Historic volcanic eruption shrunk the mighty Nile River

Researchers found that Iceland's Laki volcanic event, a series of about ten eruptions from June 1783 through February 1784, significantly changed atmospheric circulations across much of the Northern Hemisphere. This created unusual temperature and precipitation patterns that peaked in the summer of 1783, including far below normal rainfall over much of the Nile River watershed and record low river levels.

The study provides new evidence that large volcanic eruptions north of the equator often have far different impacts on climate than those in the tropics. “While considerable research has shown that eruptions in the tropics influence climate in the Northern Hemisphere winter, this study indicates that eruptions in high-latitudes produce changes in atmospheric circulation in the Northern Hemisphere summer,” said lead author Luke Oman, Rutgers University, New Brunswick, N.J. Using a sophisticated computer model developed by NASA's Goddard Institute for Space Studies, New York, the researchers linked the Laki eruptions to a cascade of effects that rippled across much of the Northern Hemisphere, altering surface temperatures that ultimately resulted in much below normal rainfall over the Sahel of Africa and record low water levels on the Nile River for up to a year. The Sahel is a stretch of land from the Atlantic Ocean to the “Horn of Africa” that includes the Sahara Desert and savanna areas with sparse vegetation.

“These findings may help us improve our predictions of climate response following the next strong high-latitude eruption, specifically concerning changes in temperature and precipitation,” said Oman. “Many societies are very dependent on seasonal precipitation for their livelihood and these predictions may ultimately allow communities time to plan for consequences, including impacts on regional food and water supplies.”

The Laki event had such a significant impact on the climate because it released large amounts of sulfur dioxide into the atmosphere. When combined with water vapor, the gas formed into tiny particles called aerosols that reduced incoming solar radiation, cooling the average temperature over Northern Hemisphere land masses by as much as 3 degrees Celsius (5.4 degrees Fahrenheit) in the summer of 1783, as simulated with the computer model. Tree ring data also showed significantly reduced tree growth in the summer of 1783, indicative of the coolest summer of the last 400 years in northwestern Alaska, while tree growth in parts of Siberia was the least in 500-600 years.

These unusually cool temperatures reduced the temperature difference between the land masses of Eurasia and Africa and the Indian and Atlantic oceans, weakening the African and Indian monsoon. A monsoon is a seasonal shift in wind direction and in this region marks the return of the rainy season. Without a significant temperature contrast between land and ocean, onshore winds weaken, reducing the inland transport of moisture and rainfall in the region.

In contrast to the cooling over Northern Hemisphere land masses, computer simulations showed the weakening monsoon led to an area of significant warming of 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) over the Sahel of Africa, southern Arabian Peninsula, and India in the summer of 1783. The researchers believe the weaker-than-normal monsoon reduced the cloud cover in the region, allowing more of the sun's energy to reach the surface, raising temperatures and further worsening drought conditions.

Computer model simulations also showed that this reduction in cloud cover was consistent with a decline in summer precipitation. “Some of the driest weather occurred over the Nile and Niger River watersheds,” said Oman. “The relative lack of cloud cover and increased temperature likely amplified evaporation, further lessening water available for run-off.”

To see what effect major high-latitude volcanic eruptions have on rainfall and river levels, the researchers used records on the height of the Nile River that date back to 622 A.D. Record low Nile River water levels occurred in 1783-1784 following the Laki event. Similarly low levels were observed after the Mount Katmai, Alaska, eruption in 1912, when the Niger River was also at a record low. And in 939 A.D. there was also low Nile River flow following the Eldgjá eruption in Iceland. “Our analysis found there is less than a 3 percent chance that the Laki and Katmai low river flow events could be attributed to natural climate variability,” said Oman.

Unlike the Laki and Katmai eruptions, similarly powerful eruptions in the tropics usually release aerosols high into the atmosphere, where they can spread around much of the globe for up to two years. As a result, tropical eruptions can influence climate around the world, but often in a different and less dramatic fashion. For instance, the eruption of Mount Pinatubo in the Philippines in 1991 resulted in upper atmospheric warming, but the aerosols also blocked heat from the sun in the lower atmosphere, cooling surface temperatures in the subtropics. This worked to reduce the difference in temperatures from north to south, altering large-scale atmospheric circulations that resulted in changes to other areas, including warmer-than-normal winters over the Northern Hemisphere.