A long period of warm, wet weather spanning several decades helped one of history’s most fearsome tyrants to conquer most of Asia and Eastern Europe and form the largest continuous land empire the world has known, a study has found.

Genghis Khan owes his place in history to a sudden shift in the Asiatic climate from the cold, arid period that immediately preceded his ascent as leader of the Mongol empire, to the warmer, wetter weather that allowed his horsemen to expand out from Central Asia.

Scientists studying ancient Siberia pine trees in central Mongolia that date back nearly 2,000 years believe that Khan’s rise to power coincided precisely with a period of unusually heavy rainfall over a couple of decades which allowed the arid grasslands of the Asian Steppe to flourish.

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Richer, more productive pastures for the herds of war horses on which the Mongols depended for their nomadic lifestyle helped Khan’s invading armies to take territory as far east as China, as far south as Afghanistan and as far west as Russia and Hungary, the researchers said.

Tree rings, which record periods of good and bad plant growth, show that the years from about 1180 to 1190, which immediately preceded Genghis Khan’s rule, suffered an intense drought that probably stoked the political turbulence that helped him to come to power.

Shape Created with Sketch. In pictures: Changing climate around the world Show all 15 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. In pictures: Changing climate around the world 1/15 Greenland Calved icebergs from the nearby Twin Glaciers are seen floating on the water in Qaqortoq, Greenland 2/15 Iran Oroumieh, one of the biggest saltwater lakes on Earth, has shrunk more than 80 percent to 1,000 square kilometers in the past decade. It shrinks mainly because of climate change, expanded irrigation for surrounding farms and the damming of rivers that feed the body of water 3/15 Greenland A boat navigates among calved icebergs from the nearby Twin Glaciers in Qaqortoq, Greenland. Boats are a crucial mode of transportation in the country that has few roads. As cities like Miami, New York and other vulnerable spots around the world strategize about how to respond to climate change, many Greenlanders simply do what theyve always done: adapt. 'Were used to change, said Greenlander Pilu Neilsen. 'We learn to adapt to whatever comes. If all the glaciers melt, well just get more land 4/15 Norway The Svalbard Global Seed Vault is seen after being inaugurated in Longyearbyen, Norway. The 'doomsday' seed vault built to protect millions of food crops from climate change, wars and natural disasters opened deep within an Arctic mountain in the remote Norwegian archipelago of Svalbard 5/15 France A technician preparing to drain a vast underground lake at the Tete Rousse glacier on the Mont Blanc Alpine mountain, to avert a potentially disatrous flood. Some 65,000 cubic metres (2.3 million cubic feet) of water have gathered in a cavity, dangerously raising the pressure beneath the mountain, a favourite spot for holiday makers in Saint-Gervais-les-Bains 6/15 Switzerland Cracked mud is picture at sunrise in the dried shores of Lake Gruyere affected by continuous drought near the western Switzerland village of Avry-devant-Pont. A leading climate scientist warned that Europe should take action over increasing drought and floods, stressing that some climate change trends were clear despite variations in predictions 7/15 USA Cattle graze on grassland that remains dry and brown at the height of the rainy season in south of Bakersfield, California. Its third straight year of unprecedented drought, California is experiencing its driest year on record, dating back 119 years, and dating back as far as 500 years, according to some scientists who study tree rings 8/15 Pakistan An aerial view shows tents of flood-displaced people surrounded by water in southern Sehwan town. United Nations Framework Convention on Climate Change (UNFCCC) executive secretary Christiana Figueres met with people displaced by last year's devastating floods. Catastrophic monsoon rains that swept through the country in 2010 and affected some 20 million people, destroyed 1.7 million homes and damaged 5.4 million acres of arable land 9/15 Australia An aerial view of flooding in North Wagga Wagga. Climate change is amplifying risks from drought, floods, storm and rising seas, threatening all countries but small island states, poor nations and arid regions in particular, UN experts warned 10/15 Honduras Damages caused by a landslide on the Pan-American highway near La Moramulca, 55 Km south of Tegucigalpa. International highways have been washed out, villages isolated and thousands of families have lost homes and crops in a region that the United Nations has classified as one of the most affected by climate change 11/15 Indonesia A resident sprays water on a peatland fire in Pekanbaru district in Riau province on Indonesia's Sumatra island. Indonesia, an archipelago of 17,000 islands, is one of the world's biggest carbon emitters because of rampant deforestation. US Secretary of State John Kerry Sunday issued a clarion call for nations to do to more to combat climate change, calling it 'the world's largest weapon of mass destruction' 12/15 Indonesia An excavator clearing a peatland forest area for a palm oil plantations in Trumon subdistrict, Aceh province, on Indonesia's Sumatra island. As Southeast Asia's largest economy grows rapidly, swathes of biodiverse forests across the archipelago of 17,000 islands have been cleared to make way for paper and palm oil plantations, as well as for mining and agriculture. The destruction has ravaged biodiversity, placing animals such as orangutans and Sumatran tigers in danger of extinction, while also leading to the release of vast amounts of climate change-causing carbon dioxide 13/15 Bangladesh Stagnant rain water with tannery waste make the Hazaribagh area in Old Dhaka as well as Buriganga River the most polluted. Each year during the seven-month long dry season between October and April the Buriganga River becomes totally stagnant with its upstream region drying up and becoming polluted from toxic waste from city industries 14/15 Bangladesh Waste water from Dhaka city drained to the River Buriganga contributes to its pollutions. On the World Water Day observed in 2007 under the theme Coping with Water Scarcity, under the leadership of the Food and Agriculture Organization of the United Nations, DrikNEWS explores some of the images of the river. UN-Water has identified coping with water scarcity as part of the strategic issues and priorities requiring joint UN action. The theme highlights the significance of cooperation and importance of an integrated approach to water resource management of water at international, national and local levels 15/15 China Heavy smog has been lingering in northern and eastern parts of China, disturbing the traffic, worsening air pollution and forcing the closure of schools. China's Environment Ministry said it will send inspection teams to provinces and cities most seriously affected by smog to ensure rules on fighting air pollution are being enforced 1/15 Greenland Calved icebergs from the nearby Twin Glaciers are seen floating on the water in Qaqortoq, Greenland 2/15 Iran Oroumieh, one of the biggest saltwater lakes on Earth, has shrunk more than 80 percent to 1,000 square kilometers in the past decade. It shrinks mainly because of climate change, expanded irrigation for surrounding farms and the damming of rivers that feed the body of water 3/15 Greenland A boat navigates among calved icebergs from the nearby Twin Glaciers in Qaqortoq, Greenland. Boats are a crucial mode of transportation in the country that has few roads. As cities like Miami, New York and other vulnerable spots around the world strategize about how to respond to climate change, many Greenlanders simply do what theyve always done: adapt. 'Were used to change, said Greenlander Pilu Neilsen. 'We learn to adapt to whatever comes. If all the glaciers melt, well just get more land 4/15 Norway The Svalbard Global Seed Vault is seen after being inaugurated in Longyearbyen, Norway. The 'doomsday' seed vault built to protect millions of food crops from climate change, wars and natural disasters opened deep within an Arctic mountain in the remote Norwegian archipelago of Svalbard 5/15 France A technician preparing to drain a vast underground lake at the Tete Rousse glacier on the Mont Blanc Alpine mountain, to avert a potentially disatrous flood. Some 65,000 cubic metres (2.3 million cubic feet) of water have gathered in a cavity, dangerously raising the pressure beneath the mountain, a favourite spot for holiday makers in Saint-Gervais-les-Bains 6/15 Switzerland Cracked mud is picture at sunrise in the dried shores of Lake Gruyere affected by continuous drought near the western Switzerland village of Avry-devant-Pont. A leading climate scientist warned that Europe should take action over increasing drought and floods, stressing that some climate change trends were clear despite variations in predictions 7/15 USA Cattle graze on grassland that remains dry and brown at the height of the rainy season in south of Bakersfield, California. Its third straight year of unprecedented drought, California is experiencing its driest year on record, dating back 119 years, and dating back as far as 500 years, according to some scientists who study tree rings 8/15 Pakistan An aerial view shows tents of flood-displaced people surrounded by water in southern Sehwan town. United Nations Framework Convention on Climate Change (UNFCCC) executive secretary Christiana Figueres met with people displaced by last year's devastating floods. Catastrophic monsoon rains that swept through the country in 2010 and affected some 20 million people, destroyed 1.7 million homes and damaged 5.4 million acres of arable land 9/15 Australia An aerial view of flooding in North Wagga Wagga. Climate change is amplifying risks from drought, floods, storm and rising seas, threatening all countries but small island states, poor nations and arid regions in particular, UN experts warned 10/15 Honduras Damages caused by a landslide on the Pan-American highway near La Moramulca, 55 Km south of Tegucigalpa. International highways have been washed out, villages isolated and thousands of families have lost homes and crops in a region that the United Nations has classified as one of the most affected by climate change 11/15 Indonesia A resident sprays water on a peatland fire in Pekanbaru district in Riau province on Indonesia's Sumatra island. Indonesia, an archipelago of 17,000 islands, is one of the world's biggest carbon emitters because of rampant deforestation. US Secretary of State John Kerry Sunday issued a clarion call for nations to do to more to combat climate change, calling it 'the world's largest weapon of mass destruction' 12/15 Indonesia An excavator clearing a peatland forest area for a palm oil plantations in Trumon subdistrict, Aceh province, on Indonesia's Sumatra island. As Southeast Asia's largest economy grows rapidly, swathes of biodiverse forests across the archipelago of 17,000 islands have been cleared to make way for paper and palm oil plantations, as well as for mining and agriculture. The destruction has ravaged biodiversity, placing animals such as orangutans and Sumatran tigers in danger of extinction, while also leading to the release of vast amounts of climate change-causing carbon dioxide 13/15 Bangladesh Stagnant rain water with tannery waste make the Hazaribagh area in Old Dhaka as well as Buriganga River the most polluted. Each year during the seven-month long dry season between October and April the Buriganga River becomes totally stagnant with its upstream region drying up and becoming polluted from toxic waste from city industries 14/15 Bangladesh Waste water from Dhaka city drained to the River Buriganga contributes to its pollutions. On the World Water Day observed in 2007 under the theme Coping with Water Scarcity, under the leadership of the Food and Agriculture Organization of the United Nations, DrikNEWS explores some of the images of the river. UN-Water has identified coping with water scarcity as part of the strategic issues and priorities requiring joint UN action. The theme highlights the significance of cooperation and importance of an integrated approach to water resource management of water at international, national and local levels 15/15 China Heavy smog has been lingering in northern and eastern parts of China, disturbing the traffic, worsening air pollution and forcing the closure of schools. China's Environment Ministry said it will send inspection teams to provinces and cities most seriously affected by smog to ensure rules on fighting air pollution are being enforced

After this period, the tree rings show a period between 1211 and 1225 of sustained rainfall and mild weather which coincided precisely with the meteoric rise of Khan’s empire, said Amy Hessl a tree ring expert at West Virginia University.

“The transition from extreme drought to extreme moisture right then strongly suggests that climate played a role in human events. It wasn’t the only thing, but it must have created the ideal conditions for a charismatic leader to emerge out of the chaos, develop an army and concentrate power,” Dr Hessl said.

“Where it’s arid, unusual moisture creates unusual plant productivity, and that translates into horsepower. Genghis was literally able to ride that wave,” she said.

The tree rings show that the normally cold, arid steppes of central Asia experienced their mildest, wettest weather in more than 1,000 years at the time when Genghis rose to power and established his enormous land empire with the help of his sons.

A study of the rings, published in the journal Proceedings of the National Academy of Sciences, shows that the climate soon reverted to its cold, dry state, which led to droughts and lower grassland productivity.

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The tree rings also show a disturbing modern trend. Since the mid-20th Century, the region has warmed rapidly and the drought years recently have been more extreme than at any time in the tree-ring record, said Neil Pederson, a tree-ring scientist at Columbia University’s Lamont-Doherty Earth Observatory, who led the study.

Each Mongolian horseman in Genghis’s army is said to have had up to five horses, which provided a supply of meat as well as transport. Higher grass yields would have also caused a boom in camels, yaks, cattle, sheep and other livestock, Dr Pederson said.

“The weather may literally have supplied the Mongols with the horsepower they needed to do what they did… Before fossil fuels, grass and ingenuity were the fuels for the Mongols and the cultures around them,” he said.

“Energy flows from the bottom of an ecosystem, up the ladder to human society. Even today, many people in Mongolia live just like their ancestors did. But in the future, they may face serious conditions,” he added.

Genghis Khan died in 1227 but his sons and grandsons continued to conquer more territory and eventually controlled what became modern Korea, China, Russia, eastern Europe, southeast Asia, Persia, India and the Middle East – before the empire began to fragment.