Permanent local climate changes as a result of a catastrophe

Question

I am currently trying to create a very low fantasy setting, with the most scientific plausibility I can, in which, with technology and "human" development being akin to those of Earth's medieval period, but, of course, with some differences. Well, none of that matters very much for my question, but I am just trying to provide some context. Do note that, in this setting, all laws of Physics as we know it apply, with a single "catch": the base elements of nature (earth, water, fire...) can more or less be controlled, though that can only be achieved through years of training and practice (and doesn't work quite like normal high-fantasy magic: there are no "elemental blasts" and nothing like that).

Well, in the middle of continent upon which I plan to make my story happen (which is somewhat like Pangea, in the sense that it is the only continent, but comprised of several tectonic plates), I intend to have some kind of grasslands (more akin to steppes, but not so arid), more or less surrounded by mountains (created by convergent plate boundaries). But I was thinking about making these grasslands turn into some kind of very harsh landscape, with little to no water and very sparse vegetation, arid but not desertic, and preferably with terrible weather (some kind of "dry thunderstorms", perhaps?).

So I was hoping someone in this very helpful website could enlighten me: what kind of natural disaster could make that change, but without impacting significantly the climate patterns of the rest of the continent? Massive volcanic eruptions could be an option, but how could I make their effects occur only in that area and not globally?

I thank you in advance for your answers.

Answer 1

The interiors of continents tend to be very dry since the water has condensed out as rain closer to the shoreline. The Western United States was considered "the great American desert" since it existed in the rain shadow of the Rocky Mountains (air forced up the Pacific side of the mountains cooled and condensed out the water; hence the Pacific rain forests, while the dry air warmed as it flowed down the eastern sides of the mountains, evaporating what surface water existed. Modern agriculture using deep wells and irrigation is what makes agriculture even possible there today).

Since your land is ringed by mountains it will be in the rain shadow regardless of which way the winds come, and natural geological processes will be pushing the mountains higher by a few millimetres a year. After a few thousand years this will become noticeable, and after tens of thousands to millions of years the mountains will have risen high enough to essentially ensure rain only occurs when the most violent storms on the oceanic sides force moist air over the mountains.

In human terms, the grasslands will have been lush in the distant past (legends from a thousand years ago, perhaps) but have been getting dryer and harsher each season for as long as the tribe/city has existed. IF poor husbandry has been practiced (overgrazing with cattle and goats are especially bad, then the land is being stripped without any change of natural renewal, accelerating the process and turning sections of the land into a dust bowl.

The migration of people will be made difficult by the presence of high mountains surrounding the highland plateau, and only the most determined and capable peole will be able to cross, to the detriment of the lowlanders living in the lush rain forests of the coastal lowlands.

Answer 2

If it's ringed with mountains, the central plain wouldn't NEED a large natural disaster. If, say, there were gaps through the mountains that had rivers (and let through clouds, etc.), and those were blocked, the plain would quickly dry out. Air cooled by the mountain tops would sink and stir up dry winds blasting across the plains.

More interesting than a natural disaster is an engineered one. Let's say that a large city lay inside the plain, accessible through the largest mountain pass running between cliffs. Being the only major city and occupying the major artery into the interior grasslands, they prosper with ample game and farming land. Other city-states take note and unite against them. Threatened by an overwhelming military force, the grasslanders collapse the pass to keep their city from being destroyed by the invaders.

Unforseen to them is the long-term ecological impact this desparate move had. Now the grasslanders are cut off from most communication and trade with the outside world - the mountains are too treacherous for large caravans, and few venture the journey on foot. The major river that ran through the pass has been dammed from the collapse, and the skies themselves bring fewer and fewer rains each year.

Perhaps there may be some underground tunnels to explore that they can use to evacuate the interior, but they would be forced to leave most of their wealth behind. Slowly, over a decade, the once proud and prosperous crown of the grasslands is reduce to a husk as dried out as the trees in their dead orchards. All those who are able have left the decaying city. Few remain, trying to scratch their feeble existence out of the dirt in the shadow of monuments built to the prosperity of ages past.

Answer 3

Your chosen area would be desert, without further explanation. Without water to coolmthe air, the land-locked weather patterns will be hot and dry, with "weather" only along the edges. Parts of Pangea had huge temperature swings, too.

That's not necessarily the case... you might have inland great lakes, mountain ranges routing jet streams, and whatever. But bad and hostile weather as a default position is the starting point of this idea.

Maybe the magical control was needed to keep it wet and clement, but it revered to desert when the magic mana was depleted, the elementals no longer on speakimg terms with the people, or a different group grabs the water first and causes a rain shadow.

The grassland is unstable, and if anything goes wrong, it turns back to desert.

So why are people living in the hostile interior? Well, maybe because all the dangerous megafauna is around the edges. Also, the isolated interior provides a tabula rosa for the engineered climate to operate on, without natural weather patterns messing things up.

You could have your land in a superbasin of overlapping mountain ranges where the plates came together. They use their elemental control to bring seasonal rain and fill an engineered lake system and/or glaciers, which provides for agreculture and moderates the weather within the giant bowl. The area is large, but elevated with a ring of mountains and subsequent drop to sea-level beyond.

Answer 4

As others have noted your location will naturally turn into a desert. Human agriculture with irrigation will vastly accelerate the process by increasing evaporation by orders of magnitude. This has actually happened in the Central Asia. With a rapid expansion of irrigated area, the process can be fast enough for your needs. This would also create a scenario where the desertification directly follows a golden age of rapid expansion of both population and economy.

The reason the area would have been habitable would probably have been a preceding ice age creating vast ice sheets n the basin starting with the mountain top glaciers. When the ice age ends the water trapped in the glaciers over millennia gets released relatively fast and creates a temporary period of lush fertility in your central basin.

There would be some kind very large lake or possibly several created by the melting water. The lakes would rapidly turn saline. Once the irrigation takes off these lakes would rapidly shrink leaving former fishing villages and trade cities land locked. See the Caspian Sea and Lake Aral for example.

Also note that the area south of the Lake Aral used to be significant center of ancient civilization. Cyrus the Great of Persia for example died there. So it probably makes a good reference for your setting.

If you want a natural disaster, you can have an earthquake break an isthmus that kept the large lakes from draining to the sea. This would make the the area lose much of its water in one vast megaflood.

Answer 5

Here's a possible real-life example, the draining of Lake Bonneville: https://en.wikipedia.org/?title=Lake_Bonneville Catastrophic, as much of the draining happened in the span of a few weeks. Could perhaps have caused change from grassland to desert, as you'd have had lake effect rain/snow from the large lake that went away, plus lots of dust from the now-exposed lakebed.

Answer 6

I'm surprised nobody has mentioned volcanoes yet. If your ring of mountains were volcanically active they could make clouds of ash which could be your dry thunderstorms. They are called "dirty thunderstorms". The entire region could be periodically smothered in a haze volcanic ash which could kill large plant life.

One other aspect of this geographical formation is that if mountains really do completely surround the area then precipitation may have no escape from the region. By that I mean there may be no rivers that flow out to the ocean. If this is the case then your region would have a salt lake or a salt pan if there isn't enough water. This could also make the area somewhat inhospitable.