Extreme climate after massive eruption of Alaska's Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom

Abstract

The assassination of Julius Caesar in 44 BCE triggered a power struggle that ultimately ended the Roman Republic and, eventually, the Ptolemaic Kingdom, leading to the rise of the Roman Empire. Climate proxies and written documents indicate that this struggle occurred during a period of unusually inclement weather, famine, and disease in the Mediterranean region; historians have previously speculated that a large volcanic eruption of unknown origin was the most likely cause. Here we show using well-dated volcanic fallout records in six Arctic ice cores that one of the largest volcanic eruptions of the past 2,500 y occurred in early 43 BCE, with distinct geochemistry of tephra deposited during the event identifying the Okmok volcano in Alaska as the source. Climate proxy records show that 43 and 42 BCE were among the coldest years of recent millennia in the Northern Hemisphere at the start of one of the coldest decades. Earth system modeling suggests that radiative forcing from this massive, high-latitude eruption led to pronounced changes in hydroclimate, including seasonal temperatures in specific Mediterranean regions as much as 7 °C below normal during the 2 y period following the eruption and unusually wet conditions. While it is difficult to establish direct causal linkages to thinly documented historical events, the wet and very cold conditions from this massive eruption on the opposite side of Earth probably resulted in crop failures, famine, and disease, exacerbating social unrest and contributing to political realignments throughout the Mediterranean region at this critical juncture of Western civilization.

Keywords: Okmok; Rome; climate forcing; ice core; volcano.

Fig. 1.

Location map. Shown are drilling sites for the six Arctic ice core records evaluated in this study (pluses), the Okmok and Mount Etna volcanoes, tree (6, 11) and Shihua Cave (12) speleothem-based climate proxy records, and the extent of the Roman provinces and Ptolemaic Kingdom in 44 BCE.

Fig. 2.

Ice core, tree ring, and speleothem evidence for the 45 and 43 BCE eruptions and climate effects. (A) Selected historical and other events (see text). (B) Model-simulated (gray) and observed (black) summer temperature anomalies from European tree ring records with original 2σ uncertainties (6). (C) Model-simulated (gray) and observed (black) summer temperature anomalies from the Chinese Shihua Cave speleothem record with original maximum temperature uncertainties (12). Shading in B and C shows annual CESM ensemble SEs. (D) Continuous NGRIP2 and discrete (2 y) GISP2 nssS (9) concentrations. (E) NGRIP2 mass-equivalent insoluble particle concentrations for medium (2.5 to 5 μm) and large (5 to 10 μm) particles. (F) GISP2 LLS measurements (13). (G) Simulated 43 and 42 BCE average air -temperature anomalies (hashing shows anomalies that are not significant [2σ]). Annual and seasonal simulations for each year are shown in SI Appendix, Figs. S6 and S7. All records were aligned at the start of the 43 BCE volcanic event to be consistent with the DRI_NGRIP2 chronology. The vertical shaded bar shows the extent of the insoluble particle (i.e., tephra) spike at the start of volcanic fallout in the NGRIP2 record.

Fig. 3.

Total alkali (Na₂O + K₂O) and silica compositions of tephra from GISP2 ice during the 43 BCE event compared with tephra from Okmok II and other potential source volcanoes. Filled circles show measurements (this study) of GISP2 and Okmok reference tephra. Shaded regions show tephra measurements from other potential first century BCE source volcanoes: Etna, Italy (21, 22); Chiltepe from Apoyeque, Nicaragua (23, 24); Masaya Triple Tuff (MTL), Nicaragua (23); A-2000, Askja, Iceland (25); White River Ash northern lobe (WRAn), Churchill, Canada (26); Furnas, Azores (27). Inset shows a tephra shard from the GISP2 sample. See SI Appendix, Fig. S5 for additional comparisons and analytical precision.

Fig. 4.

Volcanically forced temperature and precipitation anomalies in the Mediterranean region from 60 to 30 BCE. (A) CESM-simulated average annual temperature and (B) precipitation anomalies for 43 and 42 BCE, with outlines of Roman provinces north (red) and south (orange) of the Mediterranean. Dots show areas where annual anomalies are not significant (2σ) relative to the 60 to 46 BCE background variability with no volcanic forcing. Also shown are time series of simulated annual and seasonal temperature and precipitation anomalies for northern and southern Roman provinces. Years with symbols are significant (2σ) relative to the background variability. Gray bars show dates of ice core–based volcanic sulfur injections (32) in the simulations, including the early 43 BCE Okmok II eruption.