Jonathan Porritt talks about 10% sense and 90% BS. His head is so far up his backside that he's lost touch with reality - he lives in some kind of alternative hypothetical world.
Whilst the world taken as a whole does have a problem with population growth, here in the UK our population growth rate is currently 0.276%. Most of that very small increase is coming from immigration - 2.17 migrants per 1000 of population, or 0.217% increase. Our net gain is therefore another country's net loss, so not affecting overall world population numbers.
As others have said, it's the ageing population that's the problem here. We currently have 16.9% of our population aged under 15, a number which is dropping every year, and 16% aged over 65 - a number which is increasing every year. Our birth rate has fallen to 10.65 /1000 whilst our death rate remains fairly steady at 10.05 /1000. What this means is that the natural increase in population IS ALREADY SLOWING in the UK. It also means that in 20 or so years' time, we are going to have an even bigger economic crisis than we already have as more of the population is going to be retired and non-earning; probably also ageing, sickening, and requiring more resources (heat/food/light/drugs, etc) whilst there will be fewer people to produce and manage these resources.
To be completely blunt, these statistics mean that the population 'growth' is not due to the numbers being born in the UK, but to the numbers not dying at the other end of their lives. I'm just waiting till Porritt follows through with his eugenics programme and suggests euthanasia for all on their 70th birthdays, on the grounds they're a total waste of resources.
The other major flaw with his argument is that the UK has one of the lowest population growth rates in the world (as above). However, it is one of the highest CO2 producers. Many developing countries have much higher population growth rates and significantly lower records of pollution. This suggest that it is not the number of people in a country, but what those people are doing that is the problem. Basically, he's barking up completely the wrong tree. Most families with three children do not use up more resources than those with two. Most families with three children find that expense is more of an issue. If you have to pay for five return plane tickets rather than four, you are more likely to decide to take the ferry and go camping in France instead of the plane to the USA.... A family of five is in fact likely to travel fewer air miles each year (and waste fewer resources) than a young professional couple with no children who take a summer holiday, a winter skiing holiday and a couple of European city breaks each year. It's all about lifestyle, not numbers.
Personally, I think Porritt gives environmental campaigners a bad name. He seems to have a tendency to pick on the wrong targets, to misunderstand the issues, and to try to enforce impractical changes that would have very little impact other than causing resentment and antipathy towards environmental causes, instead of tackling the major causes of environmental change. I wonder how much environmental damage the war in Iraq has had, with planeloads of troops being sent out and flown back, arms manufacturing increasing, CO2 and other pollution from explosions, the waste of resources in the country, the need for massive rebuilding efforts, etc? But according to Porritt's logic, it's probably a good thing since at least it's controlloing population growth. The guy has totally the wrong end of the stick.