One that no one here has mentioned yet, but everyone should be talking about:

The U.S. will run out of easily mineable domestic deposits of phosphorus within the next 18 to 30 years. Why is this catastrophic? Let me count the ways…

1. Phosphorus is an element. It cannot be synthesized outside the heart of a star. When it is gone, it is gone. You can’t just go whip it up in a lab when you run out. The way we apply phosphorus to our crops ensures that only 10–20% actually gets into the plants we fertilize with it. The remainder gets locked up chemically in the environment or gets washed into the watershed and down into the depths of the ocean, where we can’t reach it. The waste of this limited resource is like buying 10 gallons of gas, putting 2 into your tank, and pouring the rest down the drain.
2. The current US methods of ‘conventional’ agriculture are highly dependent on heavy applications of phosphorus to maintain yields. Since phosphorus is highly reactive, it quickly binds with things in the environment and becomes unavailable to plants. Phosphorus is usually the single most limiting factor in plant nutrition. For example, it has been found that the yield potential of corn (maize) is determined by how much Phosphorus is in the plant’s tissues by the five-leaf stage. This means that the maximum amount of grain a corn plant can produce is determined by the time the plant has developed five leaves.
3. We have already reached peak Phosphorus globally. Price increases are on a track similar to that of oil. The major difference between these two limited resources is that if you can’t afford to buy gas, you don’t drive. If you can’t afford phosphorus, you don’t eat. Once people can’t afford to eat, civil unrest ensues.
4. A whopping 75% of the world’s known phosphate reserves are controlled by one man: the King of Morroco. Morocco is in a considerably unstable part of the world. The bulk of the rest of the reserves are in China and Russia, neither of which are very friendly to the US. Once we run out of our own phosphorus and are forced to pay the same prices that other countries do, you can bet that cost will be passed down to the consumer. Once again, when people can’t afford to eat, civil unrest ensues.
5. Putting too much phosphate fertilizer down on the majority of our croplands over the past 75 years has killed off the microbes that evolved to recycle phosphorus to plants and keep the soil alive. It can take 15 to 20 years to restore those microbes and to restore soil to its proper biologically active state. If we run out of phosphorus in 18 years and haven’t even started the process of restoring our soil and changing how we grow our food, we will reach a food crisis that could very well be the end of our civilization as we know it. EDIT: I realize this is a bold statement, so I want to explain it a bit more. The US exports a HUGE amount of grain (wheat, soybeans, corn, etc.) to other countries. When we begin to have trouble feeding ourselves, there will be a cascade effect, since this means we will no longer have a surplus to sell. Other countries who buy from us will be unable to feed their people, too. As Marie Antoinette found out the hard way, hungry people are desperate and angry people. Wars, revolutions, mass migrations and subsequent government collapse are likely scenarios here. The likelihood of this catastrophe occurring is also very great, because while the majority of the US land mass is given over to agriculture, the majority of Americans are utterly ignorant about how their food is grown. For an in-depth look at how this type of collapse occurs, read the book Dirt: The Erosion of Civilizations by David R. Montgomery. (Available at Amazon)
6. Biofuels, especially ethanol made from corn feedstock, are an absolute boondoggle of the highest order. We are using two very limited resources (oil and phosphorus) to grow an extremely inefficient replacement for one of those limited resources. How does that make any sense?

Here’s a ‘top 10’ list for you, with guesstimates of probability. It’s enough to scare the pants off of any sensible person…

In decreasing order of probability, some of the catastrophic things that might occur in the next 50 years (say, 2020 to 2070) in the United States are:

1. A major earthquake on the west coast, consisting of at least one temblor between magnitude 8 and 9+, as well dozens or hundreds of aftershocks in the 5–7 range. Although building codes in the last 40 years have mandated substantial improvements in earthquake resistance, there are still millions of older buildings that would suffer greatly, resulting in death tolls in the tens or hundreds of thousands. Business interruptions in the agriculture, high-tech and entertainment industries would have a significant economic impact both nationwide and globally. Estimates vary widely, but the average seems to be about a 50% probability over the next 50 years.
2. Large-scale severe drought throughout much of the midwest, lasting anywhere between three years up to two decades. If it’s toward the shorter side, say, 3 or 4 years, food prices will be driven way up, but supplies will still be marginally sufficient as many farmers can rely entirely on ground water extraction. If it’s toward the longer side, say, 15 or 18 years, much of the Green American Miracle will collapse. Again, estimates of probability vary widely, but no less than NASA pegs it at 25–35%, if climate change continues at projected rates over the next 50 years.
3. A large increase in the number and severity of major storms, mostly hurricanes and nor’easters. Most of the east coast of the USA is highly vulnerable to such storms, which contain winds and create storm surges that can do stunning amounts of damage. Cities like Miami, New York or Boston could suffer very serious damage, perhaps enough to cripple them for months. The costs could bankrupt major insurance carriers, and require billions of dollars of government assistance to keep them from becoming completely insolvent. Cities would likewise require vast sums of money to rebuild and to create better coastal storm protections. This alone could have a severely depressive effect on the economy. Meteorologist’s predictions vary greatly (naturally, we’re talking about the weather), but the probability of hugely destructive storms coming in freight-train like sequences is way too high for comfort, about 20–30%.
4. Serious geopolitical instability resulting in direct or indirect large-scale economic or military conflict. Russia is becoming increasingly hostile, almost certainly with expansionism in mind. China is becoming a superpower and militarizing like crazy. North Korea has nukes. India and Pakistan still hate one another, and they both have nukes too. The middle-east remains a hotbed of political, economic and religious conflict. For the last 70 years the USA has been in several large military conflicts, and yet if you walked down the streets or into the supermarkets of any American town, you couldn’t tell we were at war unless you specifically thought to ask. We’ve had the luxury of not being impacted on an ongoing daily basis by any of it. That may not continue, there is a non-trivial chance that a truly serious conflict could develop that might result in genuine hardships across the country, including high body counts, severe rationing, cyber attacks, major redirection of federal budgets, and more. There’s no way to assign a probability to any of that, but it’s sure not zero.
5. Runaway climate-change. Regardless of what’s causing it, we know the planet is getting warmer. What we don’t understand well enough is how the myriad of different climatic factors will interact. Hotter temperatures can mean more water vapor, resulting in clouds that cool us down. But since most of the heat is being adsorbed by the oceans, it could also mean the quasi-catastrophic melting of ices at the poles and Greenland, the relatively sudden release of vast quantities of methane from the hydrates on continental shelves, or any of dozens of other domino effects that in total could cause sea level to rise globally at a much faster rate than is currently projected. It’s unlikely, but not out of the question that most of our coastal cities will have to build very serious and stunningly expensive protections, and that low-lying areas such as the state of Florida might have to be abandoned completely by 2070. Again, it’s impossible to assign a probability, but it certainly isn’t zero.
6. Severe economic depression. As other answers have pointed out, the USA has gone into debt in a very big way. This is risky. We’ve been there before and have managed to step back from the edge without trashing ourselves, but if any of the other things on this list happen, it might be impossible to recover in a reasonable period of time. Stated simply, we have no savings, no reserves, so if anything really big and bad happens, we might find ourselves with more economic stress than we can handle.
7. Biohazards of either natural or man-made origin. We are rapidly approaching the end of the antibiotic miracle that began with the discovery of penicillin, with no suitable replacements on the horizon. Our extensive air, land and sea transport systems mix the germ pot globally. Genetic experiments are becoming vastly more sophisticated with potent techniques like CRISPR. We’d better be very, very careful that we don’t let evil genies escape their test tubes. All it will take to throw the world into crisis is for one contagious virus with a high mortality rate to jump the species barrier into humans. The economic consequences could be as great as the medical effects.
8. Dystopian scenarios, mostly based on or around things like big data and artificial intelligence gone bad. We’re already a serious distance down the road toward a surveillance society, and if the power of all that data ever falls into evil hands… well, to put it bluntly, we’re completely f*cked. Again there’s no way to assign a probability, but if history has taught us anything, we know that governments don’t always remain stable and fair. In fact, they never do.
9. A terrorist nuclear detonation. The death count would be horrible, but that’s just the beginning of the trouble. Even just one detonation in a US metropolitan area would cause the declaration of Martial Law within hours as the search for other possible devices gets underway. Borders would all but close, international travel and trade would grind to a halt. Our globalized economy would collapse, and take our liberties and standard of living with it. It would suck almost beyond description. Even just one.
10. And last but hardly least, a major solar flare. This is not very probable, the really big ones are quite rare, but they do happen. A big one would be enough to destroy huge parts of our power grid, and plunge us into blackouts for months. It could also destroy a serious fraction of our electronics, including mobile phone systems, the GPS and media satellites, computers, power plants, and a heck of a lot else. If the USA were facing the sun at the time of arrival of a really big flare, we could even see most cars and trucks die as their computer chips are fried. The entire food delivery network would be placed under severe stress, if it functioned at all.