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Banner photo: An International Space Station photo of the lower Nile River, Egypt and nearby locations

Hydroclimatic Impacts of Persistent Explosive Volcanism

On June 15, 1991, Mount Pinatubo, a volcano in the Philippines, erupted, injecting approximately 18.5 million metric tons of sulfur dioxide (SO2) into the stratosphere, creating sulfate aerosols that caused a mean global cooling of 0.5°C (˜1°F) at surface for a couple of years, in one of the largest and best observed eruptions of the 20th century. Scientists examine this event to study what the potential impact might be of large eruptions in the future.

Aerosols and Sunlight:
Probably without even realizing it, everyone is familiar with the interaction of aerosols with solar radiation. It is the strikingly beautiful scene of red and orange colors lighting up the sky when you look over a polluted city at sunset. Aerosols, depending on their size, change the direction of incoming solar rays, an effect known as scattering, sending some back to space. Aerosol scattering creates the colorful hues of sunset, effectively reducing the amount of solar radiation that reaches the surface where we live. Volcanic aerosols greatly amplify this effect because their size is optimal to maximize scattering.

Fortunately, rain removes these aerosols and their hues relatively quickly. In general, tropospheric aerosols have a residence time in the atmosphere of only a few days. Sulfate aerosols in the stratosphere though, the layer just over the troposphere, can stay aloft for years. The stratosphere is very dry, leaving gravity and the background circulation as the only means to remove these contaminants, a very slow process indeed for such small particles.

Separately, volcanic eruptions have also inspired a geoengineering idea in which the artificial injection of reflective aerosols into the upper atmosphere could provide a "solar radiation management" to mask greenhouse gas-induced global warming. However, large volcanic eruptions affect many more aspects of the Earth system beyond surface temperature. What would be the impact on human civilization? Could we adapt to changes of climate, temperatures, and even the hydrological cycle, all of which could interfere with food production and supply?

Although climate models can simulate the answer to these questions, there is only the single Mount Pinatubo example in the satellite era to inform such models. The magnitude and impact of other volcanoes must rely on simulations of atmospheric aerosols produced from past globally disruptive events, and then scientists study the resulting climate impacts in the context of history.

A real-world case of the consequences of persistent volcanic forcing appears in historical archives of Ptolemaic Egypt (305-30 BCE; see Fig. 1). This period in Egypt was a time of cultural and scientific advancement, greatly aided by the reliable annual flood of the Nile River, which (paired with seasonal human damming of these fertile waters) allowed a large, reliable food crop to be grown. The Ptolemaic era also suffered intense power struggles with rival powers and internal revolt and unrest; 12 of these 26 Ptolemaic-era revolts began close in time to volcanic eruption dates, suggesting volcanoes disrupted this society.

Two photos. At left is a papyrus scroll that describes an ancient Egyptian crop harvest. At right are the two sides of a coin minted early in the Ptolemaic era

Figure 1. A) Ancient Egyptians had a highly organized agriculture system, and this papyrus bears an administrative correspondence regarding a crop survey dated to before 190 BCE (Image credit: P. Yale I 36 (CtYBR inv. 1647)); B) A copper coin from Ptolemaic Egypt, showing the likeness of Ptolemy II, King of Egypt, 309–246 BCE, and who ruled 283–246 BCE. The coin, made of copper, was minted in Alexandria between 285–246 BCE. (Image credit: Ruth Elizabeth White Fund, Yale Art Gallery 2004.6.4054).

How exactly does the link between eruptions and poor Nile flooding work?

Climate scientists from Columbia University and the NASA Goddard Institute for Space Studies (GISS) partnered with historians, climate scientists, and hydrologists from Ireland to California to form the Yale Nile Initiative to explore the links between explosive volcanism and hydroclimate responses across the Nile River basin during the Ptolemaic era.

Within the decade of 168 to 158 BCE, four large volcanoes erupted, and the climate impacts reverberated across the world. Although we do not know exactly where these volcanoes were, we can infer their latitude from "fossil" volcanic sulfate layers preserved in the polar ice caps. The first volcano was tropical in origin and about 125% the size of the 1991 Pinatubo eruption. The rest of the decade had evenly spaced eruptions of one-third the size of the 1991 Pinatubo eruption. Ptolemaic Egypt had exceptionally poor Nile flooding in 169 BCE and further failures in 166 and 161 BCE. These flooding failures and related agricultural crises (crop failures) may have laid the groundwork for internal revolts and unrest. The Ptolemaic 160s BCE decade was peppered with uprisings and power struggles, including the invasion of Seleukid King Antiochus IV during the 6th Syrian war (170 and 168 BCE) and the intervention of self-interested Romans to stop the Seleukids.

Could these four volcanos of the 160s BCE have caused a failure of the Nile Flood by impacting rainfall of its headwaters in Ethiopia and the Great Lakes? What other impacts did they have? What lessons can we learn?

GISS ModelE incorporates our knowledge of how the climate system works — including predictions for how aerosols interact with climate, the biosphere, the cryosphere, and the ocean. We set it to a baseline of the Ptolemaic Era, then injected SO2 into the virtual atmosphere (Fig. 2).

Line plots of simulated surface air temperature and atmospheric opacity resulting from sulfur dioxide injection

Figure 2. The blue lines in the lower panel show the simulated increases in atmospheric opacity (or aerosol optical depth, at 550nm wavelength) after the repeated injection of SO2 from the "volcanic quartet" which caused the simulated global surface cooling shown in the top panel (see the black lines). (Image credit: Singh et al. 2023)

Persistent volcanic aerosols from the eruption quartet created a cascade of modeled impacts on temperature and rainfall of the 160s BCE, with the impacts of the initial volcanic eruption in 168 BCE being sustained for over a decade (Fig. 2). These aerosols hindered the northward migration of the tropical rainfall belt during the northern hemisphere summer and monsoon season over East Africa as well as the South and East Asia monsoons in the northern hemisphere, including rainfall over the Ethiopian highlands, the headwater of the Nile (Fig. 3). Annually, Nile River water flow was reduced as much as 38% after the first and largest eruption and up to 18% after the subsequent three eruptions.

This Ptolemaic "volcanic quartet" provides a natural lesson for us looking towards the future as we consider geoengineering ideas such as solar resource management. Prolonged aerosol forcing not only decreases temperature, but also causes adverse climate impacts through altering rainfall patterns, potentially diminishing food security for many nations. We did not study here the direct impact that a change from direct sunlight to scattered sunlight (diffuse radiation) might have on crop yields and regional ecosystems. However, our study points to the need for continuing research into the potential adverse impacts at regional and sub-regional scales from any deliberate effort at solar radiation management.

Four map plots of North Africa and adjacent regions showing simulated decrease in monsoon reason rains resulting from volcanic eruptions

Figure 3. This graph shows the decrease in monsoon season (June to September) rainfall for the first year after each of the eruption over Africa. The blue line outlines the modern Nile River basin extent; brown colors indicate drying areas. (Image credit: Singh et al. 2023)


Related Link

Yale Nile Initiative: Partnership of climate scientists from Columbia University and the NASA Goddard Institute for Space Studies (GISS) with historians, paleoclimate scientists, and hydrologists from Ireland to California form the Yale Nile Initiative to explore the links between explosive volcanism and hydroclimate responses across the Nile River basin during the Ptolemaic era.

Reference

Singh, R., K. Tsigaridis, A.N. LeGrande, F. Ludlow, and J.G. Manning, 2023: Investigating hydroclimatic impacts of the 168–158 BCE volcanic quartet and their relevance to the Nile River basin and Egyptian history, Clim. Past, 19, 249–275, https://doi.org/10.5194/cp-19-249-2023.

Contact

Please address all inquiries about this research to Dr. Ram Singh.

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