Ironically, you can find alarmists complaining over population will doom civilization all through history.
There is a danger if the power structure prevents change necessary to adapt.
Frankly, humanity has a margin of 100 to 150 years, based on current doubling rates and implementation of Environmental amplification - thickening the life bearing volume of the finite Earth. Environmental preservation is suicidal and genocidal.
Humanity needs to harness technology and increase the habitat for man as well as wildlife.
In parallel with that, humanity needs to colonize outer space, via giant vivariums / orbital habitats. Use robotic factories, seeded upon any Earth passing asteroid, and let them geometrically construct hulls, machinery, etc. Then it is only a matter of collecting the hulls and commissioning them with raw materials and crew. All known elements are already in space. The only thing lacking is life - which we will bring.
Conservative estimates that there is enough material and power for trillions of colonies, in all manner of orbits - around the sun, planets, and moons.
Just think - if population doubling rate remains constant at 50 years, humanity may pass 6.520 E+21 in 4000 A.D.
That's 6.52 Sextillion people. Woo-hoo.
The BIG BANGING Theory
YEAR . . . Population. . Doubling rate (years)
2014. . . . 7.200E+09 . . . . 50 . . . . . . . 60 . . . . . . . . 70
Future . . . Yrs. Diff. . . . Population
2050 . . . . 36 . . . . . . . . 1.186E+10 . . 1.091E+10 . . 1.028E+10
2100 . . . . 86 . . . . . . . . 2.372E+10 . . 1.945E+10 . . 1.687E+10
2200 . . . . 186 . . . . . . . 9.488E+10 . . 6.173E+10 . . 4.542E+10
2300 . . . . 286 . . . . . . . 3.795E+11 . . 1.960E+11 . . 1.223E+11
2400 . . . . 386. . . . . . . 1.518E+12 . . . 6.222E+11 . . 3.291E+11
2500 . . . . 486 . . . . . . . 6.072E+12 . . 1.975E+12 . . 8.858E+11
3000 . . . . 986 . . . . . . . 6.218E+15 . . 6.372E+14 . . 1.252E+14
4000 . . . . 1986 . . . . . . 6.520E+21 . . 6.629E+19 . . 2.500E+18
5000 . . . . 2986 . . . . . . 6.837E+27 . . 6.896E+24 . . 4.994E+22
6000 . . . . 3986 . . . . . . 7.169E+33 . . 7.174E+29 . . 9.973E+26
Space Colonization Basics The key advantage of space settlements is the ability to build new land, rather than take it from someone else. This allows a huge expansion of humanity without war or destruction of Earth's biosphere. The asteroids alone provide enough material to make new orbital land hundreds of times greater than the surface of the Earth, divided into millions of colonies. This land can easily support trillions of people.
Of course, their estimate is wildly conservative and timid.
http://en.wikipedia.org/wiki/McKendree_cylinderA McKendree cylinder is a type of hypothetical rotating space habitat originally proposed at NASA's Turning Goals into Reality conference in 2000 by NASA engineer Tom McKendree.[1] As with other space habitat designs, the cylinder would spin to produce artificial gravity by way of centrifugal force. The design differs from the classical designs produced in the 1970s by Gerard K. O'Neill and NASA in that it would use carbon nanotubes instead of steel, allowing the habitat to be built much larger. In the original proposal, the habitat would consist of two cylinders approximately 460 km (290 mi) in radius and 4600 km (2900 mi) in length, containing 13 million square kilometers (5.1 million square miles) of living space,[1] nearly as much land area as that of Russia.
As originally proposed, the McKendree cylinder is simply a scaled-up version of the O'Neill cylinder. Like the O'Neill cylinder, McKendree proposed dedicating half of the surface of the colony to windows, allowing direct illumination of the interior.[1] The habitat would be composed of a pair of counter-rotating cylinders which would function like momentum wheels to control the habitat's orientation.[1]
McKendree Space Colony (290 mi x 2900 mi)
5,284,158.84 sq. mi. (outer most shell)
76560 decks (10 ft)
2 x 10,127,094,564.45 sq.mi.
= 20,254,189,128.9 sq.mi.
Capacity :1,012,709,456,445 (@50 per sq.mi.)
1.01 trillion people
For without space colonization there can be no world peace, and without world peace we cannot afford space colonization.
Once humanity is free from the limits of Earth, we have millennia of expansion in our own solar system.
In time, our descendants can colonize near by star systems, filling them, and expanding outward.