1 billion is a rough estimate based populations prior to the massive boom in the industrial revolution that saw massive changes in the way agriculture is done. It could easily be 2 billion or so, but definitely not 7.8 billion. You can see the population history here[0.]

To see the powerful impact of fossil fuels on carrying capacity you'll notice there's an important inflection point around 1920-1930. This is because of the advent of the Haber process[1] which allows us to use fossil fuels to create nitrogen based fertilizers.

Lest you doubt the impact of the Haber process just look at trends in corn yield per acre since then [2]. It's truly remarkable. Additional gains there are from other industrialized, fossil fuel driven agricultural process.

The Haber process requires hydrocarbons. In the wikipedia article you can see that it consumes 3-5% of the worlds natural gas production and 1-2% of the global energy supply.

We have completely disrupted the natural nitrogen cycle [4] and so would be unable to produce anywhere near as much food without fossil fuels. Because we have disrupted this cycle it's not even obvious that we could go back to a world of pre-fossil fuel agriculture.

So those are just some bit of information about my claims but let's take a look at yours:

> we could cover all our energy needs with renewable sources within a few decades if we really wanted to... this seems mostly non-disputed to me

This is wildly disputed, and I don't know anyone who credibly believes this without invoking "magic" future technology.

For starters we haven't replaced fossil fuels with "renewables" at all so far. We've just used them to supplement our energy needs. You can see here [4] that global fossil fuel consumption has continued to rise.

Then it is important to separate electricity from the more general subject of energy. Currently only 20% of global energy usage is electricity generation [5]. So even if you replaced the entire grid with renewables over night you would still be missing the vast majority of energy demands.

We currently have no viable pathway for renewable energy in transportation. Alice Friedman has more notes on this than I could ever fit in a comment [6]. Transportation inherently requires high energy density fuels, and outside of passenger vehicles, battery technology does not have the density required for industrial shipping.

It worth looking at our national energy flows to get a good sense of just how little of the energy we use comes from renewables [7].

But even if we look just at the electrical grid, in the US, we have some very obvious problems with "all" our needs. As you probably know, wind and solar are intermittent power sources that requires fossil fuel powered "peaker" plants to provide energy in down times.

This had two problems. One you need energy storage technology that we do not currently have (you cannot use grid scale lithium batters, pumped hydro has geological constraints, molten salts only work with concentrated solar, compressed air requires decommissioned oil field, etc).

The other problem is that even if you had perfect storage you need to now more than double the total energy production so you can fill those batteries.

The should be enough sources for you to get started, but I have feeling I'll still get down votes and "hand wavy" explanations of how it will all work out.

0. https://en.wikipedia.org/wiki/World_population#/media/File:W...

1. https://en.wikipedia.org/wiki/Haber_process

2. https://www.agry.purdue.edu/ext/corn/news/timeless/YieldTren...

3. https://en.wikipedia.org/wiki/Human_impact_on_the_nitrogen_c...

4. https://ourworldindata.org/grapher/global-energy-substitutio...

5. https://www.iea.org/reports/world-energy-outlook-2019/electr...

6. https://www.resilience.org/resilience-author/alice-friedeman...

7. https://flowcharts.llnl.gov/content/assets/images/charts/Ene...