The writings of scholars in 10-century Baghdad reveal periods of extremely chilly weather, and even snowfall, in the city - valuable information for climatologists today, Melanie Swan writes.
The 10th century was the height of the Islamic Golden Age, but in Baghdad the winters were harsh. The Tigris froze, becoming a new path for horseback travellers. The water in the public bathhouses was solid, too. Even animals' urine froze where it fell.
It had rarely been so cold and the population was hit hard, with many dying.
Food was scarce and hundreds of palm trees perished.
Some of the summers were scarcely better. Where usually the harsh heat drove the population out on to their roof terraces for overnight respite, instead they huddled inside to escape the cold.
These details, culled from the literature of the time, are now being used by meteorologists to piece together a new picture of Iraq's climate more than a millennium ago.
"Arabic documentation is really useful to understand the climate variability in times and regions few studied until today," said Dr Fernando Dominguez Castro, a physicist at the University of Extremadura, Spain.
Dr Castro's work on Baghdad's climate between the years 816 and 1009 was published in last month's Weather journal.
In 762, the Abbasid caliphate, the third Islamic caliphate, moved its capital from Damascus to the new city of Baghdad.
It was a highly bureaucratic regime, keeping extensive records, but from the ninth century on civil strife and military invasions brought great destruction to the city and those records.
Many documents are thought to have been destroyed during wars such as the one in 1258. As the Mongols put a violent end to the caliphate, a writer of the time noted that "the Tigris flowed alternately red [from the blood of the killed inhabitants] and black [from the ink of the plundered manuscripts thrown into the river]".
There was also a less deliberate destruction.
While the replacement of papyrus with paper had allowed written records to become much more widespread, they did not last in the same way. Paper documents are far more fragile, and many have disintegrated over the centuries.
But some literature survived. And although references to weather in the ancient writings were scant - most concentrated on political, religious and social matters - some were more forthcoming.
Particularly useful were the works of the Sunni scholar Ibn Kathir, who wrote on issues such as creation and the lives of the Islamic prophets; the historian Ibn Al Athir; and the Islamic scholar Ibn Al Jawzi. Each makes more than 20 references to climate.
Others, such as Hamza Al Isfahani, Hilal Al Sabi and Al Hamadhani, mention it only two or three times.
While not enough to produce a formal "series" of climatic variables, between them these writings give some very useful pointers, recording in passing events that indicate quite specific temperature markers.
Animal urine freezes only when the temperature drops below minus 0.45°C. And Ibn Al Jawzi wrote about exactly that, between 998 and 999.
"This year the cold was extreme, with clouds covering the sky and continuous strong winds," he wrote. "Thousands of palm trees in the rural areas of Madinat Al Salam [Bahgdad] perished, and those that escaped were weakened and needed some years to recover their previous loftiness and their whole body."
More than a century earlier, in May 855, Ibn Al Athir noted: "This year winds from the country of the Turks affected many people, who died because they caught the coolness of the winds and suffered from the cold."
In November 902, Al Tabari noted a sudden drastic change in weather.
"People in Baghdad were attending the afternoon prayer wearing summer gowns," he wrote. "A wind from the north blew, cooling the atmosphere to the point that people were obliged to sit around fires to keep warm, wearing winter clothes. Cold intensified until water froze."
Several writers mention one cold snap in particular.
In 920, Ibn Al Jawzi wrote: "In July of this year, the weather became so cold that people left the roof terraces [where they usually slept] and wrapped themselves in blankets.
"Later on, in winter, a strong hail poured down, damaging palm trees and other trees. There was also a great snowfall."
"The picture of typical summer nights in Baghdad is one of people migrating to terrace roofs for their relative comfort in hot weather," writes Dr Castro.
For people to seek warmth inside during summer, he says, was "indeed an extraordinary event".
Extraordinary, and useful. The recommended temperature range for sleep is 18 to 22°C - so it seems likely the temperature on those nights was below 18°C, nine degrees less than the coolest Baghdad usually gets in July.
But why did it get so cold? Dr Castro suggests it could have been the result of a major volcanic eruption.
There were two large eruptions around around that time, though their precise times are not known. The volcano Ceboruco in Mexico blew its top around 930, give or take 200 years, while Guagua Pichincha, in Ecuador, erupted around 910, give or take 100 years. However, work with other sources would be required to assess whether either was the cause.
The result, though, is clearer.
Together the ancient papers describe a period of 50 years - the second half of the 10th century - during which Baghdad was hit more often by significant climatic events - hailstorms, droughts and floods - than today, and endured more intense cold, evidenced by five snowfalls in a span of 40 years.
By contrast, since 1954 the average temperature in the city has fallen below zero just once, for two days in January 1964. And the snowfall in Baghdad in 2008 was the first in living memory.
Not only was that 50-year period of turbulent weather chillier than now, it appears to have been colder than the century before it and the 50 years that followed.
Dr Castro hopes other researchers will take a similar approach to other cities in the region. "Ancient Arabic documentary sources are a very useful tool for finding eyewitness descriptions which support the theories made by climate models," he says.
"The ability to reconstruct past climates provides us with useful historical context for understanding our own climate. We hope this potential will encourage Arabic historians and climatologists to work together to increase the climate data rescued from across the Islamic world."
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