The human race has thrived on what is effectively borrowed time.
Fossil fuels are, everyone agrees, finite in supply. After we deplete our resources, they won't be replenished within the probable lifespan of the human race.
"Carrying capacity" is the maximum population an ecosystem can support before becoming unsustainable. You don't see it right away, but over time exceeding the carrying capacity of an ecosystem will cause the population to crash. For example, if an island has enough vegetation to sustainably feed 200 deer, you could get 201 deer and there wouldn't be an immediate destruction, but eventually the island would be stripped bare and the entire population would die out.
Once our reserves are depleted, the ability of the human race to feed itself will be restricted. We'll be suddenly trapped by the natural carrying capacity of the planet's ecosystems. It is essential that before then, the human race become technologically advanced enough to push the natural carrying capacity upwards to create enough food without fossil fuels, and equally essential that the human race manage their size to lower the target carrying capacity we'll need to reach with technology.
If the population keeps growing at the current rate and technology to increase the natural carrying capacity of our farming ecosystems without fossil fuels continues to be ignored, humanity will be destroyed.
We've got renewable power today. It's no magical source of infinite energy, even though it works extremely well for providing cheap renewable energy to places blessed to have the geography and the infrastructure.
Today, we exist in the numbers we do only because fossil fuels power our expansion. Without them, we'd have to rely on biofuels, which history shows us can't even provide enough power for a population a fraction of the current size.
Before fossil fuels were used to heat homes, wood was. That was the cause of deforestation in England -- with a much smaller population than today. After wood became impractical, coal was used. Similarly, after whale oil became much more difficult to procure, natural gas was used to light lanterns. Fossil fuels offset the fact that renewable sources of energy were all used in an unrenewable fashion. This devastation of renewable sources of energy was brought about by a population much much smaller than the population inhabiting the same area today. That's exactly the problem. Once the fossil fuels disappear, the population that was already sucking the natural renewable resources of the island dry is orders of magnitude larger, and will suffer. Even fish stocks have been decimated, leading to the collapse of fishing economies, like Newfoundland.
Many people will respond with their favourite pet vapourware technology. Year after year, we continue to be promised a flying car, but we don't get it. Don't rely on vapourware to provide energy for a population 6 times greater than the one that deforested England and brought whales to the brink of extinction.
I believe in technology, but I don't believe in miracles. I don't believe that technology is a perfect machine that will always provide us exactly the solution we desire. Much of the incredible advancement of the past couple centuries has been the elimination of biofuels in favour of cheaper fossil fuels.
We're far past the carrying capacity of natural or man-made means of collecting solar energy. At our current rate, the amount of food we need just to feed outselves will double by 2080. This is before we think about the amount of food that will have to be grown for conventional biofuels. The problem is that using biofuels with current technology is terribly inefficient. "In fact, even if the entire corn crop in the United States were used to make ethanol, that fuel would replace only 12 percent of current U.S. gasoline use."
There's no free lunch -- literally. No matter how you roll the numbers, there's a limited amount of energy hitting the earth, and once we've used up the fossil fuels, we've got to live with that energy and find ways to use it efficiently to keep our species going. Even look at the articles giving alternative ways to produce nitrogen fertilizer -- biofuels! Where do you think the nitrogen in those biofuels will come from? Nitrogen fertilizer! We need to manage the population because the free ride is going to end. Our choices are either be ready for it, or face extinction.
Our currently installed base of industry cannot support our race.
The reason we can support 6 billion people is nitrogen fertilizer. Nitrogen fertilizer is a "wonder drug" which causes plants to grow incredibly well. It's used around the world, but to give an idea of the effectiveness of nitrogen fertilizer, it was recently introduced to villages in China that couldn't support themselves. Suddenly they became capable of exporting huge amounts of crops using these chemicals. Nitrogen fertilizer is created almost entirely using natural gas feedstock. Thus, food is effectively a product of fossil fuels. Without nitrogen fertilizer, we wouldn't be able to grow enough to feed everyone.
Ammonia is the most important chemical containing nitrogen. It's a fundamental ingredient in fertilizer. Today, the raw ingredients to create ammonia are natural gas, steam, and air. As of 1982 (and industrial plants don't move very quickly, so it likely hasn't dramatically changed), natural gas, air, and steam is reacted with heat over a catalyst, creating the perfect combination of nitrogen and helium to produce ammonia, with water and CO2 waste products removed.
Electrolysis is a possible alternative method. It isn't competitive at the moment, just like most hydrogen from electrolysis processes (50-70% efficiency, greatly depending on size, and no matter what you're talking about obscene amounts of energy). At the moment, a fraction of a percent of the world's hydrogen supply is produced through electrolysis because of the problems involved.
So where are the problems?
1. Synthetic ammonia is the most important source of ammonia, which in turn is the most important source of nitrogen.
2. Current synthetic ammonia production for fertilizer relies almost exclusively on natural gas. In 1980, 14,686 kilotonnes of nitrogen contained in ammonia was created using the natural gas process. By contrast, all other sources combined accounted for 80 kilotonnes of nitrogen.
3. Current ammonia plants are optimized exclusively for the use of natural gas as a feed stock. Substantial re-tooling would be required to use another feedstock.
4. Massive amounts of energy is required to use electricity as a substitute for hydrocarbons. This will likely require proximity to generators and will preclude the use of existing processing plants, which can run entirely with a water and natural gas supply. Current electrolysis plants are located geographically very close to hydroelectric dams.
5. Whether this is possible at all is highly dependant on other industries which are similarly addicted to petroleum and coal.
5a. The creation of cement currently involves a lime kiln which burns oil, natural gas, or crushed coal.
5b. The creation of steel involves the use of carbon created with hydrocarbons.
5c. The creation of many plastics directly requires crude oil feedstock.
5d. The purification of many metals requires acids with similar issues. Hydrochloric acid is produced almost entirely directly or indirectly from hydrocarbons. The nitric acid process uses ammonia directly as a feedstock.
5e. Many mines use gasoline, natural gas, diesel, and oil to produce power, and couldn't run without the mobile energy source.
5f. The cardboard boxes that these things come in rely on availability of Kraft paper, which uses oil or natural gas to recover process chemicals.
5g. The production of glass requires lime, which is created using a kiln, which uses oil or natural gas
5h. Wood fibres for structural components or for paper, heavily use fossil fuels as a portable source of power to remain viable.
It's easy to say "Oh, we'll just switch to something new", but that shows a naivety about just how important they are. Entire industries rely on oil and natural gas, and there's no substitute. Once oil started going up, everything did. Metals shot up, wood shot up, fibres shot up, food shot up. Imagine if there wasn't enough to go around now. Where do you allocate your resources? Do you make ammonia to feed people now, or do you make cement so you can build new ammonia plants so you can feed people tomorrow? For those who say "go nuclear", how exactly do you intend to build new nuclear plants without any process feedstock for the building materials, without a portable source of energy for mines in the far north to use?
Let's examine the energy cost of three heavily fossil-fuel subsidized critical industries, ammonia, Portland cement, and iron.
The reaction that creates Nitrogen is as follows:
1/2N2 + 3/2H2 = NH3
Therefore, for every 1g of nitrogen required, 215mg of hydrogen will be required.
The practical energy required for a hydrogen electrolysis unit is 50kwH/kg
The world production of ammonia in 2006 was 124,000,000,000kg.
So assuming of all that, (.215/(1+.215))*124,000,000,000 ~= 21,942,386,831kg of hydrogen would be required.
Therefore, assuming no other losses, the total power required to supply hydrogen to the world's present ammonia production and not have people starving to death in the streets is roughly ~21,942,386,831kg * 50kwH/kg = 1,097,119,341,550kWH
It takes about 2000kwH/tonne of low-grade iron.
World production is approximately 1,544,000,000t
Therefore, to maintain current iron production would require 3,088,000,000,000kWH.
Now let's replace the kilns in cement factories.
Currently, the low-end of cement kiln energy usage is 5.3GJ/tonne
Google tells me 1 GJ = 277.777778 kWh
Therefore, an equivilent electric kiln would need to use 1,472.22222kWh/tonne
World production of cement is 1,462,470,000 tonnes/yr
Therefore, assuming 100% efficiency, replacing the kilns in the cement industry would take 2,153,080,833,330kWh
Our energy use to convert these three industries to electricity will cost 6,338,200,174,880kWh.
Let's see how much power we have to play with:
2,600,000,000,000kWH generated in 2005 from Nuclear Power
3,266,511,001,300kWH generated in 2005 from hydroelectric and renewable electricity
So without hydrocarbons, we're looking at 5,866,511,001,300kWH of total installed electricity generating capacity.
In other words, we can't even convert these three industries to current nuclear
 Shreve's Chemical Process Industries, p. 305