African Sahara 'greening' can alter Northern Hemisphere climate, modeling study finds

Africa's Sahara Desert may be considered a vast expanse of barren sand with limited vegetation, an extreme environment for plants and animals to thrive, but life always finds a way. Indeed, vegetation growth in the desert has waxed and waned over millennia, with periods of enhanced growth termed "greening."

One such time of flora expansion occurred 5,000–11,000 years ago, during the first half of the Holocene. Increased solar radiation during boreal summer (June to August) due to the summer solstice coinciding with Earth's perihelion orbit (when the planet is closest to the sun) led to changes in seasonality across the tropics, mid and high latitudes.

Consequently, strengthening of monsoons in the Northern Hemisphere is thought to have played a significant role in increasing humidity across Africa, initiating conditions conducive to the growth of evergreen shrubs in the Sahara.

New research published in Climate of the Past has used climate model simulations to reconstruct the impact of this Saharan greening on the mid-latitudes during the middle Holocene. Dr. Marco Gaetani, Associate Professor at IUSS School for Advanced Studies in Pavia, Italy, and colleagues' numerical simulations identified Saharan greening having a year-long impact on atmospheric circulation in the Northern Hemisphere, especially during boreal summer when the African monsoon develops.

As such, regions in the Northern Hemisphere experienced abnormal climate, being warmer and drier in Scandinavia and North America, colder winters and warmer summers in western Europe, overall warming in central Europe, colder and rainier in the Mediterranean, plus warmer winters and colder summers with increased rainfall throughout the year in central Asia.

To account for these changes, the research team found that Walker Circulation (an atmospheric loop with rising air to the west as air moves westerly at higher latitudes and sinking air to the east as air moves easterly around the tropics) shifted westwards during the middle Holocene. This had important consequences for jet streams, causing the North Atlantic component to intensify and alter track in summer, followed by the North Pacific in winter.

They also identified a change in the North Atlantic Oscillation (where alterations in surface sea level pressure across this ocean basin lead to modified temperature and precipitation patterns on nearby continents) which shifted from positive to negative across both boreal winter (December to February) and summer months.

Consequently, there were warmer and drier summers in the eastern Mediterranean, northern Africa and polar North America, but cooler and wetter summers in northern, central and eastern Europe and eastern North America.

All of these climate modifications had longevity over thousands of years due to an 80% reduction in dust emission, as well as albedo (a unitless measure of how well the Earth's surface reflects solar energy, with 0 being black and 1 being white) declining from 0.30 for desert down to 0.15 for shrubs, enhancing tropical warming. There was also increased water recycling due to the presence of more vegetation, therefore keeping drought conditions at bay.

The interconnected nature of the atmosphere and oceans means there is more scope to explore the consequences of Saharan greening across both terrestrial and marine realms in the future.

Recent reports have highlighted the role of climate change in altering long-established weather patterns, with the northern migration of weather systems across Africa leading to heavier rainfall and vegetation corridors forming across the Sahara, influencing the distribution and survival of life across this previously comparatively barren landscape.