Published: September 19, 2008 12:03 am      

How many humans can live on Earth?

Are we close to the limit?

A key estimate is to find the carrying capacity of the Earth for humans. During most of history (began 5,000 years when writing developed in Middle East), the human population has been in the millions, usually increasing by a few per cent per century. In the year 1 CE or 1 AD, there were about 300 million people. (This is about the current population of the United States.)

It took more than a thousand years for the population to grow to 400 million (1250 CE). At the beginning of our modern era in 1500, there were 500 million humans. About 1810, human numbers had reached a billion (1,000 million).

By 1927, world population reached 2 billion. In 1974 (47 years later), there were 4 billion people. In 1999, the world’s population passed 6 billion. The current estimate for our human population is 6.7 billion people.

If there are no major agricultural collapses or global plagues or WWIII, it seems likely that there will be 8 billion people on this planet a few decades from now. The human population is doubling about every 40 years. Can this growth continue at the current rate? How many people can our world sustain?

This question lends itself well to a back of the envelope estimation. If we assume that everyone subsists on 2,000 plant kilocalories a day and current fields are utilized, we can get an idea of how many people can be fed — this will be the most humans the Earth can support on a sustainable basis.

Two thousand kilocalories or Diet Calories would be a major cut back in total day calories for most Americans. Other kinds of food (ex. meat) rely on much more land or expense per kilocalorie. The global fish harvests are in decline with more and more fish being grown on fish farms, where the fish there are fed with small fish from the ocean.

Consider the current amount of land under cultivation, about 2 billion hectares (a hectare is a square 100 meters on a side or 10,000 meters). During the course of a year, a hectare devoted to organic grain and vegetables, harvested by hand typically yields 6.3 million kilojoules or 1.5 million kilocalories.

So the yield from all the fields now under cultivation would be 3 million billion kilocalories. If each person’s food were to average 2,000 kilocalories per day or 0.73 million kilocalories per year, then current fields could support a population of 4.110 billion people, 2.5 billion people less than our current population of 6.7 billion humans.

The reason why we have more people than this limit is the use of nitrogen-rich fertilizers on fields to substantially increase their crop yields (the “Green” revolution). But these fertilizers rely on fossil fuels for their manufacture.

Once the fossil fuels become too expensive, scarce or not available, then nitrogen rich fertilizers will no longer be available; crop yields will drop down to the level of 4 billion people and mass starvation will take the lives of hundreds of millions of people. So our rapid draining of fossil fuels (petroleum and natural gas used in farm machinery, food transport and fertilizer production) is setting humanity up for a great die-off, which will rival the Black Plague in Medieval times.

Fall and two dim planets

Tomorrow is the first day of fall, with the sun shining equally on both northern and southern hemispheres. Were it not the for the refraction or bending upward of sunlight, we would have 12 hours of sunlight and a like period of darkness (absence of sun) tomorrow.

But the sun’s light is bent upward at sunrise and at sunset, so there are about eight minutes more of sunlight than darkness. These numbers are for a flat horizon facing both East and West (which few readers will have).

Technically, on Thursday, there is an even split between daylight and darkness. By the then the sun will have moved farther south, causing the sunrise time to advance four minutes and the sunset time to fall back four minutes.

Now that the moon is in the early morning sky, the evening stars can be seen more easily. This also gives us an opportunity to spot Uranus and Neptune with binoculars in the evening sky. The planet Neptune is higher and best seen as it gets dark. To the left of Jupiter is Capricornus, the Seagoat.

(A seagoat is a mythical creature with a front body of a goat and the rear like a fish. It is the disguise of Aphrodite (Venus) as she fled the monster Typhon.) Capricornus resembles an upside down cocked hat (like Napoleon wore).

On the east tip of Capricornus is Delta. To Delta’s right is Gamma. Using binoculars, go above Delta and Gamma to find three dimmer stars in a (diagonal) row. Neptune is a dim point of light to the right of the top of the three star row. Neptune’s distance is about as far from the three-star row as Delta and Gamma are apart. A powerful telescope will reveal a tiny disk about 2 arc seconds across (at a distance of 3 billion miles).

Uranus is in the star group Aquarius, to the left of Capricornus. You will need to wait till 10 p.m. before Aquarius is high enough above the horizon to be viewed easily. Aquarius’ most conspicuous feature is a small Y formed by four stars. To the left and below the Y is Aquarius’ water jug. The jug is tilted so water is spilled to the left.

Just above the top of the jug and to the left is Uranus, just barely visible to the eye and well seen in binoculars. Once again, a powerful telescope is needed to see Uranus’ disk.

Continuing at Frostburg State Planetarium is “Best Stars Through the Seasons” with free programs on Sundays at 4 p.m. and 7 p.m. Our public 45 minute programs are in Tawes 302, just off the front entrance of the building, which faces away from the Lane Center and towards the Compton Science Center.

There is convenient parking nearby, including even close handicapped parking with a nearby ramp. Following our planetarium program, visitors are invited to tour the Science Discovery Center (also free) in Compton across the street. In this facility is a great display of preserved animals from five continents.

Readers are invited to email at rdoyle@frostburg.edu.