There is a lot of debate about the ecological problems we face and how to deal with them. Some proposed solutions, such as radical ante-natal policies (i.e. widespread abortion and contraception) would not be acceptable from an Orthodox Catholic perspective, and also radically lowering the birth-rate below replacement level, as many developed nations have found out, creates grave economic and social problems (rapid population ageing, shrinking workforce, requirement for very high levels of immigration, stagnating economies and innovation, problems with funding pensions, healthcare, education and other services reliant on taxes), so this is not a realistic or good solution to the problem of population.
I think an important factor in the ecological problems is not just the number of people, but the fact our modern economic way of life puts great strains on the planetās environment and resources. Despite much progress, the capitalist economy as it is now structured in its global form consumes vast amounts of natural resources and generates a lot of waste and pollution, especially as developing countries try to emulate the Western consumer lifestyle. Seeing the world as a limitless resource which can be used and exploited to no limit is certainly in my view just as responsible for the global environmental problems as much as overpopulation.
In the next century, better methods of using the planetās resources and living a life of reasonable quality will be needed. I donāt think retreating into apocalyptic fantasy about Christ magically coming to save us will help, instead something more like Hildegard of Bingenās vision will be required; respecting the creation as Godās beloved work of art, and doing our best to be responsible stewards of it and co-creators made in Godās image.