A short written history makes us complacent
According to scientists, the earth is about 4.5 billion years old, while scholars believe that written history has been in existence for a mere 5 thousand years. What this means is that we have an extremely small period of time to use as a reference point for what lies ahead as we sit comfortably in on our home here on planet Earth.
What we don't know about earthquakes, volcanoes, and incoming earth objects from written history we've learned from evidence recorded in the earth's crust. And that evidence is foreboding.
Man-made concerns from global warming to nuclear accidents, war or terrorism, and diseases, are other things that cause us concern.
Many of the phenomena outlined below occur relatively regularly and are more expected as natural when they do occur. Other events occur in 500 or 1,000 year increments or much more, and a a result, we tend to not fully acknowledge the fact that such events will surely occur again, whether in our lifetime or not, no one knows for sure.
Common weather-related threats
Living on this big world of ours we are very familiar with common threats such as weather. Almost daily there is at least a minor threat from extreme weather such as extreme heat or cold, or extreme drought or precipitation somewhere on our planet. And extreme winds and floods, often associated with hurricanes, typhoons, blizzards, cyclones, and tornadoes, occur many times each year.
Short cycle earthquakes and volcano threats - less common but not too far from our minds
Then we have less common earthquakes and volcanoes that occur on cycles of 10s to 100s of years. These relatively short cycles (relative to 500 to 1000 year + cycles or much more) are historically recorded in most cases. The more recent written recordings and especially visual recordings, are concrete reminders to us all that the earth is restless, dynamic, and indeed threatening. Scenes of the great March 2011 Japan earthquake and tsunami or the Indian Ocean tsunami that occurred December 26th 2004 are stark and vivid reminders to us all.
Relatively recent and large volcanic eruptions, not of "super" class (though super is
not a scientific designation), include the Krakatoa eruption of 1883,
1815 Tambora, and 1991 Pinatubo. Some of these large eruptions have been
even linked to worldwide temperature declines* (Nasa).
Longer cycle earthquakes and volcanoes - potential for global impact
The earth's history includes evidence of many large earthquakes and especially volcanoes that if occurred today could wipe out a large number of Earth's inhabitants. From the Yellowstone Super Volcano eruptions that occurred 2.1 million years ago, 1.3 million years ago, and 600,000 years ago* (USGS) to many others. Unlike the volcanoes that we are more accustomed to, these eruptions created calderas up to 53 miles wide. Contrast this with the Mount St. Helen's crater, 1.8m miles across at maximum N-S direction* (USGS) or Iceland's Eyjafjallajokull volcano with a crater of 1.9 to 2.5 miles in diameter. Again, these larger volcanic eruptions have occurred in the past and will occur again, it is just a matter of time.
Asteroids, Comets, or Meteorites of recent memory
The February 15, 2013 Chelyabinsk meteor that raced through the atmosphere and left a visual trail through Russia was another reminder that the earth is also threatened from space. This object was more of a surprise because it remained undetected until it entered the atmosphere with an explosion, bright flash, and accompanying shock wave. According to scientists, the meteor was initially 17 to 20 meters across and was the largest known object to enter our atmosphere since the 1908 Tungusta object that destroyed a large forest in Siberia. And while about 95% of objects at least 0.6 miles across that threaten the earth have been identified, far fewer, perhaps only 5% of the smaller, yet potentially destructive and earth intersecting objects have been identified. Scientists are now working to identify 90% Near Earth Objects (NEOs) larger than 140 meters* (NEO Program Site), but clearly, smaller objects could be very devastating.
Longer period extinction event objects
It is now widely accepted that 65 million years ago an object 3-6 miles wide struck the earth near the Yucatan Peninsula. The object (asteroid or meteorite) wiped out about 70% of all the species on earth, including the dinosaurs. The event is also referred to as the K-T event or Cretaceous-Tertiary Mass Extinction event, where we see an associated sudden change in the fossil record* (Nasa). Clearly, if such an object struck the earth today, it would be difficult for any of us to survive. And for this reason, many scientists around the world are working hard to identify and track the larger objects.
Global warming or cooling
The warming of the planet Earth and the consequences that include droughts, floods, more extreme weather, and coastal inundation and population displacement has occupied most of the conversation. What many have missed is the threat of global cooling and glacial periods. The last glacial period ended about 12,000 years ago and at it height, about 22,000 years ago, saw glaciers advancing as far south as Missouri and covering what is now New York city an estimated 100s of feet of ice if not more.
Disease outbreaks
There are also reminders that disease could threaten the human population of the earth. From H1N1, H7N9, H5N1, Coronavirus infections, their many mutations, and more. The 1918-1919 pandemic flu or Spanish flu is a not-to-distant reminder of the the consequences that a highly contagious and virulent flu can be devastating. The 1918-1919 flu killed 50 million worldwide and nearly 675,000 in the US. About 20-40% of the entire population became ill* (Flu.gov Pandemic Flu History). Adjusting for the population of today we might see in the neighborhood of 300 million deaths worldwide and many millions of deaths in the United States. And while medical advances will be of great help to us if such a flu threatens in these days, the more rapid spread due to far advanced transportation will mean that the disease will spread much faster, potentially infecting many more people, before it is noticed.
Nuclear
It was not so long ago that the threat of a global nuclear war was not far from our minds. After the Cold War and the understanding of the concept of mutual destruction (you launch on me than I launch on you) the threat may not be as present in our minds, but with the increasing number of countries poised to obtain nuclear weapons and the fragile stability of nuclear nations we might not get too comfortable.
The Sun and Coronal Mass Ejections (CMEs)
We all enjoy basking in the warmth of our nearest star, the Sun. The Sun's size truly dwarfs the size of the Earth. 1.3 million planet Earth's could fit inside the Sun* (Space). And though the Sun might seem relatively harmless, Coronal Mass Ejections (CMEs) are a real threat, especially during the 11-year cycle when the number of sunspots is at its greatest. Sunspots at times will give birth to CMEs, and if the CME is large enough and directed towards Earth, consequences could be disastrous, damaging electronic equipment including power transformers.
On September 1st 1859 the largest geomagnetic storm ever recorded, now called the Carrington Event, hit the earth with such a magnitude that Auroras were seen in tropical latitudes and telegraph operators were shocked by their instruments. Many believe that if something similar to the Carrington Event occurred today, we could suffer serious damage to our electrical grids and plunge us into many years of chaos and widespread death due to the lack of electricity to run all manner of things that we take for granted and the form the backbone of what keeps most us so well-taken care of. Transportation, communication, heating, cooling, and many related things that run on electricity would be severely impacted. With such wide-spread damage, countries would be ill-prepared to bring the electrical grids back online for years. Banking and food transportation disruption are two things that would certainly cause chaos all by themselves. Some have said that a "Carrington Level Event" occurs every 500 years* (ABC), but we don't really know and such an event could happen any time.
Well there you have it, a large array of possibilities that could give you a bad day.
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