• World Over-Population:  Already, there are many areas around the planet that can not sustain the human populations, resulting in starvation, poverty, disease, crime, war, economic instability, pain and misery.  The world population is currently growing at an amazing 211,000 people per day (as of year 2006; that's 77 million per year; or about 1 billion per 13 years; that's all births less deaths per day)!

• Arable Land and Population:  Here's something to think about (with regard to population and arable land) and the impact on the environment.  
  
  The Earth's Diameter is 7926.41 miles.
  
  The surface area of Earth is  (pi  x  Diamater²) = 197.38 million square miles (where pi = 3.14159265358979).
  
  However, the Earth only has 57 million square miles of land (that's 36.48 billion acres; there are 640 acres per square mile).
  However, there are only 12 million square miles (7.68 billion acres) of arable land.

The world population in year 1 A.D. was 250 million people.

The world population in year 1492 was 500 million people.

The world population in 1804 was 1.0 billion people.

The world population in 1922 was 2.0 billion people (doubled in 118 years; increasing on average by about 23,000 per day).

The world population in 1959 was 3.0 billion people (increased by 1.0 billion in only 37 years; increasing on average by about 74,000 per day).

The world population in 2006 was 6.68 billion people (more than doubled in 47 years; increasing now by 211,000 persons per day!).

The world population by 2039 could be 8.0 to 13 billion.

In 1959, there were 12.16 acres per person, world-wide (i.e. 36.48 billion acres / 3 billion people).
In 2006, there were 5.46 acres per person, world-wide (i.e. 36.48 billion acres / 6.68 billion people).
By 2039, there may be only 2.81 acres per person, world-wide (i.e. 36.48 billion acres / 13 billion people).

The U.S. has 3.794 million square miles, of which 3.54 million square miles is land area (for a fast growing U.S. population of 300 million people as of the end of year 2006).
That is only 8.09 acres per person in the U.S.
However, only about a quarter of that is arable land.
That means there are only about 2.02 acres per person of arable land in the U.S.

However, consider that there is only 12 million square miles (7.68 billion acres) of arable land on the planet.
And, ignore for a moment that arable land is being lost at a rate of 38,610 square miles per year.
That is, lets assume no arable land is being lost for the next 33 years.  Then . . .

In 2006, there was 1.15 acres of arable land per person, world-wide (i.e. 7.68 billion acres / 6.68 billion people).
By 2039, there may be only 0.59 acres of arable land per person, world-wide (i.e. 7.68 billion acres / 13 billion people).
 
However, arable land is being lost at the alarming rate of over 38,610 square miles (24.7 million acres) per year.
Therefore, by  2039, there may be only 0.53 acres of arable land per person, world-wide (i.e. 6.865 billion acres / 13 billion people).
At the current rate of loss of 38,610 square miles per year of arable land, and even if the population didn't grow any larger, ALL arable land could be lost in only 310 years (12 million square miles / 38,610 square miles per year)!
  
Now consider the following:  the sea-level is rising due to rising temperatures and melting ice. 
The island nation of Tuvalu is slowly being submerged. The people of Tavalu have requested permission to move to immigrate to New Zealand.
The islands in the Chesapeake Bay are being submerged.
Bangladesh's lowlands farmlands are being submerged and saltwater is seeping into fresh water systems.
NOTE:  For about every foot rise in the sea-level, the inland flooding is about 100 feet.  Many people live along coastlines all around the world.

Starting to get the picture ?

• Over-Fishing of the Oceans:  We get our food from the land and from the oceans.
  The oceans are already severely over-fished.
  The planet Earth has limits.
  Anyone who thinks over-population isn't a problem needs to explain away the obvious implications of the numbers above.

• Global Warming:  Pollution and damage to the atmosphere, land, and water is a growing problem.  Billions of tons of green-house gases are being released into the atmosphere per year.  Depletion of the ozone is destroying our protection against harmful radiation and global warming.  The current scientific consensus is that most global warming since 1950 is most likely attributable to human activities. The extent of this consensus was the subject of a study (published in December 2004 in the journal Science) of  928 refereed abstract scientific articles in the Institute for Scientific Information citation database identified with the keywords "global climate change".  This study concluded that 75% of the 928 articles either explicitly or implicitly accepted the consensus view.
  
  

• Carbon Dioxide Emissions