I'll be writing quite about about population because I think it is the most important environmental issue facing the United States of America and the world.
There are no long-term technology fixes with the world's population as high as it is and continuing to grow. I believe environmentally friendly technologies should be pursued vigorously, but I believe they cannot be sufficient, in themselves, to prevent disaster in the long run. You see this most glaringly in the current debate as to whether nuclear power is one of the proper responses to global warming. The underlying assumption is that the world's population cannot be levelled off or decrease: it must grow. With that assumption carbon dioxide levels will keep growing no matter how much we switch from coal to nuclear or solar. And nuclear is an extremely dangerous way to go.
Population figures having a margin of error, and historic ones being largely guesswork, I'm going to use the ones from Wikipedia. Pre-civilization estimates of total global human population tend to 5 million people. In the era of the Roman empire it had increased to 300 million, but then for centuries it grew relatively slowly. Around 1800, when the industrial revolution was well underway, world population hit 1 billion. In 1900 it was 1.65 billion, but by 2000 it hit 6 billion. Estimates are that there are now 6.5 billion people in the world.
While there are still a few hunter-gatherers in the world, and billions of farmers with low levels of consumption, there are also billions of people who consumer far more than all but the wealthiest people did just 2 centuries ago. No 18th century English knight consumed as much energy as a two-car middle-middle class American family.
Damage has accumulated over the centuries. Forests have dissappeared, water tables have dropped, top soil eroded away, and species have gone extinct.
I think that even with good technology, the long-term carrying capacity for humans on earth, assuming a healthy environment for them to live on, is probably in the vicinity of 1 billion people. I admit that is just my guess.
So how do we get from here to there? There is the easy way and the hard way.
The easy way is to think short-term, do nothing about reducing birth rates globally, and wait for nature to take its course. Some day the human population will crash because of war, famine, or disease. Most likely all three at once. There have been regional and short term population declines before; the magnitude in the future will simply become greater.
The hard way is to make reasonable policies that can be applied on a global scale. The simple benchmark would be: one child okay, two children maximum per couple. If that simple policy were adopted human population might level off and start to gently decline before the world is totally wrecked.
It will be hard to get consensus on this because lunatics rule. Many religious leaders are against small families. So are militarists, industrialists, and even the real estate industry. Even leftists and environmentalists in the US have been cowed: they are afraid to call for what is necessary because they are afraid they will be called racists. Well, okay, we know from experience calling for population control gets you called a racist in this country.
While I will argue for a reasonable population policy, time is slipping away. The more likely future is continued explosive population growth followed by a great crash, with much of nature extinct before the crash occurs.