7 cataclysmic ways the world might end

From supervolcanoes to nuclear winters, discover the scientific theories in danger of becoming reality

1. Supervolcano eruption

Supervolcanoes are the leviathans of volcanism. Defined by their ability to blast more than 1,000 cubic kilometres (240 cubic miles) of material into the air, they are a thousand times larger than the 1980 Mount Saint Helens eruption – the most destructive volcanic eruption in recorded US history. Geologists have never witnessed a supervolcanic eruption, but by looking at remnants of previous cataclysms, they can piece together alarming details. These eruptions rain debris and fiery destruction on a geographical region as large as Europe, but it’s the gases they inject into the stratosphere that could spell disaster for humanity. During a super-eruption, a scalding plume of gas would belch almost to the edge of space. Levelling off, it would spread out around the globe, forming a veil of sulphate aerosols that would persist for several years and trigger a volcanic winter.

Such catastrophic super-eruptions are rare; the last we know of occurred 27,000 years ago in New Zealand. But they are inevitable.

The veil would reflect and absorb incoming solar radiation, warming the upper atmosphere and preventing heat from reaching the surface. The result would be extreme instability in the climate system. Surface temperatures would tumble rapidly, leading to agricultural collapse and famine. Some even speculate that these conditions could lead to the onset of an ice age. Such catastrophic super-eruptions are rare; the last we know of occurred 27,000 years ago in New Zealand. But they are inevitable. Critically, we have no idea when the next one will strike and absolutely no way to prevent it.

Volcanic winter. Global climate in the aftermath of a supervolcanic eruption. Click to see more.

2. Nuclear winter

With the power to demolish entire cities in seconds, nuclear bombs are the most devastating weapons on the planet. In many ways, those annihilated in the first moments of a blast could be considered the lucky ones. In the 1980s, prominent scientists including Carl Sagan warned that a nuclear war between the US and the Soviet Union could drive the globe into a catastrophic nuclear winter. Incinerated cities and forests would send heaving clouds of Sun-blocking ash into the stratosphere – it could take years for particles to be rained out.

In a worst-case scenario, it is theorised that 99 per cent of the Sun’s light would be blocked for several months, resulting in noontime twilight and the halting of photosynthesis. Surface temperatures could plummet tens of degrees below normal levels for years or even decades, bringing crippling arctic conditions to the entire globe. Plants, animals and humans would perish in the darkness.

nuclear winter
Nuclear winter: How nuclear conflict could wreak havoc on global climate and the environment


3.Asteroid impact

Asteroids are hunks of rocky space debris, left over from the creation of the planets, which whizz around our Solar System, orbiting the Sun. From time to time they cross paths with us and, while impacts on the scale of the infamous ten-kilometre (6.2-mile) dinosaur-destroyer are rare, an asteroid a fifth the size could spell disaster for civilisation. With energy greater than 10 million Hiroshima bombs, the impact shock would flatten everything within a 300-kilometre (186-mile) radius. Dust and debris would cause an ‘impact winter’ and most living things would perish. An ocean strike would trigger monumental tsunamis, obliterate entire coastlines, and inject seawater into the atmosphere – destroying huge swathes of the ozone layer and exposing survivors to devastating levels of UV radiation.

7 cataclysmic ways the world might end
Don’t worry, NASA has a plan to save us


4. Runaway greenhouse effect

The greenhouse effect is essential to life as we know it. Just like a glass greenhouse lets in light but traps heat, insulating gases in our atmosphere protect us from the deathly cold of space. But since the Industrial Revolution, humans have upset the delicate balance of the atmosphere. Concentrations of carbon dioxide (CO2), released when fossil fuels are burned, and other ‘greenhouse gases’, have risen at an alarming rate, forming a thick blanket around Earth, trapping excess heat and nudging global temperatures upwards.

Scientists warn of a tipping point – a temperature beyond which the problem can no longer be dialled back

While warmer weather might be welcome in some places, ‘feedback loops’ complicate the effects of higher temperatures. Increased evaporation will cause denser cloud cover, exacerbating the warming effect because clouds themselves are strong insulators. Longstanding carbon ‘sinks’ – rocks and oceans that pull CO2 out of the atmosphere – will become unstable and release their stores, accelerating the problem still further. Scientists warn of a tipping point – a temperature beyond which the problem can no longer be dialled back. If we reach this point, a runaway greenhouse effect would cause temperatures to soar to several hundred degrees Celsius, boiling the oceans and making life on Earth impossible.

7 cataclysmic ways the world might end
Human activities set in motion an unstoppable warming of the planet


5. The Sun dies

The Sun supplies the energy for almost all of life on Earth, but all good things come to an end. When the Sun’s time comes and it starts to run out of fuel, its core will collapse as the outward force – due to fusion – can no longer balance the strong inward force of gravity. At the same time, its outer envelope will inflate, expanding the star into a red giant, and engulfing the orbits of Mercury, Venus, and – potentially – Earth. Eventually, the dying Sun will transform into a dense white dwarf surrounded by a dazzling planetary nebula. Humans won’t be around to see this; the Sun’s fuel supply will start to run low about five billion years from now, but Earth will be inhospitable long before.

7 cataclysmic ways the world might end
Red giant Sun. The view from Earth as the Sun grows old


6. Global pandemic

A pandemic is an outbreak of infectious disease that spreads throughout much of the globe. Human history is punctuated by debilitating pandemics and, despite medical advances, it’s only a matter of time before we see another. Today’s standards of sanitation and medical research help us stay one step ahead of most infectious agents, but widespread international travel and increased population densities make it much easier for a global pandemic to threaten us all.

Emerging diseases pose the biggest problem, since they involve unknown pathogens with no existing vaccinations.

In 2003, SARS (severe acute respiratory syndrome) – a serious form of pneumonia – spread to six of the world’s seven continents within months, infecting an estimated 8,000 people and killing 750. More recently, Ebola – a grisly disease ravaging West Africa with a death toll over 11,000 – threatened to go pandemic in late 2014 after cases popped up in travellers arriving back in North America and Europe. Emerging diseases pose the biggest problem, since they involve unknown pathogens with no existing vaccinations. The most dangerous are highly contagious but have delayed symptom onset, meaning that infected people unwittingly spread the disease to many others before realising they are sick. Shape-shifting diseases that mutate fast are almost impossible to vaccinate against. In today’s hyper-mobile, city-dominated world, a deadly disease combining these three features could spell doom for the human race.

7 cataclysmic ways the world might end
The deadliest pandemics in human history


7. Gamma-ray burst

Gamma-ray bursts (GRBs) are the brightest events in the universe. Produced by the explosion of massive stars, they emit focussed beams of intense gamma radiation. They can last anywhere from a fraction of a second to several hours, and can release as much energy in ten seconds as the Sun will produce in its entire lifetime. If the Earth were unlucky enough to get caught in a GRB’s almighty death beam, the effects would be catastrophic. It would trigger atmospheric chemistry that would destroy the ozone layer – leaving life on the surface exposed to deadly ultraviolet radiation.

gamma ray
Anatomy of a long GRB: Step-by-step guide to the brightest electromagnetic events in the universe. Click to see more.


The full version of this article first appeared in Issue 78 of How It Works, written by Ceri Perkins

If you enjoyed this try:

Asteroids: The ultimate guide to killer space rocks

The eruption of Mount St Helens

Why do we need renewable energy?