Sustainability has become a one-word slogan. In the best sense, it means technology and practices that we can use essentially forever without further degrading the Earth. In practice, it means subsidized technologies that cost a lot of money and consume fossil fuels in non-obvious ways, while damaging the earth in "traditional" pre-industrial ways.
Before agriculture, herding, and fire, atmospheric CO2 was 180 parts per million. There is nothing sacred about that number; if we had somehow sustained that, we might very well be in an ice age now. There are probably significant negative ecological consequences of preventing an ice age, but I won't go into them here, I have an irrational dislike of ice ages. Atmospheric was about 300 ppm around 1900, long before fossil fuel use skyrocketed. It is around 380 ppm now, after massive industrialization and massive conversion to cropland. The important point is that we have been increasing CO2 for thousands of years, without fossil fuels.
The vast expansion of agriculture (40% of the land surface under cultivation) to feed 7 billion people is a significant portion of that. Tillage (for weed control and seed planting) is responsible for much of the agriculture-related CO2 increase, the replacement of complex ecosystems with fragile mono-crops contributes more. In most places, agriculture is a recent (tens to hundreds of years) and drastic change in land use - we need look only at the "fertile" crescent in the middle east to show how NON-sustainable agriculture can be.
In the wild, nature recycles everything. During the Carbonaceous geological period, plants evolved lignin to strengthen wood, and it took millions of years for nature to evolve termites to eat and recycle it. Meanwhile, all that undigested wood piled up and was buried to make coal deposits. Recycling failures can have extreme consequences.
Farms are incomplete ecosystems. The best farms close the loop as much as possible, but by design their production leaves the system, turning into sewage and ash thousands of kilometers away. Humans cannot afford to completely close the recycling loop, though they can extend the lifetime of the land from tens to perhaps thousands of years.
Hence, "plant-sourced energy", biofuel, is unlikely to ever be perpetually renewing. Even assuming 100.00% recycle of everything besides the hydrocarbon, we are removing the energy that insects, bacteria, and fungi use for their own recycling efforts. Given the massive amount of hydrocarbon energy injected into a farm with existing land-depleting agriculture, the chances are small that we can draw energy from hypothetical non-depleting practices.
We are blind to the long-term effects of most of what we do with land. Similar blindness affects many other fashionable "sustainable" consumption activities. We must accumulate far more knowledge before we can safely assume any form of land-modifying process, even traditional forms of agriculture, are truly "sustainable".
Let's choose a more robust definition of "sustainability", in the context of continuous human habitation:
Perpetual sustainability means a large and stable human population with a high standard of living sustained for a geologically long time on a healthy planet with abundant wildlife at prehistoric levels.
That is a dauntingly high goal, and the "geologically long time" part is impossible to measure or analyze. So replace "Perpetual" with "Megayear". Assume that over many hundreds or thousands of years, our scientific knowledge and ability to detect and mitigate slowly accumulating damage will become adequate for the task. Let's rephrase in measurable terms:
Megayear sustainability: 10 billion people, 10 kilowatts each, for 1 million years.
If we restore the bio-productivity of the planet to prehistoric levels (20,000 Terawatts of sunlight into plants, 200 Terawatts feeding the rest of the bacteria, fungi, and animals) and restore degraded land, we might manage to draw 0.5 Terawatts for food from it, with clever management and advanced and properly encapsulated biotechnology. There is no way we can draw 100 Terawatts of manufactured energy from the biosphere, or even divert sufficient land to build some as-yet-inconceivable terrestrial solar power collector. Most desert is alive, too.
Nature spent 3.7 billion years developing a functional ecology. We could completely unravel that, and turn the Earth into another Venus, in a few centuries. We can accelerate that destructive process if our "cures" are worse than the diseases we are curing.
This is not a game of "who's the bad guy". We will not restore the earth with witch hunts. If a witch hunter screams "kill the enemy", please forebear, if you kill him another will take his place. Our "natural" instincts are to behave like barbarians, and we must use science and reason, not passion and revenge, to guide us towards a truly sustainable future.
Instead, this is a call to use our minds to enhance nature, not rob or compete with it. There is plenty of energy out there, in forms that nature cannot process. We should draw our manufactured energy from those sources, and share some of that energy with the rest of life on the planet. I am arrogant enough to believe that with time and understanding, we can add to the abundance of life on this planet. For now, we should stop damaging the planet further, and stop calling fashionable or traditional versions of that damage "sustainable".