Events that happen only on geological timescales
In geological history there have been many extinction events, and in each case the majority of species have been annihilated, but so far there have always been a few life forms that have survived. Genetic diversity is impressive and highly varied. This is great news, as it allows the survivors to evolve to fill the ecological niches that followed the extinction event. The most familiar and frequently cited example is the meteor impact in the Yucatan peninsula. It has gained a widespread press because some 65 million years ago it probably caused, or contributed to, the demise of the dinosaurs (and nearly everything else). A word of caution is that in that same era there was ongoing intense volcanic activity in the Indian subcontinent, which was also a major contributor to a mass extinction. The physical size of the dinosaurs was impressive, and we have bones and skeletons that have captured our imagination. Especially for children, the dinosaurs fit the image of fairy-tale dragons, and so are really exciting. However, their disappearance was not the first massive extinction event, and certainly will not be the last.
Meteor impacts have happened regularly throughout the history of the earth; so far at least ten major craters have been found from meteors and other bodies crashing into our surface (others craters may have been covered over or vanished). In terms of diameter, the top ten largest ones we know about have diameters of at least 200 kilometres (—120 miles) and happened on timescales from 2 billion to 35 million years ago. Many others look less impressive but include far more recent events. In fact, there is absolutely no reason we may not have large-scale events in the future. It is just that the frequency is so low that we assume it will never happen in our lifetime. Our catalogue of the top ten major impacts will have missed out many equally large collisions with meteors that crashed into the ocean. Earth’s surface is roughly three quarters covered with water so, at a stroke, I am claiming the number of big hits should be multiplied by 4. To me it seems odd that others do not seem to emphasize this. Perhaps the blast of water into the atmosphere, rather than rock and debris, has less damaging effects, although there would be immense tsunami waves hitting coastal regions.
In all cases, land or sea, the problem with a meteor that happens to bump into us is that it need not be very large, but because the impact speeds will be immense, so will the kinetic energy. So the famous Yucatan impact has a crater area of around 5,400 square kilometres. For me, this is meaningless, as I have no grasp of the scale, but in recognizable terms this is around the area of Wales (in Europe) or New Hampshire or New Jersey (in the USA). Now I am impressed.
Many other large meteor impacts have taken place, and small meteors are quite common. Indeed, they may even have been the source of water on the planet. The current problem, even for a medium-size impact, is not from the damage occurring at the initial collision site, but from the debris thrown into the upper atmosphere. This ejected material will circulate the world for some time (months or years), as once it reaches the stratosphere it is not washed down by clouds and rain. The obvious effect is to block sunlight, and so cause cooling of the surface, and this in turn means crop failures that destroy harvests and agriculture for many years. This is exactly the same problem that arose from major volcanic eruptions that blocked sunlight, lowered the temperature, and caused famine. One such example was the Icelandic eruptions in the 1780s that contributed to European crop failures for several years. Additional problems at that time were caused by shifts in the Pacific Ocean current, the El Nino, which brought drought to South America and a European summer drought in 1787, followed by excessive spring rains. These all contributed to the food shortages that probably catalyzed the French Revolution.
Climate-driven droughts—either from meteors, volcanoes, or changes in the ocean currents—have all resulted in famines, leading to loss of several civilizations or empires, and have been the underlying causes of revolutions. Examples range from droughts that are thought to have destroyed both the Aztecs and Incas in the Americas, led to the collapse of empires in Cambodia, and caused desertification of the Sahara, plus contributing to social collapse in many other regions.
With our twenty-first century fixation on electronics, and both satellite- and ground-based communications, the stratospheric debris will be considerably more devastating. It will obliterate much or all of our electronic communication systems. So, in this sense, as with the solar flare scenario, we will be in a far weaker position to survive in our present lifestyle than our ancestors. Yet again I am not predicting some dramatic doomsday scenario—only that without preparation and forethought, we are exposing ourselves to major communication and information loss, and severe consequences for our continued existence. Our hope is that survivors may have knowledge from simpler technologies that they could exploit. This is currently feasible, as we have written records but, as we move to totally electronic storage and dependence on other high-technology gadgets, then even this opportunity will be lost.
A very positive view is that in the past humans have not been immune to global catastrophes, and it is thought that early hominid populations plummeted on more than one occasion as a result of natural disasters; clearly they recovered and expanded. So my optimistic guess is that we would recover, even if at a very different level of civilization. We may be physically weak, but we are a resilient species, and at least we do not have to contend with dinosaurs who might wish to eat us.