I used to think that compound growth, of the consumption, was the worrying factor. Sure we might find more, but even finding MUCH more wouldn't help:
Ages ago I saw an excellent lecture on exponential growth, but I've forgotten the source; you probably know it:
Some bugs are in a jar, their population doubles every minute, their current state is not known / important, but the jar will be full at midnight.
When do they start to notice? at 3 minutes to midnight the jar is only a little over 10% full ...
Just before midnight they find 3 more jars - three times as much resource as they ever had before. But at 2 minutes past midnight they will be full ...
With oil consumption increasing at 7% p.a. (I've plucked a figure, and I know it is high ... but emerging economies have populations wanting, and able to afford, more consumer goods) that's a doubling every 10 years, rather than every minute
Still a pretty short time ... with constant & steady growth the amount we consume in the next doubling interval is the same as the total that we have ever used
But the things that are changing my mind are:
As renewables come on stream they reduce requirement for Oil. With, say, 10-20% renewables that's a huge reduction in the demand for oil.
So oil lasts longer ... but ...
... sellers are no longer making as much money as they used to, so are likely to lower their price. That makes the Renewable tech less affordable, fewer people choose to make that spend, and as a consequence implementation of renewables is delayed. Meanwhile sale and extraction of Oil takes off again, hastening the point of Peak Oil.
This scenario can be seen with bacteria colonies too. They grow extremely fast until the run out of space or food and then crash. Much of this scenario is playing out with the world's food and clean water supply right now. Hans Rosling argues that world population is in the process of stabilizing:
Global population growth, box by box
But IMO the stability point is too high. With a bit over 7 billion people, we're using up a lot of critical resources faster than we can produce them. Australia, Russia, the United States, and (I believe) Canada are all net grain exporters. Their grain is just barely able to feed the masses in the poorest countries. Many of these countries are completely dependent on the grain exporters to keep their population from starving.
A few years ago the world had a grain crisis when Australia had a bad drought, then Russia's grain crop was badly damaged by fires just about the time Australia started producing again, and the year after that American production was down because a very bad winter caused terrible spring flooding in the Midwest that reduced grain exports from the US for a year.
It didn't help that Monsanto accidentally made a species of rice extinct when a GMO rice they were testing got loose and cross pollinated with the medium grain rice grown in California ruining the crop and making medium grain rice unavailable for a while. It's still harder to find than other types of rice.
The first year of the crisis the US was able to balance the drop by pouring supplies stored in grain elevators onto the market, but that only worked for a year, maybe 2 before the excess grain supply was gone.
First rice prices worldwide spiked, then other grains spiked as the problem got worse. It triggered the Arab Spring. That started over protests about rising grain prices. Families were literally starving because they couldn't afford enough grain for a subsistence diet.
Right now the US is having a problem storing all the excess grain, but that's just masking a looming problem that could hit the world at any time. It doesn't even take anything as large as climate change, just a few bad crop years in multiple countries and the grain supply will be tight again.
On top of that, most of the world is running out of fresh water. Industrialization in China has increased fresh water use at a time when their underground aquifers are almost exhausted. Both China and India have been able to grow enough to feed their population, but they have done it through underground water supplies. In both countries the biggest food growing regions are a long ways from the coast, so desalinizing water on the coast and pumping it inland would be a major project and consume a lot of energy (even if they did it with solar power). China is currently on the deepest aquifers they can tap, water doesn't stay liquid at deeper depths and that steam is usually contaminated with all sorts of things that wouldn't be good for crops.
India's use of fossil water isn't as far along yet, but they are only about 10 years behind China.
There are only three places on Earth that have more fresh surface water than they need: some parts of the northern US, large parts of Canada, and Siberia. Everyplace else is facing a water crisis in the near future because they have used up their surface water and in a lot of places their underground supply too.
What will happen when China and India are no longer able to grow enough of their own food and need to import it?
This is all if the climate stays the same. If the world climate continues to warm as a lot of people predict, that would probably be the best long term scenario for the human race. It's bad news for coastal cities and other low lying areas which will find themselves below sea level. That requires a mass migration, but the warming will open up huge swaths of Siberia and Canada to both farming and migration. There are a lot of environmental concerns with millions of people migrating into these currently pristine areas, but there will be someplace for them to go.
If the climate does continue to change, I think it's much more likely we are going into a new period of glaciation. Over the last 2 million years, the world has been around 6 C cooler than it is now 90% of the time with brief periods of 10-20,000 years of warm like the current epoch in between. All of human history has happened during one of those warm periods. The ice records show world temperatures often spike upwards just before glaciation starts, and it's possible we are in one of those spike right now.
Over the last 2 million years Earth has been a lot warmer than it is now. 130,000 years ago (during the last warming period) global temperatures were warm enough a coral reef formed in what is not the Everglades. The average world temperatures for the last three warm periods were warmer than the current warm period and temperatures during the current warm period have been warmer than they are now. We don't have many records for it, but human civilization started right about the time global temperatures got really hot around 6000 years ago. There was also a warming during the Roman Empire that allowed Hannibal to cross valleys in the Alps that were ice free at the time, but aren't now. Around 1000 Ad world temperatures were warm enough that the Vikings established a self supporting colony in Greenland. The records of that colony show how it was slowly wiped out as the world cooled off again.
We have put a lot of CO2 into the air and IMO we have also done significant damage to the biosphere cutting down trees and poisoning the oceans. Records from plants collects in the early 19th century show the massive volcanic eruption in modern day Indonesia dumped massive amounts of CO2 into the atmosphere, but the biosphere sucked most of it up in a couple of years. The ability of the biosphere to suck up excess CO2 is compromised.
But it all comes back to the world population again. Like a bacteria colony at the limits of its food supply, it seems sustainable, but there are a lot of underlying factors that are strained to the limit and the entire population is facing crisis if any of those underpinnings fail.
Oil is a factor in the world today. The entire world would ignore the Middle East and their squabbles if the place wasn't rich in oil. Look at how much concern the developed world pays to wars in Africa? They rarely even make the news in the US and get only cursory coverage in other countries. Oil production and transportation also contributes to world pollution, not just air but the oceans and land too.
We can make noises about the Brazilians cutting down the rain forests, but the US did it first in our own country. When Europeans got here, it was said a squirrel could make it from New England to the Mississippi without ever touching the ground. As Europeans conquered the continent, it wasn't just the native peoples pushed out of the way, but that massive forest was wiped out too.
The oceans' biospheres are collapsing in large part because we have a lot of mouths to feed and factory trawlers are scooping up massive numbers of fish leaving huge gaps in the food chain and sending the entire system into chaos. Runoff from farming is also contributing by killing off the bottom of the food chain.
I can't find it now, but I read an article about 3 years ago written by someone who occasionally sails yachts from one country to another delivering them to new owners. His article was written about his experience a year after Fukoshima when he took one yacht from Australia to Japan and then another from Japan to the US. It was the first time he had been out in the Pacific in about 10 years and the degradation of the environment was scary. They normally would supplement their diet with fish caught along the way, but they had trouble catching anything. He also found the amount of debris in the water even a year after Fukoshima to be amazing. Enough that even being as careful as possible, the yachts arrived at their destinations with damage from hitting unseen objects under the surface.
Most of us have little experience what the oceans are like out away from shore, I have never been in blue water. But from what I've read and seen in documentaries, the condition of the world oceans is very bad and ultimately the problem is there are too many of us trying to make it on a planet that just can't sustain 7 billion humans for the long term.
That was a lot longer than I intended. Sorry about that...