What the Malthusian doomers fail to point out is that our technology is constantly evolving. If our technology, industry and "ecological footprint" was static and never changing, then I would be concerned. However, this is not the case, as those things are dynamic and ever-changing. Just in the last 30 years, computers and household appliances have grown more powerful and more energy efficient. Recycling is much more widespread than it was 30 years ago. Perhaps more importantly, our industrial and manufacturing processes are many times cleaner and more efficient.
I live in Ohio, the heart of industrial production here in the states. I've worked at a number of large factories and several fortune 500 companies. I've been able to see how things are made and watch the entire process from start to finish. I've also seen the waste and pollution of these companies get cleaned up and drastically improved over the years. 30-40 years ago the rivers around here were so polluted due to industrial runoff that the bald eagles and great blue herons had all died out in this region. Today the rivers have been cleaned up enough for them to come back. I've seen a number of bald eagles and great blue herons, something my parent's generation had never seen before. Due to improved technology, smarter industrial practices and better environmental regulations the local ecosystem is recovering.
I'm more concerned about countries like China, where they lack the environmental controls we have here, and are now repeating some of the same mistakes we made in decades past. Of course, this doesn't mean the end of the world, merely that it will take some time and effort to clean it up.
Also, the 3 or 4 earths is highly suspect. According to the Kardashev scale
our entire civilization has not even used 1 earth worth of energy yet. And before somebody mentions finite resources, yes I understand there are limited resources here on earth. What I also understand is that humans have an amazing capacity to adapt, create new resources and evolve our civilization further.