The mother of all apocalyptic horrors, climate change is the greatest threat facing the planet, say many scientists.
With climate change, weather can go to extremes, increasing drought in some areas, changing the distribution of animals and diseases across the globe.
The range of changes would lead to political instability, famine, ecosystem destruction and other changes – Earth would become uninhabitable.
The asteroid is the basis of disaster movies, but scientists are really worried that a rock from space could bring about the end of the Earth. A meteorite impact wiped out the dinosaurs, and in the case of Tunguska, a massive meteoroid damaged about 2,000 square kilometers of the Siberian forest in 1908. More frightening, perhaps, is that astronomers know of only a fraction of the space rocks that come around the solar system.
New deadly pathogens emerge every year: recent pandemics include SARS (severe acute respiratory syndrome), bird flu, and, most recently, a coronavirus called 'mers' originating in Saudi Arabia. Because of highly interconnected global economies, a deadly disease could spread rapidly.
Natural diseases are not the only fear. In 2011, the scientific community was outraged when researchers created an airborne version of the H5N1 bird flu virus in the lab. The results raised fears that such deadly diseases could inadvertently escape from the lab or be deliberately released, leading to a global epidemic.
Although bacterial threats are dangerous, fungal threats are even more frightening. There has been a new fungal amphibian disease with devastating effects. Chytrid fungus is wiping out frogs across the United States, scientists say. A fungus equally fatal to humans would be disastrous. Although bacteria are deadly, antibiotics are plentiful. But very little is known about the treatment of fungal infections.
Beyond the deafening rhetoric of North Korean leader Kim Jong Un, massive stockpiles of nuclear weapons around the globe could wreak havoc if they fell into the wrong hands. Last year, the Bulletin of the Atomic Scientists, a journal on global security founded in 1945 by former Manhattan Project physicists, moved the Doomsday Clock to five minutes to midnight. This clock shows how close humanity is to destruction through nuclear or biological weapons, or global climate change.
The rise of robots
The movie "Terminator" may be science fiction, but the killing machines are not far from reality. The United Nations recently called for a ban on killer robots - apparently because of experts' concerns that some countries have developed them. Many computer scientists think of the 'singularity', the point at which artificial intelligence would approach human intelligence, or surpass it many times over. Whether these robots will be benevolent helpers, humanity's problem is still up for debate. But a lot can go wrong with super-intelligent robots armed with firearms.
Fears of an overpopulated globe have existed since the 18th century, when Thomas Malthus predicted that population growth would cause mass starvation and the destruction of the planet. With the global population at 7 billion and growing, many consider population growth to be one of the major threats to the planet. Of course, not everyone agrees: population growth, most think, will stabilize in the next 50 years.
The all-encompassing effect
Although any of the above scenarios could happen, many scientists think that an effect of multiple events is more likely. For example, global warming may increase pathogens, while also causing changes in climate. Meanwhile, ecosystem decline would make it harder to produce food without bees pollinating crops or trees. So instead of an epic catastrophe, some relatively small factors would worsen life on Earth, until it was completely degraded. In this scenario, the fall of the Earth will not be dramatic, immediate, like being attacked by a tiger, but like being bitten to death by ducks.