concern is that the effects we have seen in recent years, with severe droughts in the southern U.S. and several other natural disasters, breed more rapid changes in the atmosphere, he said.  

"If we melt the ice of the ocean, the ocean is suddenly dark, rather than white. Then, it's no longer reflecting the heat, it's absorbing it.… Several of our worst possible cases this century have been coming true in the last five years. If that is not a wakeup call, I don't know what is."