At their peak, the Mongols  were a formidable force.

With very few exceptions, they succeeded  in taking over everywhere they attacked, forming the largest continuous  land empire in human history. So what was powerful enough  to make the Mongols retreat?  Turns out, the Earth itself.

The few times they were defeated  tended to have one thing in common:  weird weather. Scientists have reconstructed  these events to answer questions  that have nagged historians for centuries. From typhoons to the Little Ice Age, here’s the lousy weather that  brought down the mighty Mongols. [SciShow Intro] In the 13th century, the Mongols began a massive  westward expansion from their capital in Mongolia.

Starting under the command of Genghis Khan and continuing with his son, Ogodei, and grandson, Batu, they conquered parts of what  are now China, Russia, and Iran. Their conquest reached across  what’s called the grassland highway, a strip of grassy plains that stretches  across Eurasia, ending in Hungary And in 1241, the Hungarian  king did what I’d probably do if I were invaded by 130,000 soldiers  of a notorious army. He fled.

Then, Batu Khan got to work setting up the  foundations of an occupying government.  It was looking like an inevitable  expansion of the empire. But the next year, the Mongols  abruptly turned around and retreated! For centuries, historians wondered  what caused this change of heart.

Nearly 800 years later, paleoclimatologists have finally found  clues to the mystery in the trees. If you count a tree’s rings, you  can figure out how old it is. But because a new ring grows each year, they can also record information about the  conditions like how warm it was that year.

These researchers looked at trees  from five sites across Europe that revealed four years of  warm summers starting in 1238. But this balmy spell abruptly ended  with a particularly cold and wet 1242. Historical accounts indicate the Mongols had  difficulty crossing flooded and muddy terrain.

I mean, have you ever tried to operate siege  engines in the middle of a muddy swamp? Apparently it’s no piece of cake. On top of that, the combination of war and  this cold weather led to a famine in Hungary.

Each Mongol soldier would  travel with about five horses. That’s a lot of mouths to feed! They relied on the bounty of the  lands they conquered to feed them all.

But flooded grasslands would have  left less area for horses to graze. So the researchers think this shift in  weather literally caused the Mongols  to get stuck in the mud  and forced them to retreat. Now it’s important to note this is a hypothesis,  and one that certainly not everyone agrees with.

But Hungary’s history might look very different if it weren’t for Mother  Earth’s muddy intervention. And something similar happened the next decade… In 1258, 120,000 Mongols pushed into Syria, only to retreat two years later. And as with their trouble in Hungary, it has always been kind of a mystery.

But their fate might have been  sealed before the attack ever began. See, a year earlier, the Samalas  Volcano erupted in Indonesia. I know.

Indonesia is a long way from Syria. But it gets even weirder. To make the connection between the two, researchers had to travel  to Greenland and Antarctica to extract long samples of ice.

They were looking at the ice in cold places where it’s preserved in layers that  identify exactly when it froze into place. And in the ice layer from 1257, they  found abnormally high amounts of sulfate, one of the big things that volcanoes  spit into the air when they erupt. From this data, we know that this  eruption of the Samalas Volcano was the largest in 2,500 years.

Which means that sulfate would  have traveled far and wide. These ice cores were solid evidence of just that. Eruptions of this size spew tiny  aerosol particles into the atmosphere, which not only get trapped in  ice for thousands of years, but also have the more immediate effects.

Of reflecting some of the incoming sunlight This means less heat from the sun gets absorbed by the Earth and atmosphere. And it can have a big impact on the climate. The same research team that studied  the Mongols’ time in Hungary looked at more tree rings and  used a computer climate simulation to figure out the volcano’s impact on  the Syrian climate after this eruption.

They found that immediately afterward, the summers became cooler and  much wetter for a few years. Here, they think the rain  was temporarily a good thing, because it increased the greenery of the region, feeding all those hungry  Mongol soldiers and horses. But when things reverted to the  pre-eruption dry conditions in 1260, the land couldn’t support an army large enough to defend the Mongols’ new territory.

This idea matches another study that  found that back in central Mongolia, the initial conquests of Genghis Khan correlated with the warmest and wettest  period in the last thousand years. These were times when favourable weather provided lots of productive grasslands  for large herds of livestock. So it appears that the eruption made  Syria seem like a great place to invade, until it reverted to a desert.

But as we’ve already seen,  you can have too much water. And that happened to the Mongols again  in one of their most famous attacks. But before we get to that, here’s a quick ad.

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The mask is adjustable and  feels like what you thought like what a cloud felt like when you were a child For 15% off your Manta Sleep masks and  accessories and a free holiday recharge kit, check out the link in the  description and use the code SCISHOW. 14 years after their attack in the Middle East, the Mongols set their sights on Japan. But it proved harder to get  to than they anticipated. Twice, Mongol fleets attempting  to invade the island were crushed by kamikaze storms, which translates to “divine wind.” And they got that name for a good reason.

In November 1274, Kublai Khan attacked Japan with 30,000 soldiers in hundreds of boats. But a third of the boats were destroyed by a massive storm that swept through, killing 13,000 of his army. So in August 1281, he sent even more.

This time, as many as 200,000 soldiers  in thousands of boats sailed for Japan. But another typhoon approached, so the fleet had to shelter in Imari Bay. They were still no match for the ferocious storm.

Half of the soldiers died, and today, archeologists find weapons and splintered  remains of ships on the seafloor. Historians have found this astonishing, because the probability of two such events  occurring at the right time and place to destroy both fleets is extremely low. Typhoons are rare in November  when the first fleet was hit, and also rare in Imari Bay where the second fleet was sheltering.

So in a 2014 study, geologists  pulled a sediment core to see if they could find  evidence of these storms. This time, they took their core from the bottom of a coastal lake in Japan. In this peaceful lake, sediment layers  accumulate gradually every year.

Except when there’s a typhoon, which washes denser material from  the nearby beach into the lake and forms a recognizable layer at the bottom. The radiocarbon dates in the  core aren’t precise enough to be sure which layers are the  exact storms from 1274 and 1281. However, they can see that this was a  period of more frequent strong typhoons than we see today.

So they think there’s a high probability that two of these typhoon layers  are from the kamikaze storms that pushed back the Mongols. It’s yet another case of foul weather  defeating an otherwise invincible army. But we still have one last example to cover.

Between 1330 and 1390, the Mongol empire started to decline, even in their longtime stronghold of China. This is known as the Chinggisid Crisis. Now, there were a lot of reasons for this.

Political instability and  the spread of the Black Death are the most commonly cited. but there is also evidence that  climate change played a role. This was the beginning of the Little Ice

Age: 500 years of particularly  variable and chilly temperatures. Besides more tree ring studies, paleoclimatologists have used pollen, stalagmites, and even bat poop or guano to  show how important climate was during this period. Looking at which types of pollen  are preserved in peat bogs tells you what plants were thriving each year. The chemical composition of stalagmites  growing up from the cave floors records how much rain fell in a year.

And speaking of cave floors, bat guano gives you an idea of what bats  were eating during a certain period. Combining these lines of evidence with historical documents, scientists believe the start of the Little Ice  Age in China was a roller coaster of climate. Periods of heavy rainfall and flooding  alternated with intense drought.

This led to crops failing and plagues of locusts, which further decimated the food supply. The famines of the 1340s were so bad there are even reports of cannibalism. The Mongol government responded by spending as much as 30% of their budget on aid for the population.

But this depleted their coffers  and devalued the currency. All of this led to social unrest and rebellions. The Mongols finally lost control of China in 1368 when Ming forces pushed them out.

The Mongols had a good run. But even the largest land empire in human history wasn’t impervious to the weather. So the next time you’re planning an empire-expanding campaign in a far-off land, you might want to check the  long-term forecast first. [ outro]