It is difficult to argue that the Mongols did not have a major effect on population centres when the result of their Imperial ambition created a minor ice age whose actual influence was felt across the world.
This is the 1st time I hear that the Mongols are credited with a world-wide ice age. So-called Little Ice Age supposedly happened either between XVI and XIX centuries or between 1300 and 1850 and, according to Wiki, the reasons are not quite clear: "Several causes have been proposed: cyclical lows in solar radiation, heightened volcanic activity, changes in the ocean circulation, variations in Earth's orbit and axial tilt (orbital forcing), inherent variability in global climate, and decreases in the human population." It is rather unclear how the last reason on the list could apply to Europe where the Mongols did not cause any significant loss of the population and how their alleged impact could last till the XIX century all the way to England.
We can consider written histories suspect in their biases, but pure scientific evidence points to a massive reduction in human population in the time of Mongol expansion.
In Asia, especially China. But not in Europe.
It is also hard to imagine that building an empire results in few casualties.
Please, define "few" and provide the numbers for the British, French and Russian empires.
An empire of such scale as the Mongols led to the destruction of many nations and peoples such as Khwarezmia
There was no nation or a comprehensive group of people called "Khwarezmia". Khwaresmid Empire was a hodge podge of the tribes and areas subdued by the conquerors of the Turic origin with the conquests continued all the way to the Mongolian invasion and even during it (Jelal ad Din's conquest of Georgia with a following massacre of Tbilisi's population).
Most of the ethnic groups populating the area survived and exist even now.
and the Tanguts sudden sharp end.
Not quite "sudden" but the "end" did not mean a complete annihilation of the nation. Wuwei surrendered and avoided destruction. Xia lost independence but nation survived " many Tangut families joined the Mongol Empire. Some of them led Mongol armies, e.g. Cha'an, into the conquest of China. After the Yuan dynasty (1271–1368) was established, the Tangut troops were incorporated into the Mongol army in their subsequent military conquests in central and southern China. ... The Tangut people living in Central China preserved their language until at least the 16th century." https://en.wikipedia.org/wiki/Tangut_people#History
Deserts and forests swallowed regions of previouly intense agricultural usage leading to an environmental event that outstripped the effect of the Black Death on the environment.
AFAIK, something of the kind happened in a single region of the CA (and they still keep blaming the Mongols) but not all over the Mongolian Empire. Most of the Northern China was back to the business as usual within years (and big part of it suffered minimal damage because it relatively easily switched to the Mongolian side) and AFAIK, conquest of the Southern China was not accompanied by the unusual massacres. In Rus (another area of the "intense agricultural usage") everything was back to normal within few years after conquest. Most of the rest were steppe and desert territories.
The dissolution of such a huge empire that allowed communication across great distances would be seen as a fall back into isolation.
So not only creation but dissolution was bad as well.
"Isolation" did nor happen even after the "dissolution" (one more tricky notion: the process was quite long and complicated). Communications became more difficult but they did not disappear all the way to Timur who did a "brilliant" job in the terms of screwing things up by looting territory of the Golden Horde.
The instabilty caused by civil wars, succession wars and wars of independence would be catastrophic. The collapse of the empire would be far more damaging than the expansion and all empires end.
Actually, it was considered as a rather good thing by the Chinese (who overthrew the Mongolian dynasty), by the Russians (who eventually got rid of the vassal dependency) , by the Iranians who got rid of Ilkhanate, etc. BTW, was this fall also responsible for some global disasters (like eruption of Krakatoa in 1888)?
I am not saying there were no benefits to the Mongolian Empire, but it would be hard to argue in its favour if they are invading you.
Sorry, but it seems that you are more than a little bit confused. AFAIK, nobody said that the Mongolian invasion was a wonderful thing to look forward to (*). It was bloody and cruel but we are talking about the XII - XIII centuries. No Hague or Geneva conventions, no Red Cross, etc. The point is that a "classic" depiction of the Mongols as exclusively negative destructive force is extremely primitive and does not stand up to any serious criticism. The contemporary (including post-conquest) sources can't be taken literally. For example, how could be an eyewitness of a complete annihilation of a city's population and how come that within few years the city is, again, a well-populated center with a lot of merchants, artisans, etc. How come that with the city being burned to the ground, population being killed to a last person, princely family and the clergy being massacred to the last person we have not only all these terrible things written in a chronicle by an eyewitness clergyman but we also know for sure that almost immediately after this conquest a hereditary prince is visiting Batu Sarai and then travels all the way to Karakorum to get Khan's patent on that depopulated princedom and that somehow his capital is still in place and he is capable of presenting significant gifts to his new overlords. At least with the old Russian chronicles one thing is easy to explain: meaning of the word "kill" was significantly different from the modern and did not mean "murder", just something bad happening.
The point is that their conquests were quite complicated events and that on quite early stage a value of the educated people (Uyghurs, Chinese, Muslims) was well understood ("We conquered empire sitting in a saddle but we can't rule it from the saddle") and destruction of the knowledge was rather occasional (and more or less usual thing within the general picture of that period) than intentional.
While their enlightened attitude to religious freedom was remarkable for the time they lived in they were still brutal conquerers with some ingeneous rulers. The story of the siege of Kuju always sticks in my head as a sign of both the Mongols honour in saluting the garrison commander and their terrifying resourcefulness in boiling their prisoners alive to use their human fat as explosives.
Nobody said that the Mongols were the earlier version of Mother Theresa but the acceptable norms of behavior had been quite different from the modern ones (**) and, not being a specialist in chemistry, I'm not sure how exactly the human fat could be converted into an explosive. Even less sure why the procedure required boiling alive: would the fat extracted from the corps be less explosive? Anyway boiling the people alive was a rather common method of execution used in Europe to deal with the poisoners and counterfeiters; IIRC, there were even nuances: a noble would be boiled in oil and commoner in water. The Mongols had been doing this since at least the time of Jamuha (who did this to the captured Temuchin's followers) but I did not see any mention of the following fat utilization. Strongly suspect that it would be easier to kill, cut the fat and then boil it but I may be completely wrong about that.
In the case you are referencing to Wiki has the following "The most grisly weapon used during the siege were fire-bombs which contained boiled down, liquefied human fat". Nothing about it being extracted from the live people and nothing about explosives; BTW, any idea how well the human fat burns?). I have no idea where this information is coming for.
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(*) Except at least for an old Chinese novel in which the lovers' problems are being resolved by the Mongolian invasion of their province: all state officials immediately had been promoted to the next rank and the lovers finally could get married and live happily ever after.
(**) The English execution for those guilty of a high treason was quite "inventive". Even in the XVII century Europe breaking on the wheel was OK while in the PLC impaling was something of "the 1st serious warning" (just kidding) with the inventive additions like 1st half-burn on a slow fire and then impale. The Ottomans had been probably even more inventive in that area and Chinese also had quite a reputation (not sure which of the descriptions are inventions of the modern authors and which did exist). Killing prisoners in mass was quite common. For example, Richard the Lion Heart ordered a mass execution of the prisoners and much later Charles the Bold order to hung the captured Swiss. Speaking of which, the written rule of the Swiss was not to stop to take prisoners: they all had to be killed on the spot (quite practical reason: taking prisoners would slow down advance of the column). Or a generally accepted international military code which existed by the time of the 30YW required to execute the defenders of a hopeless position because they are causing unnecessary losses.