Cross-section of a stalagmite from Hoti Cave in present-day Oman. Holes are from sampling for uranium dating, scratch marks from sampling for isotope analysis. (Photo: Timon Kipfer, University of Basel)
An extended period of severe drought contributed to the decline of the ancient South Arabian kingdom of Himyar in the course of the 6th c. CE. Prof. John Haldon of CCHRI together with researchers from the University of Basel led by Prof. Dominik Fleitmann, also associated with CCHRI, have reported these findings in the journal Science. Combined with political unrest and war, the droughts left behind a region in disarray, thereby contributing to the conditions which framed the eventual emergence of Islam on the Arabian Peninsula.
On the plateaux of Yemen traces of the Himyarite Kingdom can still be found today: terraced fields and dams formed part of a particularly sophisticated irrigation system that transformed the semi-desert into fertile fields. Himyar was an established part of South Arabia for several centuries and its influence politically and militarily extended well into the central Arabian peninsula and on occasion beyond.
Despite its former strength, however, during the early 6th century CE the kingdom entered a period of crisis that culminated in its conquest by the neighboring kingdom of Aksum (now Ethiopia). A previously overlooked factor, namely extreme drought, may have been decisive in contributing to this situation. Palaeoecological evidence permits a much better appreciation of these developments, findings recently reported by Professor Dominik Fleitmann and Professor John Haldon and colleagues in the journal Science.
Petrified water acts as climate record
The team, led by Fleitmann, analyzed the layers of a stalagmite from the Al Hoota Cave in present-day Oman. The stalagmite’s growth rate and the chemical composition of its layers (see box) are directly related to how much precipitation falls above the cave. In tropical and sub-tropical regions, there is a correlation between the amount of precipitation and its isotopic composition, also known as the “amount effect”. The more it rains, the more the ratio between the lighter and heavier oxygen isotopes, 16O and 18O, shifts in favor of the lighter 16O in the precipitation. These changes are recorded in the stalagmite from Oman, as it is formed from dripping rainwater. Based on isotopic measurements of the stalagmite’s limestone layers, it is possible to determine the exact ratio of 16O and 18O and, in combination with uranium dating, to reconstruct how much it rained at what point in time. The shape and isotopic composition of the deposited layers of a stalagmite represent a valuable record of historical climate.
Isotopic analysis of the stalagmites layers allows researchers to draw conclusions about annual rainfall amounts. The team was able to show not only that less rain fell over a longer period, but that there must have been an extreme drought. Based on the radioactive decay of uranium, the researchers were able to date this dry period to the early sixth century CE.
Historical significance
Whether there was a direct temporal correlation between this drought and the decline of the Himyarite Kingdom, or whether it actually didn’t begin until afterwards was not possible to determine conclusively from this data alone. The team therefore analyzed further climate reconstructions from the region and combed through historical sources in order to narrow down the time of the extreme drought, which lasted several years. Helpful sources included, for example, data about the water level of the Dead Sea and historical documents describing a drought of several years in the region and dating to 520 CE, which do indeed connect the extreme drought with the crisis in the Himyarite Kingdom.
Fleitmann noted that “Water is absolutely the most important resource. It is clear that a decrease in rainfall and especially several years of extreme drought could destabilize a vulnerable semi-desert kingdom.” Furthermore, the irrigation systems required constant maintenance and repairs, which could only be achieved with tens of thousands of well-organized workers. The population of Himyar, stricken by water scarcity, was presumably no longer able to ensure this laborious maintenance, aggravating the situation further.
Political unrest in its own territory in the later fifth and early sixth century and a war between its northern neighbors, the Byzantine and Sasanian Empires, spilling over into Himyar, further weakened the kingdom. When its western neighbor Aksum finally invaded Himyar and conquered the realm, the formerly powerful state definitively lost significance.
Location of Hoti Cave relative to the kingdoms of Aksum and Himyar
Turning points in history
Extreme weather events are often thought of as having relatively short-term impacts, but a number of historical examples show that they can lead to substantial political and often socio-economic instability. Haldon placed emphasis on the complex nature of society-environment-climate causality. He stressed that the team’s findings do not mean to suggest that the drought directly brought about the emergence of Islam. But there seems little doubt that such an extended period of very severe aridity was an important contributory factor in the context of the upheavals in the Arabian world of the sixth century.