Scenarios 3 - 4
Scenario 3: Famine
It is not necessary that all crops worldwide fail at once, although there are situations (nuclear winter, volcanic explosions, asteroid impacts) in which this could happen. It is only necessary that enough crops fail worldwide that prices skyrocket, and/or that transportation costs increase to the point where getting food from where it is grown to the mouths of seven billion hungry mouths (or more to the point: my hungry mouth) becomes infeasible.
The real danger of climate change is not global warming but increasing climate volatility. Some areas might face unexpected drought; others unexpected flooding or heavy, heavy rains. Unexpectedly late frosts (In the middle of July? In August?) can wipe out fields of tender seedlings and their associated harvests. Honeybee populations are already plummetting thanks to Bee Colony Collapse disorder; without enough pollinators it does not matter how good the weather is. Or maybe we get sideswiped by a different disaster; nobody thought the pine beetles would wipe out British Columbia's forests either.
The world food system might be resiliant enough to take one year's worth of crop failure, although many will suffer (as they did in 2008). A couple of subsequent years of crop failures would likely be disastrous.
Or maybe we avoid the crises that make headlines. Maybe we continue with business as usual. People make more and more babies, fewer and fewer people die of old age, and the food demands of the human population continue to outpace the capacity of the industrial food system to feed it. Organic growers and locovores continue to spin stories about feeding the world's population while their tomatoes and cucumbers fail year after year.
Whatever the causes, food prices rise dramatically. As in 2008, price increases hit the most vulnerable countries first, but since I don't live in a vulnerable country I don't care. Then food prices start rising domestically and the most vulnerable here go malnourished. I worry about spending more and more of my paycheque to feed myself, but food activists applaud; they hate cheap food and hate that I had been able to feed myself a nutritious diet on a modest salary. But sooner or later prices rise to the point where even the food activists feel the pinch of the "real cost of food". A bag of rice that used to cost me $8 costs me $15, then who knows? $40? $100?
As people start dipping into their savings to put food on the table people's desperation grows. We all get thinner and pull our belts tighter. The storytellers tell us to grow our own vegetables, but urban agriculture is a losing proposition. Even in the good old days one vegetable thief had the ability to destroy the spirit of a community garden; how secure will our young vegetables be when everyone is hungry? Those who have high fences and guns might be able to protect their vegetables; as a renter who is not allowed to own a gun I fall into neither category.
People are resiliant. Even in Zimbabwe and North Korea people stay alive. But I am not resiliant. My abundant body fat might help me for a few months. Then what? Then it is survival of the fittest. It is fighting for spots in line at the grocery store, or worse.
Whatever resources we have will be scraped clean. We harvest every fish in the ocean; shoot every mammal in the forest, maybe even chop down every tree for fertilizer and cooking fuel. Then erosion and ecosystem instability kick in.
The hunger is bad, but in more cases than not the hunger does not kill by itself. Hunger distorts my thinking so I make more bad choices. Maybe one of those choices finishes me off. More importantly, malnutrition weakens immune systems, making me much more susceptible (and much less able to fight off) the diseases that ravage the population.
Some people would cheer on such a population plunge. Fewer mouths to feed mean more food to go around -- unless the mutual mistrust we have sown during the crisis germinates and bears fruit, or we have ravaged our surroundings to the point where we cannot rebuild.
Scenario 4: Infectious Disease
Famine and war are two excellent ways to foster the spread of disease, which is why they hang out with disease as three of the four Horsemen. But spreading disease remains easy without other catastrophes. We take airplane travel for granted; few of us would put our travel schedules on hold without enforced quarantine.
When the pig flu or bird flu or toflu hits I certainly will not skip work pre-emptively. I will continue shopping and probably continue attending talks. My housemates will continue their regular lives as well. If the disease is easily transmitted via aerosol or even hand-to-hand contact via contaminated surfaces, it will be easy for me to get sick.
And when I am sick? Despite being notoriously paranoid about illness, I do not have a good record of taking time off when I am sick. Partially this is because I tend to get sick for weeks at a time; can I afford to stay home for weeks at a time? As far as I know I have not caused widespread death from this poor habit yet; I may not be so fortunate in the future.
The ideal illness would be easily transmitted (like the common cold) but slow-acting (like HIV), hard to treat (like HIV or the common cold) and invariably fatal (like Ebola?). We would carry it around, transmitting it without knowing about it. We have had our scares over the years; so far we have avoided the killer pandemic; we may not be so fortunate in the future. Even a non-ideal illness of sufficient severity will be "bad enough".
When I get sick I will have nobody to take care of me. Many of us will be paranoid, and nobody will want to take care of an infected person -- especially if prevention measures remain unclear.
I expect panic will be as much a factor as the disease itself. If the disease is exotic or severe then the newspapers and media outlets will whip up the paranoia, just as they did in the early days of the AIDS crisis. Those who look infected will be barred from society; they might be lynched by mobs. Many people in urban centres will attempt to flee to the country, where they will be shot by frightened country-dwellers. The survivalists who are stocking up on toilet paper and ammo while the rest of us surf the Internet might make it; but I am no survivalist, have no means of transportation, and no where to go. If this scenario comes to pass I hope I will be one of the early ones to go, and I hope I will not infect too many others via my selfish actions.