This is a great topic, and one could discuss these issues forever. The answers are not always so cut and dried, and sometimes contradictory though. A few examples:
Much is made of the problem of CO2 buildup and global warming, yet some estimates show there has been a 10-15% increase in crop yields over the last 50 years due to increased CO2 in the atmosphere.
Decreasing or even just sustaining world populations puts unsustainable pressure on social programs as the population ages and there are fewer young people working to sustain these programs. We need to keep making children with slightly increasing populations at least, or we will fade away
Developing countries such as China and India are growing their energy usage at a rate that far outranks the US or Europe, yet how can one expect them to cut back when huge sections of there populations do not even have basic services such as electricity, clean water etc., yet.
I for one believe that only science and technology can save us; increasing food yields, finding alternative energy sources, sustainable sources of all foods including fish (probably through farming, done properly) etc.
A pet peeve of mine is much of the junk science out there such as the attitude to genetically altered foods such as wheat - despite the fact that there is no evidence that these foods have any ill effects on those who eat them or on the environment, junk scientists however have been able to get these foods banned even from countries where significant parts of the population are at below subsistance level. We have been genetically altering foods since the science of genetics started with Mendel - just splicing one plant to another is genetic manipulation.
Yes I know, I have wandered off topic, but in the long run using in my very, very humble and insignificant opinion, snart use of science and technology is going to be far more effective than trying to convince people to stop eating certain things, stop having children, don't fly to the Pacific on a dive trip, etc.