> i think we need to define limits in terms of energy

Ok. Let's do that. The earth receives about 10,000 exajoule per day (3850000 per year) [1]. Daily human caloric intake is 2500 calories [2]. Let's say inefficiency and the fact that you actually need food, not pure energy, let's say that means 20% efficiency, so 12500 calories per human daily. 12500 calories is about 50k joules. Things like running a tv/computer/... would be peanuts on top of that so they "come free".

So divide one by the other. The earth, without space based power, can keep about 1e18 humans alive on this planet using just earth-based solar power.

Plus you have to keep into account what humans actually do : they convert chemical energy into heat. If we cover the planet in solar panels, why couldn't we recover most of that human-generated heat ? Let's say we can recover 99% of energy that we put into humans back out, that's another factor of 100 right there.

We could use nuclear to increase that number to 1e30 or so for a millenium and if you count fusion based power, the numbers become completely ridiculous.

To put that number in perspective, take it like this : when every human alive today breeds into a population the size of today's entire human population, then we'd be near the "pure solar" carrying capacity of just the planet itself. If we were to beam power from space to the planet, we wouldn't run into the next limit until 1e50 at least.

So we are very, very, very far away, even on an exponential population curve, from the carrying capacity of the planet. When fitting an exponential curve to some sample data (very inaccurate, but good enough for a ballpark figure), I get that if the growth since WWII (1.36% per year, starting at 2000 with 6 billion people), then we'd get to 1e18 a little after the year 3400.

> we broke those limits by displacing a lot of other life

Only for a few measurements. There are a lot of problematic measurements of life that are inaccurate and therefore full of noise, and can be outright deceptive. Some of those measurements look bad : number of species, for instance. But how relevant is that number ? Sure, if it keeps dropping our zoos will become boring, but what will actually happen ? The truth is nobody knows, and there doesn't seem to be a reason, aside from nostalgia, to assume it's bad. One "real metric" that is at least measurable and I believe is meaningful : the total amount of life is total biomass, and it is pretty well known that humans have increased that by a lot.

> at some point however that fact will effect us unpredictably

It seems to me to be pretty much a given that lots of things can affect the human race unpredictably, and given how many such factors there are, it seems a safe bet that a few are indeed going to affect us (though the vast majority won't). Malthusianism and global warming are such factors, malthusianism has been proven false 3-4 times at least, and many limits to growth theories both before and after have been (I like the economic "debt limit" ones, stating that we can never have an economy larger than the value of all gold, then we passed that, then money became "loaned gold", and the next theory was that there was some factor that limits how big the economy can be n * total_gold, n was to be 10, then 100, now it's at least 100 million, and they're still at it), I wonder what the fate of global warming will be.

If you believe in Darwin and evolution, this is a positive. We will adapt to whatever effect, predictable or unpredictable, starts affecting us.

[1] https://en.wikipedia.org/wiki/Solar_energy
[2] http://www.webmd.com/diet/estimated-calorie-requirement