Naturally altering the climate in order to create lush areas in a wasteland - Is it plausible, and if so how?

Question

In essence: I'm playing around with a few ideas relating to a post apocalyptic United States, and one of the concepts I've come up with is a world of mostly dusty deserts and other regions mostly devoid of life, and, more importantly, are nearly impossible to live in - you can't grow food there and the conditions are not ideal for long-term settlement. However - not all is lost for humanity, as some large areas (say perhaps the largest would be about the size of Vermont, but far often smaller areas closer in size to Delaware or Rhode Island) grow green and lush, with an environment that is usable for permanent settlement.

My question is: How, realistically, can this occur, and what criteria, or features of a location, if any, would alter the transformation from into a wasteland devoid of life to a location lush with it?

Here's what I'm looking for:

• The occurrence that would would cause a situation like this to arise. By this, I mean what series of changes to the regions climate would have caused this situation. Factors such as temperature, that kind of thing.
• Why this situation is continuing, and the world wouldn't revert back to it's pre-disaster state. Here, I'm looking for reasons why the wastelands don't become able to support life again, or if they do become able, it happens so slowly that it barely matters.
• The criteria, or features, of a location that would cause it to develop into one of the "lush" areas. Again, details such as temperature, elevation, perhaps the amount of water in the area - those details would be really helpful.
• Any other features about your scenario that you think might be important would also be helpful - I'd like my world to be at least scientifically plausible, and I'd hate to leave out possible important details.

Speaking of important details:

I think there's a few details I owe you regarding to what I was thinking about.

• I'd prefer the disaster to be something that could occur naturally - I'm not averse to a human-created set of circumstances leading to the disaster, but I'd rather the disaster not be something that occurs directly due to technology - instead of nanites altering the climate through the change of weather patterns, the nanite foundries produce pollution that causes climate change.
• Along the same lines, I'd rather the disaster not directly be caused by humans, but by a natural series of circumstances that were perhaps helped along by humans - by this, I mean I don't want something that humans deliberately did, as that's not part of the ideas I want to explore with this concept.
• In terms of timeframe, I'm perfectly happy for the change to be something that gradually occurred over the course of centuries - I don't particularly mind how long it takes to happen, as long as it happens.
• I'm not too fussed over the exact details of the wasteland - as long as it's unable to support life, and is problematic to even travel in for a short time, I think anything should work.

For those of you who decide to answer, I'd like to thank you in advance for helping me figure out how this would work. If you see any problems with my question, feel free to ask me to clarify them, and I should be able to reply within a day or so.

Answer 1

Look at Egypt! Although Egypt is a desert, parts of it are still fertile because of the Nile River Delta. The Nile River Delta covers 25,000 sq km. of land area (roughly the size of Vermont, which is 24,906 sq km.) Prior to the construction of the Aswan High Dam, the Nile would flood its delta each year, allowing for fertile soil in an otherwise desert environment.

Deltas form when a sediment-carrying river:

• reaches a body of water, such as a lake, ocean, or reservoir
• contacts another river that cannot remove the sediment quickly enough to stop delta formation
• travels to an inland region where the water spreads out and deposits sediments

A major river may reach a lake and form a delta much like the Nile Delta in your story.

Another way to have a fertile region in an otherwise desert environment is to place a river traveling through the desert that originated in a high-elevation region where snow melts each spring. This will create a floodplain that floods with water each year, much like the Nile Delta and allow for a small fertile region.

The Tigris-Euphrates river system consisting of the above floodplains measures 35,600 sq km. This is much larger than Delaware (5,060 sq km.) or Rhode Island (3,139 sq km.)

A third way that this could occur is through an oasis formed through a large underground river. This will allow for small areas of fertile land, but no larger than cities.

The drying of the area could have originated through Milankovitch cycles. The Earth's exact position to the Sun varies over time, making certain areas drier (since you preferred a natural cause). This is how the Sahara actually dried (the Sahara was once fertile land much like other parts of the world and fertile land is now constrained to deltas or oases.)

Answer 2

Global warming

It is theorized (and to some extent proven) that global climate change would lead to polarization of climactic extremes. Basically, dry areas would become drier, and wet areas would become wetter. I don't know how that fit into a "natural change" requirement, but it is also theorized that climate change can go into runaway mode, and even with human activity totally ceased, the change would persist for centuries.

Thus, in the new world Great Plains would go into a permanent "Dust Bowl" mode and become deserts or semi-deserts. At the same time, rainfall in coastal areas like Florida and New Jersey would increase, producing even more lush vegetation.

This approach, however, would not produce a sharp desert/oasis divide that you may be looking for. Appalachia would be wet, but these mountains are not tall enough to produce real desert on continental side. In the middle of the continent rivers like Mississippi and Missouri will still flow, and agriculture along their banks would still be very possible.

Answer 3

Irrigation Farming

There are a lot of places today that cannot sustain crops based on the natural environment, but have thriving agriculture anyway due to irrigation. Southern Alberta and California are examples. Southern Alberta is essentially a desert terrain, but has a huge agriculture industry due to irrigation farming.

In your post-apocalyptic world you could achieve this with some kind of aqueduct bringing water from a reservoir, which would give you your limited regions that qualify, or you could pump the water, requiring energy. Maybe the only areas that can sustain greenery are those where people have cobbled together enough solar or wind power, or someone is still running a nuclear power plant and the areas around it are green and lush, because with enough power you can do all kinds of things to sustain agriculture in areas that would otherwise not support it.

If it's a really hot world, with enough energy you could also use water misting to lower ground temperatures around people to make it feel cooler and more tropical. Las Vegas does this in some areas.