His solution: let the state decide the level of consumption by using some 'scientific way' to allocate resources. That way, every body will only consume according to his/her need. When I pointed out that that exactly command economy is about, he said, "well, may be, but in the past, they didn't use science to allocate the resources"...
I kept asking him how do we know that we already pass the threshold, so he gave examples of 'catastrophic' result when someone introduced a new species to the sea. But, he conceded that we don't really know how far men contribute to many natural phenomena today. He also conceded to my argument, however, that 'carrying capacity' assumes some constant use of factor production(s) and technological change. In other words, 'carrying capacity' is dynamic. And, if that is the case, then his argument of passing the threshold become somewhat moot.
During the debate, we kept getting to the point that we both agree that we don't know much about the impact of environmental change, and yet, I pointed out to him, he kept choosing the negative scenarios.