By piecing together clues from Arabic historical records, Spanish researchers have assembled a rough chronology of climatic events in the Mesopotamian region from the years 816 to 1009.
The records, which focus on the Iraqi city of Baghdad, depict a markedly colder region where snowfall and summer cold waves occurred with far more frequency than they do today.
"The period between 902 and 944 had a high number [of cold waves] if we compare them to current weather data," said Fernando Dominguez-Castro, a researcher in the physics department at the University of Extremadura in Spain. Dominguez-Castro and his colleagues recently published their findings in the journal Weather.
During the 193 years covered by the documentary sources under study -- the first two centuries of what is sometimes called Islam's 'Golden Era' -- researchers found examples of 10 cold spells with temperatures cold enough to freeze water, with six of those events occurring within a 42-year period.
By contrast, temperature records from Baghdad International Airport over the past 50 years record only one two-day period when temperatures dropped below 32 degrees Fahrenheit.
Today, Baghdad is ranked among the hottest capital cities in the world. But according to ninth-century Berber geographer Al-Ya'qubi, the region was an ideal place for agriculture during the Golden Era. Its hot summers and cool winters provided ideal climatic conditions for bountiful harvests, he wrote.
A puzzle of history and geography
Baghdad, founded as the capital of the Islamic Empire by the second Caliph Abu Ja'far Abdallah al-Mansur in 762, was a hub of culture, commerce and learning for hundreds of years before it was sacked by Mongol armies in 1258.
Thousands of books and scrolls were destroyed during the siege, along with most of the city. According to historical legend, the invading Ilkhanate army used books from the sacked libraries to build a bridge across the Tigris River. Yet enough texts have survived to allow researchers to assemble a rough chronology of climatic events that affected the region during the past 200 years of the first millennium.
In one instance, historians recorded an unexpected drop in temperature during July 920, when night temperatures fell as low as 13 degrees Fahrenheit below the monthly average. Baghdad's citizens, accustomed to passing the warm summer nights on their rooftops, were forced to retreat into their houses.
As the example illustrates, when meteorological phenomena are mentioned in the records, it is usually in the case of climate extremes and their effects on the local population, said Dominguez-Castro.
"Climate information recovered from these ancient sources mainly refers to extreme events which impacted wider society, such as droughts and floods," he said. "However, they also document conditions which were rarely experienced in ancient Baghdad, such as hailstorms, the freezing of rivers or even cases of snow."
Those anecdotes help fill in the gaps in what is already known about the era's climate. Researchers believe that although the cooler weather can be partially attributed to natural climatic cycles, volcanic activity might also have played a part.
Two significant volcanic events -- a world away, in Central and South America -- occurred during the period under study. The first was the eruption of the Guagua Pichincha volcano in Ecuador in 910, the second the Ceboruco volcano in Mexico around 930.
Although experts believe there is a strong chance these events could have influenced weather in the Middle East, they stress that more research is needed to confirm the hypothesis.