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Saturday April 21st 2018

Timescales of Human Interest

Canals in Amsterdam, Netherlands. March 2010.

In order to bound the problem of rising sea level, consider various timescales of the planet and life. Let’s start at the longest timescales and work to the shorter.

Timescales
Universe. The latest estimate for the age of the universe is 13.82 billion years. No one knows how long the universe will be around for. For this analysis, let’s assume that the universe is halfway through its life, and has another 14 or so billion years to go. The yardstick for the life of the universe is marked off in billions of years and the maximum is about 28 billion.

Species. Our species homo sapiens has been around for about 200,000 years. Other members of the human family have been around for as much as 1,000,000 years but have since gone extinct, some as recently as 40 to 70 thousand years ago. For most of our existence, modern humans were not the only human species, but shared the Earth with other humans species.

Eventually homo sapiens will also go extinct. When, is unknown. For this analysis, let’s assume that homo sapiens is about halfway through it’s time on Earth, so it will be around for another 200,000 years, for a total of 400,000 years. The timescale of our species is measured in 100,000’s of years. Maybe out to a maximum of 1 or 2 million years.

Human Lifespan. Human lifetime is measured in decades, with maximum lifetime of about 100 years. The average age of females at birth of their first child is 21 years and the last at 31 years, giving a mean of 25.5 year per generation. [2] Let’s call one human generation is about 25 years. That would be 16,000 generations of homo sapiens for a 400,000 year existence.

After many tens of thousands of years as nomadic hunter-gatherers, the development of agriculture led to human civilization. Agriculture in the form of wheat and barley, in the Fertile Crescent began about 9,000 B.C. Other crops were developed in other places, such as potatoes in South America (8,000 B.C.), rye in Europe (7,000 B.C.), corn in North America (2,700 B.C.), chickens in South Asia (6,000 B.C.).

Civilization. Scholars date the beginning of civilization to a time after the development of agriculture, about 3,200 B.C., some 5,000 years ago. For this analysis, lets say that the current civilization will last for another 5,000 years before some major calamity puts an end to it. The time scale of human civilization can be measured in thousands of years, up to about 10,000 years.

Humans occupy regions of the Earth and organize themselves into hierarchical communities in habitable regions, e.g. family, tribe, clan, race, city-state, kingdom, country, empire. Kingdoms and counties exercise control over some bounded area with populations concentrated in cities which often lie on key transportation routes, such as rivers. Empires are groups of states, kingdoms or countries with a single supreme authority.

Empires. There have been many empires throughout the history of this civilization. Empires come and go. The first ancient Egyptian empire lasted for roughly 550 years. The second ancient Egyptian empire lasted for roughly 400 years. The Roman empire lasted for 507 years from 31 B.C. until 476 A.D. We could go on and on listing empires. Several ancient empire lasted for just over 1,000 years, but the average length of time of an Empire has been about 350 years [3] Let’s say that the lifetime of Empires is measured in centuries, with the maximum on the order of 1,000 years.

Cities. Next we look at cities. Cities are places where human population has concentrated. They are the center for trade, political power, society, and manufacturing. They often form at strategic sites such as rivers, natural harbors, mountain passes, key transportation breakpoints, etc. Some cities, such as Athens and Damascus have been continuously occupied sites for as much as 10,000 years. Cities, while the center of an Empire, often outlast the old Empire and become the center for the subsequent Empire. The lifetime of cities can be measured in 1,000’s of years, with a maximum of about 20,000 years.

Improvements. Cities are made up of human-built structures–buildings, roads, bridges, canals, quays, piers, jetties breakwaters, aqueducts, sewerage, power lines, communication lines, radio towers, etc.  The structures, called improvements, are built on some plot of land out of building materials that will deteriorate and decay. The improvements thus have a limited Useful Life, which depends on the materials and workmanship. Structures built out of stone have a much greater useful life than structures made out of wood.

A few buildings, often Cathedrals,  have been in use for well over 1,000 years. e.g. Hagia Sophia in Constantinople.  But due to obsolescence and deterioration, most improvements have a much shorter useful life. Technology advancements make older buildings obsolete in terms of structural integrity, electrical, plumbing, and communication features. It is often more cost-effective to tear down older structures and build new ones in accordance with the latest engineering standards.  In the United States, structures are often obsolete after only 50 years. E.g. professional sports stadiums in many American cities are torn down after a few decades of use. E.g. Shea Stadium (1964-2009) and numerous others. [4]

Useful Life of Improvements. When a new improvement is proposed, investors must purchase the land and then build the structure. Investors make an assumption of the useful life of the improvement, for example 25 or 50 years. If the structure can be expected to last that long, the investment is made. Useful Life varies with the kind of structure from a few decades for some dwelling units to well over a hundred years for some infrastructure. Let’s assume that the average useful life for improvements is 100 years.

The timescales of interest are summarized in Table 1.

Table 1. Timescales of Human Existence
Item Timescale
(years)
Age
(years)
Maximum
(years)
Universe 109 13.82 x 109 28 x 109
Milky Way Galaxy 109 13.21 x 109 26 x 109
Solar System 109 5 x 109 10 x 109
Planet Earth 109 4.543 x 109 10 x 109
Mammals 106 65 x 106 130 x 106
Primates 106 50 x 106 100 x 106
Human Family 100,000 1,000,000 2,000,000
Homo Sapiens 10,000 200,000 400,000
Human 10 71 100
Generation 21 25.5 31
Civilization 1,000 10,000 20,000
Empire 100 350 1,000
City 100 500 5,000
Useful Life of Improvements 10 100 500

Sea-Level and Tides

For improvements near the shore, variation in Mean Sea Level is of interest. The sea level rises and falls with tidal forces from the Moon and Earth’s daily rotation. In most places, there are two high tides and two low tides each day, at about 6 hour intervals. The size of tides varies widely with location, phase of the Moon, and weather. Greater tides usually occur during New Moon and Full Moon phases and are called Spring tides. Lesser tides, called Neap Tides, occur on Quarter Moon phases. In some locations, tidal range is a couple of feet, or even just a few centimeters. In other places, total ranges is several meters. The greatest tidal range on Earth is at the Bay of Fundy in Canada with high-to-low range of 16 meters!

Structures built at the shore have to account for the maximum high tide and flooding that can be expected over the useful life of the structure. Structures are often built to withstand the 100 year maximum flood. This is the greatest tide that would occur once per hundred years, on average.

NEXT: We consider Variations in Global Sea Level

———————————————-
References:

[2] How long is a generation? By Donn Devine, CG, FNGS

[3] How Long Did the Empires of Ancient Civilizations Last?

[4] Rest In Pieces: 50 Demolished Sports Stadiums We Love

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