According to Sir Martin Rees, author of Our Final Century, astronomer royal and professor of cosmology and astrophysics at the University of Cambridge, humans only have a 50-50 chance of making it through the 21st century without serious setback. "Some natural threats, such as earthquakes and meteorite impacts, remain the same throughout time, while others are aggravated by our modern-interconnected world. But now we also need to consider threats that are human induced."

So what are the greatest threats to humans and can we do anything about them? Below, 10 scientists talk about their greatest fears and explain how society could be affected. Afterwards we estimate each threat in two ways: first, the chance of it occurring in our lifetime (the next 70 years); and, second, the danger that it would pose to the human race if it did happen (10 = making humans extinct, to one = barely having an impact on our lives).

1: Climate Change

Nick Brooks is a senior research associate at the Tyndall Centre for Climate Change Research at the University of East Anglia:

"By the end of this century it is likely that greenhouse gases will have doubled and the average global tempera­ture will have risen by at least 2C. This is hotter than anything the Earth has experienced in the last one and a half million years. In the worst case scenario it could completely alter the climate in many regions of the world. This could lead to global food insecurity and the widespread collapse of existing social systems, causing mass migration and conflict over resources as some parts of the world become much less habitable. I don't think that climate change will sound the death knell for humans, but it certainly has the potential to devastate."

Chance of temperatures rising more than 2C (the level considered to be dangerous by the European Union) in the next 70 years: High

Danger score: 6

2: Telomere erosion

Reinhard Stindl, a medical doctor at the University of Vienna, says every species contains an "evolutionary clock", ticking through the generations and counting down towards an inevitable extinction date:

"On the end of every animal's chromosomes are protective caps called telomeres. Without them our chromosomes would become unstable. Each time a cell divides it never quite copies its telomere completely and throughout our lifetime the telomeres become shorter and shorter as the cells multiply. Eventually, when they become critically short, we start to see age-related diseases, such as cancer, Alzheimer's, heart attacks and strokes.

"However, it is not just through our lifetime that telomeres get shorter. My theory is that there is a tiny loss of telomere length from one generation to the next, mirroring the process of ageing in individuals. Over thousands of generations the telomere gets eroded down to its critical level. Once at the critical level we would expect to see outbreaks of age-related diseases occurring earlier in life and finally a population crash. Telomere erosion could explain the disappearance of a seemingly successful species, such as Neanderthal man, with no need for external factors such as climate change."

Chances of a human population crash due to telomere erosion during the next 70 years: Low

Danger score: 8

3: Viral Pandemic

Professor Maria Zambon is a virologist and head of the Health Protection Agency's Influenza Laboratory:

"Within the last century we have had four major flu epidemics, along with HIV and Sars. Major pandemics sweep the world every century, and it is inevitable that at least one will occur in the future. At the moment the most serious concern is H5 avian influenza in chickens in south-east Asia. If this virus learns to transmit from human to human then it could sweep rapidly around the world. The 1918 influenza outbreak caused 20m deaths in just one year: more than all the people killed in the first world war. A similar outbreak now could have a perhaps more devastating impact.

"It is not in the interests of a virus to kill all of its hosts, so a virus is unlikely to wipe out the human race, but it could cause a serious setback for a number of years. We can never be completely prepared for what nature will do: nature is the ultimate bioterrorist."

Chance of a viral pandemic in the next 70 years: Very high

Danger score: 3

4: Terrorism

Professor Paul Wilkinson is chairman of the advisory board for the Centre for the Study of Terrorism and Political Violence at the University of St Andrews:

"Today's society is more vulnerable to terrorism because it is easier for a malevolent group to get hold of the necessary materials, technology and expertise to make weapons of mass destruction. The most likely cause of large scale, mass-casualty terrorism right now is from a chemical or biological weapon. The large-scale release of something like anthrax, the smallpox virus, or the plague, would have a huge effect, and modern communications would quickly make it become a trans-national problem.

"In an open society, where we value freedoms of movement, we can't guar­antee stopping an attack, and there is a very high probability that a major attack will occur somewhere in the world, within our lifetimes."

Chances of a major terrorist attack in the next 70 years: Very high

Danger score: 2

5: Nuclear war

Air Marshal Lord Garden is Liberal Democrat defence spokesman and author of Can Deterrence Last?:

"In theory, a nuclear war could destroy the human civilisation but in practice I think the time of that danger has probably passed. There are three potential nuclear flashpoints today: the Middle East, India-Pakistan and North Korea. Of these, North Korea is the most worrying, with a hair-trigger, conventional army that might start a war by accident. But I like to believe the barriers against using a nuclear weapon remain high because of the way we have developed an international system to restrain nuclear use.

"The probability of nuclear war on a global scale is low, even if there remains the possibility of nuclear use by a rogue state or fanatical extremists."

Chance of a global nuclear war in the next 70 years: Low

Danger score: 8

6: Meteorite impact

Donald Yeomans is manager of Nasa's Near Earth Object Program Office at the Jet Propulsion Laboratory in California:

"Over very long timescales, the risk of you dying as a result of a near-Earth object impact is roughly equivalent to the risk of dying in an aeroplane accident. To cause a serious setback to our civilisation, the impactor would have to be around 1.5km wide or larger. We expect an event of this type every million years on average. The dangers associated with such a large impactor include an enormous amount of dust in the atmosphere, which would substantially shut down sunlight for weeks, thus affecting plant life and crops that sustain life. There would be global firestorms as a result of re-entering hot ejecta and severe acid rain. All of these effects are relatively short-term, so the most adaptable species (cockroaches and humans, for example) would be likely to survive."

Chance of the Earth being hit by a large asteroid in the next 70 years: Medium

Danger score: 5

7: Robots taking over

Hans Moravec is a research professor at Carnegie Mellon University's Robotics Institute in Pittsburgh:

"Robot controllers double in complexity (processing power) every year or two. They are now barely at the lower range of vertebrate complexity, but should catch up with us within a half-century. By 2050 I predict that there will be robots with humanlike mental power, with the ability to abstract and generalise.

"These intelligent machines will grow from us, learn our skills, share our goals and values, and can be viewed as children of our minds. Not only will these robots look after us in the home, but they will also carry out complex tasks that currently require human input, such as diagnosing illness and recommending a therapy or cure. They will be our heirs and will offer us the best chance we'll ever get for immortality by uploading ourselves into advanced robots."

Chance of super-intelligent robots in the next 70 years: High

Danger score: 8

8: Cosmic ray blast from exploding star

Nir Shaviv is a senior lecturer in physics at the Hebrew University in Jerusalem, Israel:

"Once every few decades a massive star from our galaxy, the Milky Way, runs out of fuel and explodes, in what is known as a supernova. Cosmic rays (high-energy particles like gamma rays) spew out in all directions and if the Earth happens to be in the way, they can trigger an ice age. If the Earth already has a cold climate then an extra burst of cosmic rays could make things really icy and perhaps cause a number of species to become extinct. The Earth is at greatest risk when it passes through a spiral arm of the Milky Way, where most of the supernova occur. This happens approximately every 150m years. Paleoclimate indicators show that there has been a corresponding cold period on Earth, with more ice at the poles and many ice ages during these times.

"We are nearly out of the Sagittarius-Carina arm of the Milky Way now and Earth should have a warmer climate in a few million years. But, in around 60m years we will enter the Perseus arm and ice-house conditions are likely to dominate again."

Chance of encountering a supernova in the next 70 years: Low

Danger score: 4

9: Super-volcanos

Professor Bill McGuire is director of the Benfield Hazard Research Centre at University College London and a member of Tony Blair's Natural Hazards working group:

"Approximately every 50,000 years the Earth experiences a super-volcano. More than 1,000 sq km of land can be obliterated by pyroclastic ash flows, the surrounding continent is coated in ash and sulphur gases are injected into the atmosphere, making a thin veil of sulphuric acid all around the globe and reflecting back sunlight for years to come. Daytime becomes no brighter than a moonlit night.

"The global damage from a super-volcano depends on where it is and how long the gas stays in the atmosphere. Taupo in New Zealand was the most recent super-volcano, around 26,500 years ago. However, the most damaging super-volcano in human history was Toba, on Sumatra, Indonesia, 74,000 years ago. Because it was fairly close to the equator it injected gas quickly into both hemispheres. Ice core data shows that temperatures were dramatically reduced for five to six years afterwards, with freezing conditions right down to the tropics.

"A super-volcano is 12 times more likely than a large meteorite impact. There is a 0.15% probability that one will happen in your lifetime. Places to watch now are those that have erupted in the past, such as Yellowstone in the US and Toba. But, even more worryingly, a super-volcano could also burst out from somewhere that has never erupted before, such as under the Amazon rainforest."

Chance of a super-volcano in the next 70 years: Very high

Danger score: 7

10: Earth swallowed by a black hole

Richard Wilson is Mallinckrodt Research Professor of Physics at Harvard University in the US:

"Around seven years ago, when the Relativistic Heavy Ion Collider was being built at the Brookhaven National Laboratory in New York, there was a worry that a state of dense matter could be formed that had never been created before. At the time this was the largest particle accelerator to have been built, making gold ions crash head on with immense force. The risk was that this might form a stage that was sufficiently dense to be like a black hole, gathering matter from the outside. Would the Brookhaven labs (and perhaps the entire Earth) end up being swallowed by a black hole created by the new accelerator?

"Using the information we already know from black holes in outer space, we did some calculations to find out if the Brookhaven particle accelerator was capable of forming such a black hole. We are now pretty certain this state of matter won't form at Brookhaven and that the Earth won't be swallowed when these particles collide."

Chance of Earth being gobbled up by a black hole in the next 70 years: Exceedingly low

Danger score: 10

· Naked Science: Clash of crises, is a free event at the Science Museum's Dana Centre on Wednesday April 20. The event is free but tickets should be prebooked on 020 7942 4040 or tickets@danacentre.org.uk. Please see www.danacentre.org.uk for full details. You can also take part in the Dana Centre's survey, investigating the nation's biggest fears for the future at www.sciencemuseum.org.uk