From "Die for Gaia, save the planet?," by Tim Worstall,

"The enviroloonies seem to have found their way out of the asylum again: this time to tell us that 70 per cent of Britons should die for the sake of Gaia. That's not quite the way they put it, of course. Rather, the Optimum Population Trust (there's a pedantic part of me that wants to tell them it's Optimal) tells us that the maximum sustainable population of the UK is 17 million: given that there are north of 60 million currently, we can only avert the coming End Times if the extra pop their clogs soonest.

It's not bad for a paper on demography, economics, the environment and their interactions written by a physicist, that is, a paper written by someone with no knowledge of any of the three basic disciplines.

...Their sad misunderstanding about the effects of technology blinds them to the truth, that by not constraining technology we don't have to constrain either affluence or population. The late great Julian Simon once calculated that we had the resources for a permanently growing economy and population for the next 7 billion years. That might be a little Panglossian, to be honest, but it's more accurate than the insistence that there should be fewer, poorer people."

Hardcore environmentalists who think like this are dangerous people. One of these days, some nutball is going to get ahold of some deadly biological plague and deliberately release it on the world to kill off the human population so that they can...

A) Have a sustainable population
B) Bring the earth back to its "natural" state
C) Make the earth safe for squirrels and hoot owls

The reality is that without mankind, there is no point to the earth's continued existence and moreover, the past history of our species suggests that we can use technology to adapt to our circumstances.

That's not to say mankind can't go the way of the dinosaurs one day, but there's very little evidence I've seen that we're anywhere near the "maximum sustainable population" of the earth.