The world can support 1.5 to 2.0 billion people continuously, in combination with a natural world. Right now we have 6.2 billion people. The total mass of vertebrates on land and in the air is now made up 98% by humans+livestock+pets, and 2% by all natural creatures. Ten thousand years ago the 98% was only under 0.1%. Civilization's rocketing growth comes from exploiting non-renewables: coal since 1800 and oil since 1900, for example. US oil peaked about 15 years ago; global supplies should peak in about 10-15 years. There are semi-practical alternatives available or at least conceivable to let us get by on renewables, but virtually no one really sees the importance.
Virtually all your correspondents focus on details of how to make humans better and more numerous. Very few examine civilization's growth and the world as would a creature from space visiting us every few thousand years. Sincerely yours,
Paul B. MacCready
Chairman/Founder of AeroVironment Inc.
The "Father of Human-Powered Flight"