Here are some potential physical menaces to the survival of the human species and some comments on their likelihood. Actually most of them are lesser calamities. None seem likely in the immediate future. Ideological menaces are both more likely and have the possibility of doing humanity greater harm. The disaster flic is a current art form, but don't hope to get facts from movies.
My opinion is that science, technology, and the wide dispersal of humanity has turned us into a very tough species. Because of journalism, humanity is readily annoyed, but we are capable of surviving any of the catastrophes to life that have occurred in past, and science is very likely to give us plenty of warning for most of them.
In discussing any menace, journalists, lawyers, and politicians, movie makers, and even scientists who have enrolled themselves in a cause, often take a worst-case approach. They say X could happen if we do Y, and therefore Y should be avoided at all costs. The anti-nukes make up worst cases as do the proponents of extreme laws about potential terrorism. So did the advocates of preventive war against the Soviet Union, and so do some present advocates of preventive war agains Islam. A worst case attitude to the possibility of an asteroid hitting the earth would require devoting most of the GDP of the whole world to defenses against it. A worst case policy to a possible pandemic would require that every country quarantine itself, and indeed every state in the US quarantine itself.
Human society is not yet technologically powerful enough, economically rich enough or politically united enough to afford a worst case attitude. On a personal level, no person can afford a worst case attitude to menaces to his own survival, e.g. to protect himself against a meteorite or an airplane hitting his house.
Most of the menaces that people or organizations see have not done much harm yet. They forsee harm, urge us to act now, are disturbed if the public is unresponsive, and predict doom. However, the public will support action when sufficient harm actually occurs. Thus the American failure to continue building nuclear power plants bothers me, and I think we have suffered from it. However, if this negligence causes a severe shortage of electricity, we will then build new plants. (Actually, building new plants seems to be in the works because of the greatly increased price of natural gas - a warning of more severe shortages to come). Human society has many feedback mechanisms - servomechanisms they are sometimes called.
To destroy humanity, the collision would have to be much larger than those that have hit the earth since 3.8 billion years ago after life started. Much smaller collisions could kill a lot, perhaps most, of the people. A repetition of the Chicxulub asteroid collision would, according to some theories, temporarily wipe out agriculture, but there is enough stored food for a large number of people to survive and re-establish an economy. One can complicate the asteroid collision with a war, as science fiction writers have done.
Suppose we just now got 500 years warning of a strike by a Chicxulub class asteroid.
Q. What should we do *now* about deflecting it?
A. Nothing.Our descendants 200 years from now will know enough more about how to deflect it because of the general advance of science and technology. They'll still have 300 years. I'm optimistic about the capabilities of our descendants.
The asteroids whose orbits cross earth's orbit are being observed and their orbits calculated, but the astronomers haven't got all of those that are small but still large enough to do damage. At least twice an asteroid has been observed for which the initial orbit calculations showed it might come dangerously close to the earth in the next hundred years. In each case further observation showed that the asteroid wouldn't come close.
According to an astronomer I asked, about 20 percent of the asteroids that cross the earth's orbit come from so far out that we can't expect to know their orbits in advance. If one of them was going to hit the earth, the amount of warning would depend on how big it was and on luck. This would justify making asteroid deflection experiments in advance. Depending on the orbit and the amount of warning, nuclear explosions might be the only way of deflecting it. The effects of nuclear explosions at different distances on various kinds of asteroid is sufficiently uncertain to justify advance experiments. At present these experiments are prevented by superstitions about nuclear explosions. One or two generations from now, people, including the media and do-good scientists, may be more sensible.
Here's a web page on why experiments with nuclear explosions should not be forbidden. The US hasn't ratified the non-proliferation treaty that forbids them, and I hope it won't, but it is still not planning any explosions. The opponents of the treaty today are mainly concerned with US national defense. My reasons are different.
Ice ages have come at intervals of 15,000 years or so and have had profound effects on life on the planet, presumably including human life. The cause is apparently changes in the inclination and eccentricity of the earth's orbit in turn caused by the gravitational effects of other planets, mainly Jupiter. How soon we are due for another ice age, if events are allowed to take their course is uncertain. There are feedback effects in the climate system as well as the primary astronomical effects. The next hundred years seems to be regarded as extremely unlikely, the next thousand years quite possible, and the next ten thousand years rather certain.
Some years ago there was a scientific conference entitled "When will the present inter-glacial age come to an end". The conference received world-wide press coverage. None of the papers mentioned by the press discussed preventing the next ice age or mitigating its effects. This inhibition about discussing prevention among many scientists is an ideological phenomenon.
My opinion is that humanity will prevent the next ice age. There are a number of possibilities including changing the earth's albedo in many ways. The apparent fact that small quantities of certain substances in the earth's upper atmosphere can affect the heat balance provides helpful tools. Studying these possibilities is of scientific and technological interest, but it seems that humanity will have plenty of time to get ready. Therefore, I advocate complacency for now about the ice age danger. Gregory Benford in his Climate controls discusses many ways of preventing the next ice age.
It might have been much worse than depicted in those scenarios. Even after the enormous destruction, the combatants might have continued to fight until one was conquered by a surviving underground fragment of the other.
The most likely nuclear wars in the present political configuration of the world are minor in comparison.
However, human society has not advanced to the point where major wars are impossible in the long term. I don't see any of the present conflicts in the world as likely to lead to major war, but I have no confidence in my or anyone else's ability to predict even the next 100 years. In the long run the likelihood of war (or any other catastrophe) wiping out the human race will be reduced by the dispersal of humanity throughout the galaxy.
Wars have been very bad, even of demographic significance. The 30 years catholic-protestant war of the 17th century killed 1/3 of the population of Germany. The holocaust killed 2/3 of the Jews of Europe (1/3 of the Jews of the world.) Whole generations of young men have been mostly wiped out by wars.
[2002 March 23: The above was written before the 2001 September 11 terrorist attack. Neither the WTC attack, nor the anthrax attack, nor the subsequent incrased speculation about terrorist attacks require altering the previous paragraph.] An interesting comparison is with the 1998 ice storm in eastern Canada and New England. Three million power poles were downed and required hundreds of utility crews from as far away has Hawaii to repair the damage. There is no way terrorists, who have blown up a few power poles occasionally, could do as much damage as that ice storm did.
Consider the terrorism againt Israel. It kills far fewer Israelis than automobile accidents but gets much more attention. Here are two reasons. (1) An appropriate response is to catch and disable the terrorists, and that arouses combative instincts in people and is much more interesting than (say) installing more traffic lights. (2) Those injured by human crime always get more attention and sympathy than those suffering equally from accident or disease. Perhaps this remark is a distraction from the main thread about menaces of various magnitudes.
Until 1999 I was rather neutral about the danger of global warming. Recently I have moved towards the skeptical side, seeing as so many scientists have been refusing to be stampeded by the politicians. Fred Singer has been a long time skeptic, and here is his tally of some fellow skeptics. He's not alone.
As I have read more about climate change, I have become convinced that I cannot learn enough about the subject to be entitled to an opinion on my own.
If necessary, global warning can be avoided.
2003 March note: Science for 2003 March 28 has a review article entitled "Abrupt Climate Change" by R. B. Alley, et. al. The article points out that abrupt climate change has occurred many times in the past and is doubtful that present science can readily anticipate such events. The article says, "In the light of these uncertainties, policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptablilty and resilience of ecosystems and economies." This article represents a considerable advance in the thinking of climatologists away from single menace theories associated with human sins.
I only just noticed that this excellent article lacks any discussion of how humanity might prevent undesired climate changes by global engineering.
Should a super-infectious mortal disease develop without the delayed onset of AIDS, we could use, at immense cost, a technology of isolation and quarantine - an extension of the P4 technology on a global scale. It would involve greatly restricting travel and transportation of goods.
At a lower level come the epidemics in human history. The black plague is said to have killed 1/3 of the population of Europe in the 14th century. The worst recent epidemic was the influenza epidemic of 1918-20, which killed 20 million worldwide, including 600 thousand Americans. European diseases are said to have killed a large fraction of the original inhabitants of the Americas after first contact. Probably either renewed plague or a repeat of the 1918 influenza would have a much lower death rate, just because of improvements in nursing.
At present the U.S. Government Centers for Disease Control with headquarters in Atlanta is pursuing some rather small scale threats from newly recognized infectious diseases. For example, there is a hantavirus spread by rodents. There have been 160 cases of which half died of pneumonia. The death rate among recent cases is about 20 percent. The major outbreaks were in the Four Corners region of New Mexico and in the Sierras in California. Another small outbreak is occurring in 1998, because the increased rains caused by El Nino increased the population of the mice that carry the disease. The point is that CDC is reduced to pursuing small threats. It's good that pursuing the small threats will keep them in practice for when bigger threats arise.
Present civilization is much more capable of surviving catastrophes than any past civilization, because every part of the system has so much reserve capacity. Here are some of the reserves.
There are also social menaces that have been most imaginatively portrayed in science fiction.
There are also imaginary menaces.Up to: Sustainability FAQ
Send comments to mccarthy at stanford.edu. I sometimes make changes suggested in them. - John McCarthyHits since 1995 December 15.